diff --git a/docs/tutorial/step1.html b/docs/tutorial/step1.html index 2468f1da8..fc66b124c 100644 --- a/docs/tutorial/step1.html +++ b/docs/tutorial/step1.html @@ -39,7 +39,7 @@
This is the first section of the FreeType 2 tutorial. It will teach you to do the following:
- +NOTA BENE: A single face object can deal with one or more size - objects at a time; however, this is something that few programmers - really need to do. We have thus have decided to simplify the API for - the most common use (i.e. one size per face), while keeping this feature - available through additional functions.
- + +A single face object can deal with one or more size objects at a + time; however, this is something that few programmers really need to do. + We have thus have decided to simplify the API for the most common use + (i.e. one size per face), while keeping this feature available through + additional functions.
+When a new face object is created, its size object defaults to the character size of 10 pixels (both horizontally and vertically) for scalable formats. For fixed-sizes formats, the size is more or less @@ -374,7 +374,7 @@
This example will set the character pixel sizes to 16x16 pixels. +
This example will set the character pixel sizes to 16x16 pixels. As previously, a value of 0 for one of the dimensions means same as the other.
@@ -452,7 +452,7 @@ Unicode character codes if it finds one. Otherwise, it tries to find support for Latin-1, then ASCII. -We will describe later how to look for specific charmaps in a face. +
We will describe later how to look for specific charmaps in a face. For now, we will assume that the face contains at least a Unicode charmap that was selected during FT_New_Face(). To convert a Unicode character code to a font glyph index, we use @@ -479,13 +479,13 @@
Once you have a glyph index, you can load the corresponding glyph image. The latter can be stored in various formats within the font - file. For fixed-size formats like FNT or PCF, each image is a bitmap. + file. For fixed-size formats like FNT or PCF, each image is a bitmap. Scalable formats like TrueType or Type 1 use vectorial shapes, named outlines to describe each glyph. Some formats may have - even more exotic ways of representing glyph (e.g. MetaFont). + even more exotic ways of representing glyph (e.g. MetaFont). Fortunately, FreeType 2 is flexible enough to support any kind of glyph format through a simple API.
- +The glyph image is always stored in a special object called a
glyph slot. As its name suggests, a glyph slot is a
container that is able to hold one glyph image at a time, be it a
@@ -497,7 +497,7 @@
The load_flags value is a set of bit flags used to
indicate some special operations. The default value
FT_LOAD_DEFAULT is 0. This function will try to load the corresponding glyph image from
the face. Basically, this means that If a bitmap is found for the corresponding glyph and pixel
@@ -524,7 +524,7 @@
for certain formats like TrueType and Type 1. The field glyph->format describes the format used to store
the glyph image in the slot. If it is not
ft_glyph_format_bitmap, it is possible to immedialy convert
@@ -536,7 +536,7 @@
face->glyph, /* glyph slot */
render_mode ); /* render mode */
- error = FT_Load_Glyph(
+ error = FT_Load_Glyph(
face, /* handle to face object */
glyph_index, /* glyph index */
load_flags ); /* load flags, see below */
@@ -506,10 +506,10 @@
-
+
The parameter render_mode specifies how to render the glyph image. Set it ft_render_mode_normal to render a high-quality anti-aliased (256 gray levels) bitmap. You can @@ -547,11 +547,11 @@ through glyph->bitmap (a simple bitmap descriptor), and position it with glyph->bitmap_left and glyph->bitmap_top.
- +Note that bitmap_left is the horizontal distance from the current pen position to the left-most border of the glyph bitmap, while bitmap_top is the vertical distance from the pen - position (on the baseline) to the top-most border of the glyph bitmap. + position (on the baseline) to the top-most border of the glyph bitmap. It is positive to indicate an upwards distance.
The second part of the tutorial will describe the contents of a @@ -572,23 +572,23 @@
There are two ways to select a different charmap with FreeType 2. The easiest is if the encoding you need already has a corresponding enumeration defined in - <freetype/freetype.h>, as ft_encoding_big5. + <freetype/freetype.h>, as ft_encoding_big5. In this case, you can simply call FT_Select_CharMap() as in
- +error = FT_Select_CharMap( face, /* target face object */ ft_encoding_big5 ); /* encoding */- +
Another way is to manually parse the list of charmaps for the face, this is accessible through the fields num_charmaps and charmaps (notice the final 's') of the face object. As you could expect, the first is the number of charmaps in the face, while the second is a table of pointers to the charmaps embedded in the face.
- +Each charmap has a few visible fields used to describe it more precisely. Mainly, one will look at charmap->platform_id and charmap->encoding_id which define a pair of values that can @@ -605,7 +605,7 @@ list. Bear in mind that some encodings correspond to several values pairs (yes, it's a real mess, but blame Apple and Microsoft on such stupidity). Here some code to do it:
- +FT_CharMap found = 0; @@ -642,9 +642,9 @@+It is possible to specify an affine transformation to be applied to glyph images when they are loaded. Of course, this will only work for scalable (vectorial) font formats.
- +To do that, simply call FT_Set_Transform(), as in
- +error = FT_Set_Transform( @@ -652,22 +652,22 @@ &matrix, /* pointer to 2x2 matrix */ &delta ); /* pointer to 2d vector */- +This function will set the current transformation for a given face object. Its second parameter is a pointer to a FT_Matrix structure that describes a 2x2 affine matrix. The third parameter is a pointer to a FT_Vector structure that describes a simple 2d vector that is used to translate the glyph image after the 2x2 transformation.
- +Note that the matrix pointer can be set to NULL, in which case the identity transformation will be used. Coefficients of the matrix are otherwise in 16.16 fixed float units.
- +The vector pointer can also be set to NULL in which case a delta vector of (0,0) will be used. The vector coordinates are expressed in 1/64th of a pixel (also known as 26.6 fixed floats).
