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Correction for alpha blending issues in texture mod compositing (#9029)

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Warr1024 authored and SmallJoker committed Oct 18, 2019
1 parent b7e3587 commit 15b6922ae6e3d4807e709fcfea0615595f6f765e
Showing with 20 additions and 2 deletions.
  1. +20 −2 src/client/tile.cpp
@@ -1811,6 +1811,24 @@ bool TextureSource::generateImagePart(std::string part_of_name,
return true;
}

/*
Calculate the color of a single pixel drawn on top of another pixel.
This is a little more complicated than just video::SColor::getInterpolated
because getInterpolated does not handle alpha correctly. For example, a
pixel with alpha=64 drawn atop a pixel with alpha=128 should yield a
pixel with alpha=160, while getInterpolated would yield alpha=96.
*/
static inline video::SColor blitPixel(const video::SColor &src_c, const video::SColor &dst_c, u32 ratio)
{
if (dst_c.getAlpha() == 0)
return src_c;
video::SColor out_c = src_c.getInterpolated(dst_c, (float)ratio / 255.0f);
out_c.setAlpha(dst_c.getAlpha() + (255 - dst_c.getAlpha()) *
src_c.getAlpha() * ratio / (255 * 255));
return out_c;
}

/*
Draw an image on top of an another one, using the alpha channel of the
source image
@@ -1830,7 +1848,7 @@ static void blit_with_alpha(video::IImage *src, video::IImage *dst,
s32 dst_y = dst_pos.Y + y0;
video::SColor src_c = src->getPixel(src_x, src_y);
video::SColor dst_c = dst->getPixel(dst_x, dst_y);
dst_c = src_c.getInterpolated(dst_c, (float)src_c.getAlpha()/255.0f);
dst_c = blitPixel(src_c, dst_c, src_c.getAlpha());
dst->setPixel(dst_x, dst_y, dst_c);
}
}
@@ -1853,7 +1871,7 @@ static void blit_with_alpha_overlay(video::IImage *src, video::IImage *dst,
video::SColor dst_c = dst->getPixel(dst_x, dst_y);
if (dst_c.getAlpha() == 255 && src_c.getAlpha() != 0)
{
dst_c = src_c.getInterpolated(dst_c, (float)src_c.getAlpha()/255.0f);
dst_c = blitPixel(src_c, dst_c, src_c.getAlpha());
dst->setPixel(dst_x, dst_y, dst_c);
}
}

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