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Diffstat (limited to 'src/sdf/ftbsdf.c')
| -rw-r--r-- | src/sdf/ftbsdf.c | 89 |
1 files changed, 89 insertions, 0 deletions
diff --git a/src/sdf/ftbsdf.c b/src/sdf/ftbsdf.c index 510273367..13dbd4511 100644 --- a/src/sdf/ftbsdf.c +++ b/src/sdf/ftbsdf.c @@ -11,6 +11,95 @@ /************************************************************************** * + * A brief technical overview of how the BSDF rasterizer works + * ----------------------------------------------------------- + * + * [Notes]: + * * SDF stands for Signed Distance Field everywhere. + * + * * BSDF stands for Bitmap to Signed Distance Field rasterizer. + * + * * This renderer converts rasterized bitmaps to SDF. There is another + * renderer called 'sdf', which generates SDF directly from outlines; + * see file `ftsdf.c` for more. + * + * * The idea of generating SDF from bitmaps is taken from two research + * papers, where one is dependent on the other: + * + * - Per-Erik Danielsson: Euclidean Distance Mapping + * http://webstaff.itn.liu.se/~stegu/JFA/Danielsson.pdf + * + * From this paper we use the eight-point sequential Euclidean + * distance mapping (8SED). This is the heart of the process used + * in this rasterizer. + * + * - Stefan Gustavson, Robin Strand: Anti-aliased Euclidean distance transform. + * http://weber.itn.liu.se/~stegu/aadist/edtaa_preprint.pdf + * + * The original 8SED algorithm discards the pixels' alpha values, + * which can contain information about the actual outline of the + * glyph. This paper takes advantage of those alpha values and + * approximates outline pretty accurately. + * + * * This rasterizer also works for monochrome bitmaps. However, the + * result is not as accurate since we don't have any way to + * approximate outlines from binary bitmaps. + * + * ======================================================================== + * + * Generating SDF from bitmap is done in several steps. + * + * (1) The only information we have is the bitmap itself. It can + * be monochrome or anti-aliased. If it is anti-aliased, pixel values + * are nothing but coverage values. These coverage values can be used + * to extract information about the outline of the image. For + * example, if the pixel's alpha value is 0.5, then we can safely + * assume that the outline passes through the center of the pixel. + * + * (2) Find edge pixels in the bitmap (see `bsdf_is_edge` for more). For + * all edge pixels we use the Anti-aliased Euclidean distance + * transform algorithm and compute approximate edge distances (see + * `compute_edge_distance` and/or the second paper for more). + * + * (3) Now that we have computed approximate distances for edge pixels we + * use the 8SED algorithm to basically sweep the entire bitmap and + * compute distances for the rest of the pixels. (Since the algorithm + * is pretty convoluted it is only explained briefly in a comment to + * function `edt8`. To see the actual algorithm refer to the first + * paper.) + * + * (4) Finally, compute the sign for each pixel. This is done in function + * `finalize_sdf`. The basic idea is that if a pixel's original + * alpha/coverage value is greater than 0.5 then it is 'inside' (and + * 'outside' otherwise). + * + * Pseudo Code: + * + * ``` + * b = source bitmap; + * t = target bitmap; + * dm = list of distances; // dimension equal to b + * + * foreach grid_point (x, y) in b: + * { + * if (is_edge(x, y)): + * dm = approximate_edge_distance(b, x, y); + * + * // do the 8SED on the distances + * edt8(dm); + * + * // determine the signs + * determine_signs(dm): + * + * // copy SDF data to the target bitmap + * copy(dm to t); + * } + * + */ + + + /************************************************************************** + * * useful macros * */ |