- +The transformation is applied to every glyph that is loaded through FT_Load_Glyph() and is completely independent of any hinting process. This means that you won't get the same @@ -682,7 +682,7 @@ compute a new character pixel size, then the other one to call FT_Set_Transform(). This is explained in details in a later section of this tutorial.
- +Note also that loading a glyph bitmap with a non-identity transformation will produce an error.
@@ -695,7 +695,7 @@We will now present you with a very simple example used to render a string of 8-bit Latin-1 text, assuming a face that contains a Unicode charmap
- +The idea is to create a loop that will, on each iteration, load one glyph image, convert it to an anti-aliased bitmap, draw it on the target surface, then increment the current pen position.
@@ -706,7 +706,7 @@The following code performs our simple text rendering with the functions previously described.
- +FT_GlyphSlot slot = face->glyph; /* a small shortcut */ @@ -716,37 +716,37 @@ .. initialize library .. .. create face object .. .. set character size .. - + pen_x = 300; pen_y = 200; - + for ( n = 0; n < num_chars; n++ ) { FT_UInt glyph_index; - + /* retrieve glyph index from character code */ glyph_index = FT_Get_Char_Index( face, text[n] ); - + /* load glyph image into the slot (erase previous one) */ error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT ); if ( error ) continue; /* ignore errors */ - + /* convert to an anti-aliased bitmap */ error = FT_Render_Glyph( face->glyph, ft_render_mode_normal ); if ( error ) continue; - + /* now, draw to our target surface */ my_draw_bitmap( &slot->bitmap, pen_x + slot->bitmap_left, pen_y - slot->bitmap_top ); - + /* increment pen position */ pen_x += slot->advance.x >> 6; pen_y += slot->advance.y >> 6; /* not useful for now */ }- - +
This code needs a few explanations:
The following code is a refined version of the example above. It uses features and functions of FreeType 2 that have not yet been introduced, and which will be explained below.
- +FT_GlyphSlot slot = face->glyph; /* a small shortcut */ @@ -795,29 +795,29 @@ .. initialize library .. .. create face object .. .. set character size .. - + pen_x = 300; pen_y = 200; - + for ( n = 0; n < num_chars; n++ ) { /* load glyph image into the slot (erase previous one) */ error = FT_Load_Char( face, text[n], FT_LOAD_RENDER ); if ( error ) continue; /* ignore errors */ - + /* now, draw to our target surface */ my_draw_bitmap( &slot->bitmap, pen_x + slot->bitmap_left, pen_y - slot->bitmap_top ); - + /* increment pen position */ pen_x += slot->advance.x >> 6; }- +
We have reduced the size of our code, but it does exactly the same thing.
- +We use the function FT_Load_Char() instead of @@ -828,20 +828,20 @@
We do not use FT_LOAD_DEFAULT for the loading mode but the bit flag FT_LOAD_RENDER. It indicates that the glyph - image must be immediately converted to an anti-aliased bitmap. + image must be immediately converted to an anti-aliased bitmap. This is of course a shortcut that avoids calling FT_Render_Glyph() explicitly but is strictly equivalent.
- +Note that you can also specify that you want a monochrome bitmap instead by using the additional FT_LOAD_MONOCHROME load flag.
We now render transformed text (for example through a rotation). + +
We now render transformed text (for example through a rotation). To do that we use FT_Set_Transform():
@@ -862,30 +862,30 @@ matrix.xy = (FT_Fixed)(-sin( angle ) * 0x10000L ); matrix.yx = (FT_Fixed)( sin( angle ) * 0x10000L ); matrix.yy = (FT_Fixed)( cos( angle ) * 0x10000L ); - + /* the pen position in 26.6 cartesian space coordinates */ pen.x = 300 * 64; pen.y = ( my_target_height - 200 ) * 64; - + for ( n = 0; n < num_chars; n++ ) { /* set transformation */ FT_Set_Transform( face, &matrix, &pen ); - + /* load glyph image into the slot (erase previous one) */ error = FT_Load_Char( face, text[n], FT_LOAD_RENDER ); if ( error ) continue; /* ignore errors */ - + /* now, draw to our target surface (convert position) */ my_draw_bitmap( &slot->bitmap, slot->bitmap_left, my_target_height - slot->bitmap_top ); - + /* increment pen position */ pen.x += slot->advance.x; pen.y += slot->advance.y; } - +Notes:
@@ -916,7 +916,7 @@ is not rounded this time. - +It is important to note that, while this example is a bit more complex than the previous one, it is strictly equivalent for the case where the transformation is the identity. Hence it can be used as a @@ -934,11 +934,11 @@
In this first section, you have learned the basics of FreeType 2, as well as sufficient knowledge how to render rotated text.
- +The next part will dive into more details of the API in order to let you access glyph metrics and images directly, as well as how to deal with scaling, hinting, kerning, etc.
- +The third part will discuss issues like modules, caching, and a few other advanced topics like how to use multiple size objects with a single face.
