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fix: text rendering did not work correctly with alpha #3934

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fix: text rendering did not work correctly with alpha #3934

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14 changes: 11 additions & 3 deletions src/libOpenImageIO/imagebufalgo_draw.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1078,7 +1078,9 @@ ImageBufAlgo::render_text(ImageBuf& R, int x, int y, string_view text,
// If the buffer doesn't have an alpha, but the text color passed
// has 4 values, assume the last value is supposed to be alpha.
textalpha = textcolor[3];
}
alpha_channel = 3;
} else
alpha_channel = -1;

// Convert the UTF to 32 bit unicode
std::vector<uint32_t> utext;
Expand Down Expand Up @@ -1162,8 +1164,14 @@ ImageBufAlgo::render_text(ImageBuf& R, int x, int y, string_view text,
float val = t[0];
float alpha = a[0] * textalpha;
R.getpixel(r.x(), r.y(), pixelcolor);
for (int c = 0; c < nchannels; ++c)
pixelcolor[c] = val * textcolor[c] + (1.0f - alpha) * pixelcolor[c];
if (alpha == 0.0)
continue;
for (int c = 0; c < nchannels; ++c) {
if (c == alpha_channel)
pixelcolor[c] = alpha + (1.0f - alpha) * pixelcolor[c];
else
pixelcolor[c] = (val * alpha * textcolor[c]) + (1.0f - alpha) * pixelcolor[c];
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You may have noticed CI failures. Basically, every test with text in it is failing with this PR.

I think this line is the part that's responsible. Compared to the old code, you have an extra multiplication of the foreground color by alpha. I believe (but admit that I am not 100% sure) that it was correct before because freetype is supplying color results that are already scaled by the glyph's alpha coverage, so now you are doubly accounting for alpha coverage in the foreground.

}
R.setpixel(r.x(), r.y(), pixelcolor);
}

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