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Fix error diffusion algorithm

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The error propagation must be limited to 0...255 values, and the new
quantization error must be calculated to that limited value. Also the
1/16 part of the Floyd-Steinberg distribution must be the remaining of
the other parts.
This commit is contained in:
David Capello 2019-04-02 13:54:39 -03:00
parent c2af88343e
commit b6d07d995f
1 changed files with 15 additions and 9 deletions
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24
src/render/error_diffusion.cpp
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@ -64,15 +64,14 @@ doc::color_t ErrorDiffusionDither::ditherRgbToIndex2D(
doc::rgba_getb(color),
doc::rgba_geta(color)
};
int u[kChannels];
for (int i=0; i<kChannels; ++i) {
v[i] += m_err[i][x+1] / 16;
u[i] = MID(0, v[i], 255);
v[i] += m_err[i][x+1];
v[i] = MID(0, v[i], 255);
}
const doc::color_t index =
(rgbmap ? rgbmap->mapColor(u[0], u[1], u[2], u[3]):
palette->findBestfit(u[0], u[1], u[2], u[3], m_transparentIndex));
(rgbmap ? rgbmap->mapColor(v[0], v[1], v[2], v[3]):
palette->findBestfit(v[0], v[1], v[2], v[3], m_transparentIndex));
doc::color_t palColor = palette->getEntry(index);
if (m_transparentIndex == index || doc::rgba_geta(palColor) == 0) {
@ -90,10 +89,17 @@ doc::color_t ErrorDiffusionDither::ditherRgbToIndex2D(
// TODO using Floyd-Steinberg matrix here but it should be configurable
for (int i=0; i<kChannels; ++i) {
int* err = &m_err[i][x];
err[ +2] += quantError[i] * 7;
err[m_width ] += quantError[i] * 3;
err[m_width+1] += quantError[i] * 5;
err[m_width+2] += quantError[i] * 1;
const int a = quantError[i] * 7 / 16;
const int b = quantError[i] * 3 / 16;
const int c = quantError[i] * 5 / 16;
// Same as d=quantError[i]*1/16 but without rounding errors
const int d = quantError[i] - a - b - c;
err[ +2] += a;
err[m_width ] += b;
err[m_width+1] += c;
err[m_width+2] += d;
}
return index;