diff --git a/docs/tutorial/step2.html b/docs/tutorial/step2.html index d8cc1505c..242546389 100644 --- a/docs/tutorial/step2.html +++ b/docs/tutorial/step2.html @@ -1,4 +1,5 @@ - +
@@ -36,9 +37,9 @@ Introduction - This is the second section of the FreeType 2 tutorial. It will teach - you the following: - +This is the second section of the FreeType 2 tutorial. It will + teach you the following: +
- + + + - 1. Glyph metrics: + 1. Glyph metrics-Glyph metrics are, as their name suggests, certain distances associated - to each glyph in order to describe how to use it to layout text. +Glyph metrics are, as their name suggests, certain distances + associated to each glyph in order to describe how to use it to layout + text. -There are usually two sets of metrics for a single glyph: those used to - layout the glyph in horizontal text layouts (like latin, cyrillic, - arabic, hebrew, etc..), and those used to layout the glyph in vertical - text layouts (like some layouts of Chinese, Japanese, Korean, and - others..). +There are usually two sets of metrics for a single glyph: those used + to layout the glyph in horizontal text layouts (like Latin, Cyrillic, + Arabic, Hebrew, etc.), and those used to layout the glyph in vertical + text layouts (like some layouts of Chinese, Japanese, Korean, and + others). + +Note that only a few font formats provide vertical metrics. You can + test wether a given face object contains them by using the macro + FT_HAS_VERTICAL(face), which is true if has vertical + metrics. -Note that only a few font formats provide vertical metrics. You can - test wether a given face object contains them by using the macro - FT_HAS_VERTICAL(face), which is true when appropriate. -Individual glyph metrics can be accessed by first loading the glyph - in a face's glyph slot, then accessing them through the - face->glyph->metrics structure. This will be detailed - later, for now, we'll see that it contains the following fields: - -
NOTA BENE: As all fonts do not contain vertical - metrics, the values of vertBearingX, vertBearingY - and vertAdvance should not be considered reliable when - FT_HAS_VERTICAL(face) is false. +
As not all fonts do contain vertical metrics, the values of + vertBearingX, vertBearingY, and vertAdvance + should not be considered reliable if FT_HAS_VERTICAL(face) is + false. + +The following graphics illustrate the metrics more clearly. First, + for horizontal metrics, where the baseline is the horizontal axis: + +The following graphics illustrate the metrics more clearly. First, for - horizontal metrics, where the baseline is the horizontal axis : - -For vertical text layouts, the baseline is vertical and is the - vertical axis: - -The metrics found in face->glyph->metrics are normally - expressed in 26.6 pixels (i.e 1/64th of pixels), unless you use - the FT_LOAD_NO_SCALE flag when calling - FT_Load_Glyph or FT_Load_Char. In this case, - the metrics will be expressed in original font units. - -The glyph slot object has also a few other interesting fields - that will ease a developer's work. You can access them though - face->glyph->??? : + expressed in 26.6 pixel format (i.e 1/64th of pixels), unless you use + the FT_LOAD_NO_SCALE flag when calling FT_Load_Glyph() + or FT_Load_Char(). In that case, the metrics will be expressed + in original font units. -
The glyph slot object has a few other interesting fields that will + ease a developer's work. You can access them through + face->glyph->???: + +
+ + + 2. Managing glyph images ++ +The glyph image that is loaded in a glyph slot can be converted into + a bitmap, either by using FT_LOAD_RENDER when loading it, or by + calling FT_Render_Glyph() afterwards. Each time you load a new + glyph image, the previous one is erased from the glyph slot. + +There are times, however, where you may need to extract this image + from the glyph slot, in order to cache it within your application, and + even perform additional transformations and measures on it before + converting it to a bitmap. + +The FreeType 2 API has a specific extension which is capable of + dealing with glyph images in a flexible and generic way. To use it, you + first need to include the ftglyph.h header file: + + ++ #include <freetype/ftglyph.h>+ + + We will now explain how to use the functions defined in this + file. + ++ a. Extracting the glyph image ++ +You can extract a single glyph image very easily. Here some code + that shows how to do it. + + ++ FT_Glyph glyph; /* handle to glyph image */ -+ - The glyph image that is loaded in a glyph slot can be converted into - a bitmap, either by using FT_LOAD_RENDER when loading it, or - by calling FT_Render_Glyph. Each time you load a new glyph - image, the previous one is erased from the glyph slot. - -There are times however where you may need to extract this image from - the glyph slot, in order to cache it within your application, and - even perform additional transforms and measures on it before converting - it to a bitmap. - +As can be seen, we have -The FreeType 2 API has a specific extension which is capable of dealing - with glyph images in a flexible and generic way. To use it, you first need - to include the "ftglyph.h" header file, as in: - -- #include <freetype/ftglyph.h> -+
We will now explain how to use the functions defined in this file: - -a. Extracting the glyph image:+It is important to note that the extracted glyph is in the same + format as the original one that is still in the slot. For example, if + we are loading a glyph from a TrueType font file, the glyph image will + really be a scalable vector outline. -You can extract a single glyph image very easily. Here's some code - that shows how to do it: +You can access the field glyph->format if you want to know + exactly how the glyph is modeled and stored. A new glyph object can + be destroyed with a call to FT_Done_Glyph. -- FT_Glyph glyph; // handle to glyph image - - .... - error = FT_Load_Glyph( face, glyph, FT_LOAD_NORMAL ); - if (error) { .... } - - error = FT_Get_Glyph( face->glyph, &glyph ); - if (error) { .... } -+ The glyph object contains exactly one glyph image and a 2d vector + representing the glyph's advance in 16.16 fixed float coordinates. + The latter can be accessed directly as glyph->advance. -As you see, we have: - -
