From 250b0c624a49ca56e221ae9778ebac747175c9bd Mon Sep 17 00:00:00 2001 From: Alexei Podtelezhnikov Date: Wed, 30 May 2018 23:29:41 -0400 Subject: [PATCH] Shorten LCD filtering docs. --- include/freetype/ftlcdfil.h | 122 +++++++++++------------------------- 1 file changed, 35 insertions(+), 87 deletions(-) diff --git a/include/freetype/ftlcdfil.h b/include/freetype/ftlcdfil.h index 2a27196cb..a71113e43 100644 --- a/include/freetype/ftlcdfil.h +++ b/include/freetype/ftlcdfil.h @@ -42,7 +42,7 @@ FT_BEGIN_HEADER * LCD Filtering * * @abstract: - * Reduce color fringes of subpixel-rendered bitmaps. + * Remove color fringes of subpixel-rendered bitmaps. * * @description: * Should you #define FT_CONFIG_OPTION_SUBPIXEL_RENDERING in your @@ -54,73 +54,42 @@ FT_BEGIN_HEADER * * ClearType-style LCD rendering exploits the color-striped structure of * LCD pixels, increasing the available resolution in the direction of - * the stripe (usually horizontal RGB) by a factor of~3. Since these - * subpixels are color pixels, using them unfiltered creates severe - * color fringes. Use the @FT_Library_SetLcdFilter API to specify a - * low-pass filter, which is then applied to subpixel-rendered bitmaps - * generated through @FT_Render_Glyph. The filter sacrifices some of - * the higher resolution to reduce color fringes, making the glyph image - * slightly blurrier. Positional improvements will remain. + * the stripe (usually horizontal RGB) by a factor of~3. Using the + * subpixels coverages unfiltered can create severe color fringes + * especially when rendering thin features. Indeed, to produce + * black-on-white text, the color components must be dimmed equally. * - * A filter should have two properties: + * A good 5-tap FIR filter should be applied to subpixel coverages + * regardless of pixel boundaries and should have these properties: * - * 1) It should be normalized, meaning the sum of the 5~components - * should be 256 (0x100). It is possible to go above or under this - * target sum, however: going under means tossing out contrast, going - * over means invoking clamping and thereby non-linearities that - * increase contrast somewhat at the expense of greater distortion - * and color-fringing. Contrast is better enhanced through stem - * darkening. + * 1) It should be symmetrical, like {~a, b, c, b, a~}, to avoid + * any shifts in appearance. * - * 2) It should be color-balanced, meaning a filter `{~a, b, c, b, a~}' - * where a~+ b~=~c. It distributes the computed coverage for one - * subpixel to all subpixels equally, sacrificing some won resolution - * but drastically reducing color-fringing. Positioning improvements - * remain! Note that color-fringing can only really be minimized - * when using a color-balanced filter and alpha-blending the glyph - * onto a surface in linear space; see @FT_Render_Glyph. + * 2) It should be color-balanced, meaning a~+ b~=~c, to reduce color + * fringes by distributing the computed coverage for one subpixel to + * all subpixels equally. * - * Regarding the form, a filter can be a `boxy' filter or a `beveled' - * filter. Boxy filters are sharper but are less forgiving of non-ideal - * gamma curves of a screen (viewing angles!), beveled filters are - * fuzzier but more tolerant. + * 3) It should be normalized, meaning 2a~+ 2b~+ c~=~1.0 to maintain + * overall brightness. * - * Examples: - * - * - [0x10 0x40 0x70 0x40 0x10] is beveled and neither balanced nor - * normalized. - * - * - [0x1A 0x33 0x4D 0x33 0x1A] is beveled and balanced but not - * normalized. - * - * - [0x19 0x33 0x66 0x4c 0x19] is beveled and normalized but not - * balanced. - * - * - [0x00 0x4c 0x66 0x4c 0x00] is boxily beveled and normalized but not - * balanced. - * - * - [0x00 0x55 0x56 0x55 0x00] is boxy, normalized, and almost - * balanced. - * - * - [0x08 0x4D 0x56 0x4D 0x08] is beveled, normalized and, almost - * balanced. - * - * The filter affects glyph bitmaps rendered through @FT_Render_Glyph, - * @FT_Load_Glyph, and @FT_Load_Char. It does _not_ affect the output - * of @FT_Outline_Render and @FT_Outline_Get_Bitmap. + * Use the @FT_Library_SetLcdFilter API to specify a low-pass filter, + * which is then applied to subpixel-rendered bitmaps generated through + * @FT_Render_Glyph. The filter affects glyph