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GRAPHICS: Improve TTF rendering on transparent surfaces. #2748

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Feb 21, 2021
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GRAPHICS: Improve TTF rendering on transparent surfaces. #2748

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GRAPHICS: Improve TTF rendering on transparent surfaces.
OMGPizzaGuy Jan 23, 2021
117c282
GRAPHICS: Update TTF render color blending to work with any surface a…
OMGPizzaGuy Feb 19, 2021
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25 changes: 17 additions & 8 deletions graphics/fonts/ttf.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -583,7 +583,7 @@ namespace {

template<typename ColorType>
static void renderGlyph(uint8 *dstPos, const int dstPitch, const uint8 *srcPos, const int srcPitch, const int w, const int h, ColorType color, const PixelFormat &dstFormat) {
uint8 sR, sG, sB;
uint8 sA, sR, sG, sB;
dstFormat.colorToRGB(color, sR, sG, sB);

for (int y = 0; y < h; ++y) {
Expand All @@ -594,16 +594,25 @@ static void renderGlyph(uint8 *dstPos, const int dstPitch, const uint8 *srcPos,
if (*src == 255) {
*rDst = color;
} else if (*src) {
const uint8 a = *src;
sA = *src;

uint8 dR, dG, dB;
dstFormat.colorToRGB(*rDst, dR, dG, dB);
uint8 dA, dR, dG, dB;
dstFormat.colorToARGB(*rDst, dA, dR, dG, dB);

dR = ((255 - a) * dR + a * sR) / 255;
dG = ((255 - a) * dG + a * sG) / 255;
dB = ((255 - a) * dB + a * sB) / 255;
if (dA == 0) {
*rDst = dstFormat.ARGBToColor(sA, sR, sG, sB);
} else {
double alpha = (double)sA / 255.0;
dR = static_cast<uint8>((sR * alpha) + (dR * (1.0 - alpha)));
dG = static_cast<uint8>((sG * alpha) + (dG * (1.0 - alpha)));
dB = static_cast<uint8>((sB * alpha) + (dB * (1.0 - alpha)));

*rDst = dstFormat.RGBToColor(dR, dG, dB);
if (sA > dA) {
dA = static_cast<uint8>((sA * alpha) + (dA * (1.0 - alpha)));
}

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Where do those formula come from?
My understanding of color blending is that it should be:

double sAn = (double)sA / 255.0;
double dAn = (double)dA / 255.0;
dR = static_cast<uint8>(sR * sAn + dR * dAn * (1.0 - sAn) / (sAn + dAn * (1.0 - sAn)));
dG = static_cast<uint8>(sG * sAn + dG * dAn * (1.0 - sAn) / (sAn + dAn * (1.0 - sAn)));
dB = static_cast<uint8>(sB * sAn + dB * dAn * (1.0 - sAn) / (sAn + dAn * (1.0 - sAn)));
dA = static_cast<uint8>(sA + dA * (1.0 - sAn));

This also means there is no need for a special case if dA is 0.
And you can of course optimise that code a bit by computing dAn * (1.0 - sAn) only once instead of 6 times.

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Mostly followed suit with managed_surface.cpp:487-490
Figured if case for dA is 0 might be better to avoid calculations, but perhaps not.
Destination alpha... I just wasn't sure the best call.

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OK, looking at https://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
If I'm understanding it correctly, what I used would be correct for dA being 255, but not complete enough for any other value.
I'll take another swing at this.

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@OMGPizzaGuy OMGPizzaGuy Feb 19, 2021 •
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Yep, the suggested code was great.
I went with calculating a normalized output alpha and converting that to a byte at the end.
I thought it would be a little more clear to anyone reading the code.

*rDst = dstFormat.ARGBToColor(dA, dR, dG, dB);
}
}

++rDst;
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