Note that unlike other FreeType objects, the library doesn't + keep a list of all allocated glyph objects. This means you will need + to destroy them yourself, instead of relying on + FT_Done_FreeType() doing all the clean-up. -It is important to note that the extracted glyph is in the same format - than the original one that is still in the slot. For example, if we're - loading a glyph from a TrueType font file, the glyph image will really - be a scalable vector outline. ++ b. Transforming & copying the glyph image +-You can access the field glyph->format if you want to - know exactly how the glyph is modeled and stored. A new glyph object can - be destroyed with a call to FT_Done_Glyph. +If the glyph image is scalable (i.e., if glyph->format is + not equal to ft_glyph_format_bitmap), it is possible to + transform the image anytime by a call to + FT_Glyph_Transform(). -The glyph object contains exactly one glyph image and a 2D vector - representing the glyph's advance in 16.16 fixed float coordinates. - The latter can be accessed directly as glyph->advance - +You can also copy a single glyph image with + FT_Glyph_Copy(). Here some example code: -Note that unlike - other FreeType objects, the library doesn't keeps a list of all - allocated glyph objects. This means you'll need to destroy them - yourself, instead of relying on FT_Done_FreeType doing - all the clean-up. - -b. Transforming & copying the glyph image+ ++ FT_Glyph glyph, glyph2; + FT_Matrix matrix; + FT_Vector delta; -+ - c. Measuring the glyph image- -You can also retrieve the control (bounding) box of any glyph image - (scalable or not), through the FT_Glyph_Get_CBox function, - as in: - +Note that the 2x2 transform matrix is always applied to the 16.16 + advance vector in the glyph; you thus don't need to recompute it. -- FT_BBox bbox; - ... - FT_Glyph_BBox( glyph, bbox_mode, &bbox ); -+ + c. Measuring the glyph image +-Coordinates are relative to the glyph origin, i.e. (0,0), using the - Y_upwards convention. This function takes a special argument, the - "bbox mode", that is a set of bit flags used to indicate how - box coordinates are expressed. If ft_glyph_bbox_subpixels - is set in the bbox mode, the coordinates are returned in 26.6 pixels - (i.e. 1/64th of pixels). Otherwise, they're in integer pixels. +You can also retrieve the control (bounding) box of any glyph image + (scalable or not), using the FT_Glyph_Get_CBox function: -Note that the box's maximum coordinates are exclusive, which means - that you can always compute the width and height of the glyph image, - be in in integer or 26.6 pixels with: - -- width = bbox.xMax - bbox.xMin; - height = bbox.yMax - bbox.yMin; -- - Note also that for 26.6 coordinates, if - ft_glyph_bbox_gridfit is set in the bbox mode, - the coordinates will also be grid-fitted, which corresponds to: - -- bbox.xMin = FLOOR(bbox.xMin) - bbox.yMin = FLOOR(bbox.yMin) - bbox.xMax = CEILING(bbox.xMax) - bbox.yMax = CEILING(bbox.yMax) -- - The default value for the bbox mode, which is 0, corresponds to - ft_glyph_bbox_pixels (i.e. integer pixel coordinates). + ++ FT_BBox bbox; -+ + Coordinates are relative to the glyph origin, i.e. (0,0), using the + Y upwards convention. This function takes a special argument, + the bbox mode, to indicate how box coordinates are expressed. + If bbox_mode is set to ft_glyph_bbox_subpixels, the + coordinates are returned in 26.6 pixels (i.e. 1/64th of pixels). + + Note that the box's maximum coordinates are exclusive, which means + that you can always compute the width and height of the glyph image, + be it in integer or 26.6 pixels with + + ++ width = bbox.xMax - bbox.xMin; + height = bbox.yMax - bbox.yMin;+ + + Note also that for 26.6 coordinates, if + ft_glyph_bbox_gridfit is set in bbox_mode, the + coordinates will also be grid-fitted, which corresponds to + + ++ bbox.xMin = FLOOR(bbox.xMin) + bbox.yMin = FLOOR(bbox.yMin) + bbox.xMax = CEILING(bbox.xMax) + bbox.yMax = CEILING(bbox.yMax)+ + + The default value for the bbox mode is + ft_glyph_bbox_pixels (i.e. integer, grid-fitted pixel + coordinates). Please check the API reference for + FT_Glyph_Get_CBox() other possible values + ++ d. Converting the glyph image to a bitmap ++ +You may need to convert the glyph object to a bitmap once you have + conveniently cached or transformed it. This can be done easily with + the FT_Glyph_To_Bitmap() function: + + +FT_Vector origin; - - origin.x = 32; /* 1/2 pixel in 26.26 format */ + + + origin.x = 32; /* 1/2 pixel in 26.26 format */ origin.y = 0; - - error = FT_Glyph_To_Bitmap( &glyph, + + error = FT_Glyph_To_Bitmap( &glyph, render_mode, - &origin, - 1 ); // destroy original image == true -+ &origin, + 1 ); /* destroy orig. image == true */ + - We will know details this function's parameters: - -
The new glyph object always contain a bitmap (when no error is returned), - and you must typecast its handle to the - FT_BitmapGlyph type in order to access its content. - This type is a sort of "subclass" of FT_Glyph that contains - additional fields: - -
- - 3. Global glyph metrics: -- -Unlike glyph metrics, global ones are used to describe distances - and features of a whole font face. They can be expressed either in - 26.6 pixels or in design "font units" for scalable formats. +Some details on this function's parameters: -- a. Design Global Metrics: -+
For scalable formats, all global metrics are expressed in font units - in order to be later scaled to device space, according to the rules - described in the last chapter of this section of the tutorial. You - can access them directly as simple fields of a FT_Face - handle. - -However, you need to check that the font face's format is scalable - before using them. One can do it by using the macro - FT_IS_SCALABLE(face) which returns true when - appropriate. +The new glyph object always contains a bitmap (when no error is + returned), and you must typecast its handle to the + FT_BitmapGlyph type in order to access its contents. This + type is a sort of subclass of FT_Glyph that contains + additional fields: -In this case, you can access the global design metrics as: +
Notice how, unfortunately, the values of the ascender and the descender - are not reliable (due to various discrepancies in font formats). - -- b. Scaled Global Metrics: -+- Each size object also contains a scaled versions of some of the global - metrics described above. They can be accessed directly through the - face->size->metrics structure. - -Note that these values correspond to scaled versions of the design - global metrics, with no rounding/grid-fitting performed.. - They are also completely independent of any hinting process. In other - words, don't rely on them to get exact metrics at the pixel level. - They're expressed in 26.6 pixels. ++ 3. Global glyph metrics +-
