forked from minhngoc25a/freetype2
[sdf] Added a basic overview of the `sdf' rasterizer.
* src/sdf/ftsdf.c: Added the citation of the original paper and added a overview of the process for generating SDF from outlines. * src/sdf/ftsdf.c (sdf_generate_subdivision): Added comment explaining how the optimization works.
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Anuj Verma
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2020-08-15 Anuj Verma <anujv@iitbhilai.ac.in>
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[sdf] Added a basic overview of the `sdf' rasterizer.
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* src/sdf/ftsdf.c: Added the citation of the original paper
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and added a overview of the process for generating SDF
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from outlines.
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* src/sdf/ftsdf.c (sdf_generate_subdivision): Added comment
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explaining how the optimization works.
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2020-08-13 Anuj Verma <anujv@iitbhilai.ac.in>
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[sdf] Fix gcc compiler warnings.
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100
src/sdf/ftsdf.c
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src/sdf/ftsdf.c
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@ -6,6 +6,76 @@
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#include "ftsdferrs.h"
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/**************************************************************************
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*
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* A brief technical overview of how the SDF rasterizer works.
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* -----------------------------------------------------------
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*
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* [Notes]:
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* * SDF stands for Signed Distance Field everywhere.
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*
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* * This renderer generate SDF directly from outlines. There is another
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* renderer `bsdf' which convert bitmaps to SDF, see `ftbsdf.c' for
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* more details on the `bsdf' rasterizer.
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*
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* * The basic idea of generating the SDF is taken from Viktor Chlumsky's
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* research paper. Citation:
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* Chlumsky, Viktor. Shape Decomposition for Multi-channel Distance
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* Fields. Master’s thesis. Czech Technical University in Prague,
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* Faculty of InformationTechnology, 2015.
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* For more information: https://github.com/Chlumsky/msdfgen
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*
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* ========================================================================
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*
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* Generating SDF from outlines is pretty straightforward:
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*
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* 1 - We have a set of contours which make the outline of a shape/glyph.
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* Each contour comprises of several edges and the edges can be of
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* three types i.e.
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*
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* * Line Segments
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* * Conic Bezier Curves
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* * Cubic Bezier Curves
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*
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* 2 - Apart from the outlines we also have a 2D grid namely the bitmap
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* which is used to represent the final SDF data.
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*
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* 3 - Now, in order to generate SDF, our task is to find shortest signed
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* distance from each grid point to the outline. The signed distance
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* means that if the grid point is filled by any contour then it's
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* sign will be positive, otherwise it will be negative. The pseudo
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* code is as follows:
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*
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* foreach grid_point (x, y):
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* {
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* int min_dist = INT_MAX;
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*
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* foreach contour in outline:
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* foreach edge in contour:
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* {
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* // get shortest distance from point (x, y) to the edge
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* d = get_min_dist(x, y, edge);
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*
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* if ( d < min_dist ) min_dist = d;
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* }
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*
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* bitmap[x, y] = min_dist;
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* }
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*
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* 4 - After this the bitmap will contain information about the closest
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* point from each point to the outline of the shape. Of course, this
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* is the most straightforward way of generating SDF, in this raster-
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* izer we use various optimizations, to checkout how they works
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* see the `sdf_generate_' functions in this file.
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*
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* The optimization currently used by default is the subdivision opt-
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* imization, see `sdf_generate_subdivision' for more details.
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*
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* Also, to see how we compute the shortest distance from a point to
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* each type of edge checkout the `get_min_distance_' functions.
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*
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*/
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/**************************************************************************
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*
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* for tracking memory used
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@ -2910,8 +2980,8 @@
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* all the directions. Anything outside the control box will naturally
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* be more than the `spread' and shouldn't be computed.
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* Lastly to determine the sign of unchecked pixels we do a single pass
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* of all the rows starting with a + sign and flipping when we come
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* across a - sign and continue.
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* of all the rows starting with a '+' sign and flipping when we come
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* across a '-' sign and continue.
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* This also eliminate the chance of overflow because we only check the
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* proximity of the curve. Therefore we can use squared distanced
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* safely.
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@ -3152,6 +3222,32 @@
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FT_UInt spread,
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const FT_Bitmap* bitmap )
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{
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/* Thanks to Alexei for providing the idea of this optimization. */
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/* */
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/* This optimiztion mode take advantage of two facts: */
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/* */
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/* - Computing shortest distance froma point to a line segment */
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/* is super fast. */
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/* - We don't have to compute shortest distance for the entire */
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/* 2D grid. */
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/* */
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/* This is how it works: */
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/* */
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/* - We split the outlines into a number of line segments. */
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/* */
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/* - For each line segment we only process the neighborhood of */
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/* the line segment. */
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/* */
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/* - Now, only for the neighborhood grid points we compute the */
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/* closest distance to the line. */
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/* */
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/* - This way we do not have to check all grid points against */
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/* all the edges. Instead for each line's neighborhood we */
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/* only compute shortest distance for that one line only. */
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/* */
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/* All in all, it reduces the number of grid point to edge check */
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/* */
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FT_Error error = FT_Err_Ok;
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FT_CALL( split_sdf_shape( shape ) );
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