bitmaps rendered through + * @FT_Render_Glyph, @FT_Load_Glyph, and @FT_Load_Char. It does _not_ + * affect the output of @FT_Outline_Render and @FT_Outline_Get_Bitmap. * * If this feature is activated, the dimensions of LCD glyph bitmaps are * either wider or taller than the dimensions of the corresponding * outline with regard to the pixel grid. For example, for - * @FT_RENDER_MODE_LCD, the filter adds 3~subpixels to the left, and - * 3~subpixels to the right. The bitmap offset values are adjusted + * @FT_RENDER_MODE_LCD, the filter adds 2~subpixels to the left, and + * 2~subpixels to the right. The bitmap offset values are adjusted * accordingly, so clients shouldn't need to modify their layout and * glyph positioning code when enabling the filter. * - * It is important to understand that linear alpha blending and gamma - * correction is critical for correctly rendering glyphs onto surfaces - * without artifacts and even more critical when subpixel rendering is - * involved. + * Only filtering along with gamma-corrected alpha blending can completely + * remove color fringes. Boxy 3-tap filter {0, 1/3, 1/3, 1/3, 0} is + * sharper but is less forgiving of non-ideal gamma curves of a screen + * (viewing angles!), beveled filters are fuzzier but more tolerant. * * Each of the 3~alpha values (subpixels) is independently used to blend * one color channel. That is, red alpha blends the red channel of the @@ -145,44 +114,23 @@ FT_BEGIN_HEADER * results in sometimes severe color fringes. * * FT_LCD_FILTER_DEFAULT :: - * The default filter reduces color fringes considerably, at the cost - * of a slight blurriness in the output. - * - * It is a beveled, normalized, and color-balanced five-tap filter - * that is more forgiving to screens with non-ideal gamma curves and - * viewing angles. Note that while color-fringing is reduced, it can - * only be minimized by using linear alpha blending and gamma - * correction to render glyphs onto surfaces. The default filter - * weights are [0x08 0x4D 0x56 0x4D 0x08]. + * This is a beveled, normalized, and color-balanced five-tap filter + * with weights of [0x08 0x4D 0x56 0x4D 0x08] in 1/256th units. * * FT_LCD_FILTER_LIGHT :: - * The light filter is a variant that is sharper at the cost of - * slightly more color fringes than the default one. - * - * It is a boxy, normalized, and color-balanced three-tap filter that - * is less forgiving to screens with non-ideal gamma curves and - * viewing angles. This filter works best when the rendering system - * uses linear alpha blending and gamma correction to render glyphs - * onto surfaces. The light filter weights are - * [0x00 0x55 0x56 0x55 0x00]. + * this is a boxy, normalized, and color-balanced three-tap filter + * with weights of [0x00 0x55 0x56 0x55 0x00] in 1/256th units. * * FT_LCD_FILTER_LEGACY :: + * FT_LCD_FILTER_LEGACY1 :: * This filter corresponds to the original libXft color filter. It * provides high contrast output but can exhibit really bad color * fringes if glyphs are not extremely well hinted to the pixel grid. - * In other words, it only works well if the TrueType bytecode - * interpreter is enabled *and* high-quality hinted fonts are used. - * * This filter is only provided for comparison purposes, and might be - * disabled or stay unsupported in the future. - * - * FT_LCD_FILTER_LEGACY1 :: - * For historical reasons, the FontConfig library returns a different - * enumeration value for legacy LCD filtering. To make code work that - * (incorrectly) forwards FontConfig's enumeration value to - * @FT_Library_SetLcdFilter without proper mapping, it is thus easiest - * to have another enumeration value, which is completely equal to - * `FT_LCD_FILTER_LEGACY'. + * disabled or stay unsupported in the future. The second value is + * provided for compatibility with FontConfig, which historically + * used different enumeration, sometimes incorrectly forwarded to + * FreeType. * * @since: * 2.3.0 (`FT_LCD_FILTER_LEGACY1' since 2.6.2) @@ -258,7 +206,7 @@ FT_BEGIN_HEADER * * weights :: * A pointer to an array; the function copies the first five bytes and - * uses them to specify the filter weights. + * uses them to specify the filter weights in 1/256th units. * * @return: * FreeType error code. 0~means success.