Unlike glyph metrics, global ones are used to describe distances and + features of a whole font face. They can be expressed either in 26.6 + pixel format or in design font units for scalable formats. -Note that the face->size->metrics structure contains other - fields that are used to scale design coordinates to device space. They're - described, in the last chapter. ++ a. Design global metrics +-- c. Kerning: -- -Kerning is the process of adjusting the position of two subsequent - glyph images in a string of text, in order to improve the general - appearance of text. Basically, it means that when the glyph for an - "A" is followed by the glyph for a "V", the space between them can - be slightly reduced to avoid extra "diagonal whitespace". - -Note that in theory, kerning can happen both in the horizontal and - vertical direction between two glyphs; however, it only happens in - the horizontal direction in nearly all cases except really extreme - ones. +For scalable formats, all global metrics are expressed in font + units in order to be later scaled to device space, according to the + rules described in the last chapter of this part of the tutorial. You + can access them directly as simple fields of a FT_Face + handle. -Note all font formats contain kerning information. Instead, they sometimes - rely on an additional file that contains various glyph metrics, including - kerning, but no glyph images. A good example would be the Type 1 format, - where glyph images are stored in a file with extension ".pfa" or ".pfb", - and where kerning metrics can be found in an additional file with extension - ".afm" or ".pfm". - -FreeType 2 allows you to deal with this, by providing the - FT_Attach_File and FT_Attach_Stream APIs. - Both functions are used to load additional metrics into a face object, - by reading them from an additional format-specific file. For example, - you could open a Type 1 font by doing the following: - -- error = FT_New_Face( library, "/usr/shared/fonts/cour.pfb", 0, &face ); - if (error) { ... } - - error = FT_Attach_File( face, "/usr/shared/fonts/cour.afm" ); - if (error) { .. could not read kerning and additional metrics .. } -+ However, you need to check that the font face's format is scalable + before using them. One can do it by using the macro + FT_IS_SCALABLE(face) which returns true if we have a + scalable format. -Note that FT_Attach_Stream is similar to - FT_Attach_File except that it doesn't take a C string - to name the extra file, but a FT_Stream handle. Also, - reading a metrics file is in no way, mandatory. +In this case, you can access the global design metrics as -Finally, the file attachment APIs are very generic and can be used to - load any kind of extra information for a given face. The nature of the - additional content is entirely font format specific. +
FreeType 2 allows you to retrieve the kerning information between - two glyphs through the FT_Get_Kerning function, whose - interface looks like: - -- FT_Vector kerning; - ... - error = FT_Get_Kerning( face, // handle to face object - left, // left glyph index - right, // right glyph index - kerning_mode, // kerning mode - &kerning ); // target vector -+ Notice how, unfortunately, the values of the ascender and the + descender are not reliable (due to various discrepancies in font + formats). -As you see, the function takes a handle to a face object, the indices - of the left and right glyphs for which the kerning value is desired, - as well as an integer, called the "kerning mode", and a pointer to - a destination vector that receives the corresponding distances. - -The kerning mode is very similar to the "bbox mode" described in a - previous chapter. It's a enumeration that indicates how the - kerning distances are expressed in the target vector. - -The default value is ft_kerning_mode_default which - has value 0. It corresponds to kerning distances expressed in 26.6 - grid-fitted pixels (which means that the values are multiples of 64). - For scalable formats, this means that the design kerning distance is - scaled then rounded. - -The value ft_kerning_mode_unfitted corresponds to kerning - distances expressed in 26.6 unfitted pixels (i.e. that do not correspond - to integer coordinates). It's the design kerning distance that is simply - scaled without rounding. - -Finally, the value ft_kerning_mode_unscaled is used to - return the design kerning distance, expressed in font units. You can - later scale it to device space using the computations explained in the - last chapter of this section. ++ b. Scaled global metrics +-Note that the "left" and "right" positions correspond to the visual - order of the glyphs in the string of text. This is important for + Each size object also contains a scaled versions of some of the + global metrics described above. They can be accessed directly through + the face->size->metrics structure. - bi-directional text, or simply when writing right-to-left text.. - -+ Note that these values correspond to scaled versions of the design + global metrics, with no rounding/grid-fitting performed. They + are also completely independent of any hinting process. In other + words, don't rely on them to get exact metrics at the pixel level. + They are expressed in 26.6 pixel format. -- 4. Simple text rendering: kerning + centering: -+
In order to show off what we just learned, we will now show how to modify - the example code that was provided in section I to render a string of text, - and enhance it to support kerning and delayed rendering. +Note that the face->size->metrics structure contains other + fields that are used to scale design coordinates to device space. + They are described below. -- a. Kerning support: -- -Adding support for kerning to our code is trivial, as long as we consider - that we're still dealing with a left-to-right script like Latin. We - simply need to retrieve the kerning distance between two glyphs in order - to alter the pen position appropriately. The code looks like: - -- FT_GlyphSlot slot = face->glyph; // a small shortcut - FT_UInt glyph_index; - FT_Bool use_kerning; - FT_UInt previous; - int pen_x, pen_y, n; +
