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Fix composite glyph transform double-scaling #2014

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Fix composite glyph transform double-scaling #2014

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24 changes: 18 additions & 6 deletions stb_truetype.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1851,23 +1851,35 @@ static int stbtt__GetGlyphShapeTT(const stbtt_fontinfo *info, int glyph_index, s
mtx[3] = ttSHORT(comp)/16384.0f; comp+=2;
}

// Find transformation scales.
// Find transformation scales. Per TrueType spec, these are used
// ONLY to rescale the offsets (mtx[4], mtx[5]) when
// SCALED_COMPONENT_OFFSET (flag bit 11, 0x800) is set. They must
// NOT scale the linear part of the transform — doing so squares
// the scale factor and is the cause of composite sub-components
// rendering at wrong sizes in COLR emoji fonts (OpenMoji, Noto
// Color Emoji, Microsoft Fluent Emoji).
m = (float) STBTT_sqrt(mtx[0]*mtx[0] + mtx[1]*mtx[1]);
n = (float) STBTT_sqrt(mtx[2]*mtx[2] + mtx[3]*mtx[3]);
// Default: UNSCALED offsets (Microsoft convention; matches modern
// font tooling). Only SCALED_COMPONENT_OFFSET (bit 11) set without
// UNSCALED_COMPONENT_OFFSET (bit 12) triggers the offset rescale.
if ((flags & (1<<11)) && !(flags & (1<<12))) {
mtx[4] *= m; mtx[5] *= n;
}

// Get indexed glyph.
comp_num_verts = stbtt_GetGlyphShape(info, gidx, &comp_verts);
if (comp_num_verts > 0) {
// Transform vertices.
// Transform vertices: v' = M·v + t. No double-scaling.
for (i = 0; i < comp_num_verts; ++i) {
stbtt_vertex* v = &comp_verts[i];
stbtt_vertex_type x,y;
x=v->x; y=v->y;
v->x = (stbtt_vertex_type)(m * (mtx[0]*x + mtx[2]*y + mtx[4]));
v->y = (stbtt_vertex_type)(n * (mtx[1]*x + mtx[3]*y + mtx[5]));
v->x = (stbtt_vertex_type)(mtx[0]*x + mtx[2]*y + mtx[4]);
v->y = (stbtt_vertex_type)(mtx[1]*x + mtx[3]*y + mtx[5]);
x=v->cx; y=v->cy;
v->cx = (stbtt_vertex_type)(m * (mtx[0]*x + mtx[2]*y + mtx[4]));
v->cy = (stbtt_vertex_type)(n * (mtx[1]*x + mtx[3]*y + mtx[5]));
v->cx = (stbtt_vertex_type)(mtx[0]*x + mtx[2]*y + mtx[4]);
v->cy = (stbtt_vertex_type)(mtx[1]*x + mtx[3]*y + mtx[5]);
}
// Append vertices.
tmp = (stbtt_vertex*)STBTT_malloc((num_vertices+comp_num_verts)*sizeof(stbtt_vertex), info->userdata);
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