As you see, this is not terribly complex :-) + /* now, draw to our target surface */ + my_draw_bitmap( &slot->bitmap, + pen_x + slot->bitmap_left, + pen_y - slot->bitmap_top ); -- b. Centering: -+ /* increment pen position */ + pen_x += slot->advance.x >> 6; -Our code begins to become interesting but it's still a bit too simple - for normal uses. For example, the position of the pen is determined - before we do the rendering when in a normal situation, you would want + /* record current glyph index */ + previous = glyph_index; + } + + +We are done. Notice that + +
+ b. Centering ++ +Our code becomes more interesting but it is still a bit too simple + for normal uses. For example, the position of the pen is determined + before we do the rendering if in a real-life situation; you would want to layout the text and measure it before computing its final position (e.g. centering) or perform things like word-wrapping. - -We're thus now going to decompose our text rendering function into two - distinct but successive parts: the first one will position individual - glyph images on the baseline, while the second one will render the - glyphs. As we'll see, this has many advantages. - -We will thus start by storing individual glyph images, as well as their - position on the baseline. This can be done with code like: - -- FT_GlyphSlot slot = face->glyph; // a small shortcut - FT_UInt glyph_index; - FT_Bool use_kerning; - FT_UInt previous; - int pen_x, pen_y, n; - - FT_Glyph glyphs[ MAX_GLYPHS ]; // glyph image - FT_Vector pos [ MAX_GLYPHS ]; // glyph position - FT_UInt num_glyphs; - .. initialise library .. - .. create face object .. - .. set character size .. - - pen_x = 0; /* start at (0,0) !! */ - pen_y = 0; - - num_glyphs = 0; - use_kerning = FT_HAS_KERNING(face); - previous = 0; - - for ( n = 0; n < num_chars; n++ ) ++ + + As you see, this is a very simple variation of our previous code where we extract each glyph image from the slot, and store it, along with the corresponding position, in our tables. - -Note also that "pen_x" contains the total advance for the string of - text. We can now compute the bounding box of the text string with - a simple function like: - - -- void compute_string_bbox( FT_BBox *abbox ) - { - FT_BBox bbox; - - // initialise string bbox to "empty" values - bbox.xMin = bbox.yMin = 32000; - bbox.xMax = bbox.yMax = -32000; - - // for each glyph image, compute its bounding box, translate it, - // and grow the string bbox - for ( n = 0; n < num_glyphs; n++ ) - { - FT_BBox glyph_bbox; - FT_Glyph_Get_CBox( glyphs[n], &glyph_bbox ); +
The same loop can be used to render the string anywhere on our display - surface, without the need to reload our glyph images each time.. We - could also decide to implement word wrapping, and only draw -- - 5. Advanced text rendering: transform + centering + kerning: -- -We are now going to modify our code in order to be able to easily - transform the rendered string, for example to rotate it. We will - start by performing a few minor improvements: - -a. packing & translating glyphs:- -We'll start by packing the information related to a single glyph image - into a single structure, instead of parallel arrays. We thus define the - following structure type: - -- typedef struct TGlyph_ - { - FT_UInt index; // glyph index - FT_Vector pos; // glyph origin on the baseline - FT_Glyph image; // glyph image - - } TGlyph, *PGlyph; -- - We will also translate each glyph image directly after it is loaded - to its position on the baseline at load time. As we'll see, this - as several advantages. Our glyph sequence loader thus becomes: - -- FT_GlyphSlot slot = face->glyph; // a small shortcut - FT_UInt glyph_index; - FT_Bool use_kerning; - FT_UInt previous; - int pen_x, pen_y, n; - - TGlyph glyphs[ MAX_GLYPHS ]; // glyphs table - PGlyph glyph; // current glyph in table - FT_UInt num_glyphs; - - .. initialise library .. - .. create face object .. - .. set character size .. - - pen_x = 0; /* start at (0,0) !! */ - pen_y = 0; - - num_glyphs = 0; - use_kerning = FT_HAS_KERNING(face); - previous = 0; - - glyph = glyphs; - for ( n = 0; n < num_chars; n++ ) + /* check that we really grew the string bbox */ + if ( bbox.xMin > bbox.xMax ) { - glyph->index = FT_Get_Char_Index( face, text[n] ); - - if ( use_kerning && previous && glyph->index ) - { - FT_Vector delta; - - FT_Get_Kerning( face, previous, glyph->index, - ft_kerning_mode_default, &delta ); - - pen_x += delta.x >> 6; - } - - // store current pen position - glyph->pos.x = pen_x; - glyph->pos.y = pen_y; - - error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT ); - if (error) continue; - - error = FT_Get_Glyph( face->glyph, &glyph->image ); - if (error) continue; - - // translate the glyph image now.. - FT_Glyph_Transform( glyph->image, 0, &glyph->pos ); - - pen_x += slot->advance.x >> 6; - previous = glyph->index - - // increment number of glyphs - glyph++; + bbox.xMin = 0; + bbox.yMin = 0; + bbox.xMax = 0; + bbox.yMax = 0; } - // count number of glyphs loaded.. - num_glyphs = glyph - glyphs; -- Note that translating glyphs now has several advantages. The first - one, is that we don't need to translate the glyph bbox when we compute - the string's bounding box. The code becomes: + /* return string bbox */ + *abbox = bbox; + } + -- void compute_string_bbox( FT_BBox *abbox ) ++ - Now take a closer look, the compute_string_bbox can now - compute the bounding box of a transformed glyph string. For example, - we can do something like: - -- FT_BBox bbox; - FT_Matrix matrix; - FT_Vector delta; - - ... load glyph sequence - - ... setup "matrix" and "delta" - - // transform glyphs - for ( n = 0; n < num_glyphs; n++ ) - FT_Glyph_Transform( glyphs[n].image, &matrix, &delta ); - - // compute bounding box of transformed glyphs - compute_string_bbox( &bbox ); -+ Some remarks: + +
The same loop can be used to render the string anywhere on our + display surface, without the need to reload our glyph images each + time. + ++ + + 5. Advanced text rendering: transformation + centering + kerning ++ +We are now going to modify our code in order to be able to easily + transform the rendered string, for example to rotate it. We will start + by performing a few minor improvements: + ++ a. packing & translating glyphs ++ +We start by packing the information related to a single glyph image + into a single structure instead of parallel arrays. We thus define + the following structure type: + + ++ typedef struct TGlyph_ + { + FT_UInt index; /* glyph index */ + FT_Vector pos; /* glyph origin on the baseline */ + FT_Glyph image; /* glyph image */ + + } TGlyph, *PGlyph;+ + + We will also translate each glyph image directly after it is loaded + to its position on the baseline at load time, which has several + advantages. Our glyph sequence loader thus becomes: + + ++ FT_GlyphSlot slot = face->glyph; /* a small shortcut */ + FT_UInt glyph_index; + FT_Bool use_kerning; + FT_UInt previous; + int pen_x, pen_y, n; + + TGlyph glyphs[MAX_GLYPHS]; /* glyphs table */ + PGlyph glyph; /* current glyph in table */ + FT_UInt num_glyphs; + + + .. initialize library .. + .. create face object .. + .. set character size .. + + pen_x = 0; /* start at (0,0)! */ + pen_y = 0; + + num_glyphs = 0; + use_kerning = FT_HAS_KERNING( face ); + previous = 0; + + glyph = glyphs; + for ( n = 0; n < num_chars; n++ ) + { + glyph->index = FT_Get_Char_Index( face, text[n] ); + + if ( use_kerning && previous && glyph->index ) + { + FT_Vector delta; + + + FT_Get_Kerning( face, previous, glyph->index, + ft_kerning_mode_default, &delta ); + + pen_x += delta.x >> 6; + } + + /* store current pen position */ + glyph->pos.x = pen_x; + glyph->pos.y = pen_y; + + error = FT_Load_Glyph( face, glyph_index, FT_LOAD_DEFAULT ); + if ( error ) continue; + + error = FT_Get_Glyph( face->glyph, &glyph->image ); + if ( error ) continue; + + /* translate the glyph image now */ + FT_Glyph_Transform( glyph->image, 0, &glyph->pos ); + + pen_x += slot->advance.x >> 6; + previous = glyph->index + + /* increment number of glyphs */ + glyph++; + } + + /* count number of glyphs loaded */ + num_glyphs = glyph - glyphs;+ + + Translating glyphs now has several advantages, as mentioned + earlier. The first one is that we don't need to translate the glyph + bounding box when we compute the string's bounding box. The code + becomes: + + ++ void compute_string_bbox( FT_BBox* abbox ) + { + FT_BBox bbox; + + + bbox.xMin = bbox.yMin = 32000; + bbox.xMax = bbox.yMax = -32000; -+ + + compute_string_bbox() can now compute the bounding box of + a transformed glyph string. For example, we can do something + like + + ++ FT_BBox bbox; + FT_Matrix matrix; + FT_Vector delta; + + + ... load glyph sequence + + ... setup "matrix" and "delta" + + /* transform glyphs */ + for ( n = 0; n < num_glyphs; n++ ) + FT_Glyph_Transform( glyphs[n].image, &matrix, &delta ); + + /* compute bounding box of transformed glyphs */ + compute_string_bbox( &bbox );+ + + + b. Rendering a transformed glyph sequence ++ +However, directly transforming the glyphs in our sequence is not a + useful idea if we want to re-use them in order to draw the text string + with various angles or transforms. It is better to perform the affine + transformation just before the glyph is rendered, as in the following code: - -
It's possible to call this function several times to render the string - width different angles, or even change the way "start" is computed in - order to move it to different place. + ++ FT_Vector start; + FT_Matrix transform; -
a.Scaling distances to device space:- -Design coordinates are scaled to device space using a simple scaling - transform, whose coefficients are computed with the help of the - character pixel size: - -- device_x = design_x * x_scale - device_y = design_y * y_scale + /* set up start position in 26.6 cartesian space */ + start.x = ( ( my_target_width - string_width ) / 2 ) * 64; + start.y = ( ( my_target_height - string_height ) / 2 ) * 64; - x_scale = pixel_size_x / EM_size - y_scale = pixel_size_y / EM_size -+ /* set up transformation (a rotation here) */ + matrix.xx = (FT_Fixed)( cos( angle ) * 0x10000L ); + matrix.xy = (FT_Fixed)(-sin( angle ) * 0x10000L ); + matrix.yx = (FT_Fixed)( sin( angle ) * 0x10000L ); + matrix.yy = (FT_Fixed)( cos( angle ) * 0x10000L ); - Here, the value EM_size is font-specific and correspond - to the size of an abstract square of the design space (called the "EM"), - which is used by font designers to create glyph images. It is thus - expressed in font units. It is also accessible directly for scalable - font formats as face->units_per_EM. You should - check that a font face contains scalable glyph images by using the - FT_IS_SCALABLE(face) macro, which returns true when - appropriate. + for ( n = 0; n < num_glyphs; n++ ) + { + FT_Glyph image; + FT_Vector pen; + FT_BBox bbox; -When you call the function FT_Set_Pixel_Sizes, you're - specifying the value of pixel_size_x and pixel_size_y - you want to use to FreeType, which will immediately compute the values - of x_scale and y_scale. -When you call the function FT_Set_Char_Size, you're - specifying the character size in physical "points", which is used, - along with the device's resolutions, to compute the character pixel - size, then the scaling factors. + /* create a copy of the original glyph */ + error = FT_Glyph_Copy( glyphs[n].image, &image ); + if ( error ) continue; -Note that after calling any of these two functions, you can access - the values of the character pixel size and scaling factors as fields - of the face->size->metrics structure. These fields are: - -
Basically, this means that you can scale a distance expressed in - font units to 26.6 pixels directly with the help of the FT_MulFix - function, as in: - -- // convert design distances to 1/64th of pixels - // - pixels_x = FT_MulFix( design_x, face->size->metrics.x_scale ); - pixels_y = FT_MulFix( design_y, face->size->metrics.y_scale ); -- - However, you can also scale the value directly with more accuracy - by using doubles and the equations: - -- FT_Size_Metrics* metrics = &face->size->metrics; // shortcut - double pixels_x, pixels_y; - double em_size, x_scale, y_scale; + /* check bounding box -- if the transformed glyph image */ + /* is not in our target surface, we can avoid rendering */ + FT_Glyph_Get_CBox( image, ft_glyph_bbox_pixels, &bbox ); + if ( bbox.xMax <= 0 || bbox.xMin >= my_target_width || + bbox.yMax <= 0 || bbox.yMin >= my_target_height ) + continue; - // compute floating point scale factors - // - em_size = 1.0 * face->units_per_EM; - x_scale = metrics->x_ppem / em_size; - y_scale = metrics->y_ppem / em_size; - - // convert design distances to floating point pixels - // - pixels_x = design_x * x_scale; - pixels_y = design_y * y_scale; -- - - b. Accessing design metrics (glyph & global): -- -You can access glyph metrics in font units simply by specifying the - FT_LOAD_NO_SCALE bit flag in FT_Load_Glyph - or FT_Load_Char. The metrics returned in - face->glyph->metrics will all be in font units. - -You can access unscaled kerning data using the - ft_kerning_mode_unscaled mode + /* convert glyph image to bitmap (destroy the glyph */ + /* copy!) */ + error = FT_Glyph_To_Bitmap( + &image, + ft_render_mode_normal, + 0, /* no additional translation */ + 1 ); /* destroy copy in "image" */ + if ( !error ) + { + FT_BitmapGlyph bit = (FT_BitmapGlyph)image; -Finally, a few global metrics are available directly in font units - as fields of the FT_Face handle, as described in chapter 3 - of this section. - -- - - Conclusion -- -This is the end of the second section of the FreeType 2 tutorial, - you're now able to access glyph metrics, manage glyph images, and - render text much more intelligently (kerning, measuring, transforming - & caching). - -You have now sufficient knowledge to build a pretty decent text service - on top of FreeType 2, and you could possibly stop there if you want. - -The next section will deal with FreeType 2 internals (like modules, - vector outlines, font drivers, renderers), as well as a few font format - specific issues (mainly, how to access certain TrueType or Type 1 tables). - + + my_draw_bitmap( bitmap->bitmap, + bitmap->left, + my_target_height - bitmap->top ); + + FT_Done_Glyph( image ); + } + } + + +There are a few changes compared to the previous version of this + code: + +
It is possible to call this function several times to render the + string width different angles, or even change the way start + is computed in order to move it to different place. + +This code is the basis of the FreeType 2 demonstration program + named ftstring.c. It could be easily extended to perform + advanced text layout or word-wrapping in the first part, without + changing the second one. + +Note however that a normal implementation would use a glyph cache + in order to reduce memory consumption. For example, let us assume + that our text string to render is "FreeType". We would store three + identical glyph images in our table for the letter "e", which isn't + optimal (especially when you consider longer lines of text, or even + whole pages). + ++ + + 6. Accessing metrics in design font units, and scaling them ++ +Scalable font formats usually store a single vectorial image, called + an outline, for each in a face. Each outline is defined in an + abstract grid called the design space, with coordinates + expressed in nominal font units. When a glyph image is loaded, + the font driver usually scales the outline to device space according to + the current character pixel size found in a FT_Size object. + The driver may also modify the scaled outline in order to significantly + improve its appearance on a pixel-based surface (a process known as + hinting or grid-fitting). + +This section describes how design coordinates are scaled to device + space, and how to read glyph outlines and metrics in font units. This + is important for a number of things: + +
+ a. Scaling distances to device space ++ +Design coordinates are scaled to device space using a simple + scaling transformation whose coefficients are computed with the help + of the character pixel size: + + ++ device_x = design_x * x_scale + device_y = design_y * y_scale + + x_scale = pixel_size_x / EM_size + y_scale = pixel_size_y / EM_size+ + + Here, the value EM_size is font-specific and corresponds + to the size of an abstract square of the design space (called the + "EM"), which is used by font designers to create glyph images. It is + thus expressed in font units. It is also accessible directly for + scalable font formats as face->units_per_EM. You should + check that a font face contains scalable glyph images by using the + FT_IS_SCALABLE(face) macro, which returns true when the font + is scalable. + +When you call the function FT_Set_Pixel_Sizes(), you are + specifying the value of pixel_size_x and + pixel_size_y; FreeType will then immediately compute the + values of x_scale and y_scale. + +When you call the function FT_Set_Char_Size(), you are + specifying the character size in physical "points", which is used, + along with the device's resolutions, to compute the character pixel + size, then the scaling factors. + +Note that after calling any of these two functions, you can access + the values of the character pixel size and scaling factors as fields + of the face->size->metrics structure. These fields are: + +
Basically, this means that you can scale a distance expressed in + font units to 26.6 pixels directly with the help of the + FT_MulFix() function, as in: + + ++ /* convert design distances to 1/64th of pixels */ + pixels_x = FT_MulFix( design_x, face->size->metrics.x_scale ); + pixels_y = FT_MulFix( design_y, face->size->metrics.y_scale );+ + + However, you can also scale the value directly with more accuracy + by using doubles and the equations: + + ++ FT_Size_Metrics* metrics = &face->size->metrics; /* shortcut */ + double pixels_x, pixels_y; + double em_size, x_scale, y_scale; + + + /* compute floating point scale factors */ + em_size = 1.0 * face->units_per_EM; + x_scale = metrics->x_ppem / em_size; + y_scale = metrics->y_ppem / em_size; + + /* convert design distances to floating point pixels */ + pixels_x = design_x * x_scale; + pixels_y = design_y * y_scale;+ + + + b. Accessing design metrics (glyph & global) ++ +You can access glyph metrics in font units simply by specifying the + FT_LOAD_NO_SCALE bit flag in FT_Load_Glyph() or + FT_Load_Char(). The metrics returned in + face->glyph->metrics will then all be in font units. + +You can access unscaled kerning data using the + ft_kerning_mode_unscaled mode. + +Finally, a few global metrics are available directly in font units + as fields of the FT_Face handle, as described in + section 3 of this tutorial part. + ++ + + Conclusion ++ +This is the end of the second part of the FreeType 2 tutorial; + you are now able to access glyph metrics, manage glyph images, and + render text much more intelligently (kerning, measuring, transforming + & caching). + +You have now sufficient knowledge to build a pretty decent text + service on top of FreeType 2, and you could possibly stop there if + you want. + +The next section will deal with FreeType 2 internals (like + modules, vector outlines, font drivers, renderers), as well as a few + font format specific issues (mainly, how to access certain TrueType or + Type 1 tables). |