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sf::Image::copy() - Fixed the incorrect alpha value being calculated for semi-transparent pixels on fully transparent pixels #1993

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merged 1 commit into from Apr 21, 2022
Merged

sf::Image::copy() - Fixed the incorrect alpha value being calculated for semi-transparent pixels on fully transparent pixels #1993

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10 changes: 7 additions & 3 deletions include/SFML/Graphics/Image.hpp
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Expand Up @@ -203,9 +203,13 @@ class SFML_GRAPHICS_API Image
/// kind of feature in real-time you'd better use sf::RenderTexture.
///
/// If \a sourceRect is empty, the whole image is copied.
/// If \a applyAlpha is set to true, the transparency of
/// source pixels is applied. If it is false, the pixels are
/// copied unchanged with their alpha value.
/// If \a applyAlpha is set to true, alpha blending is
/// applied from the source pixels to the destination pixels
/// using the \b over operator. If it is false, the source
/// pixels are copied unchanged with their alpha value.
///
/// See https://en.wikipedia.org/wiki/Alpha_compositing for
/// details on the \b over operator.
///
/// \param source Source image to copy
/// \param destX X coordinate of the destination position
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19 changes: 13 additions & 6 deletions src/SFML/Graphics/Image.cpp
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Expand Up @@ -240,12 +240,19 @@ void Image::copy(const Image& source, unsigned int destX, unsigned int destY, co
const Uint8* src = srcPixels + j * 4;
Uint8* dst = dstPixels + j * 4;

// Interpolate RGBA components using the alpha value of the source pixel
Uint8 alpha = src[3];
dst[0] = static_cast<Uint8>((src[0] * alpha + dst[0] * (255 - alpha)) / 255);
dst[1] = static_cast<Uint8>((src[1] * alpha + dst[1] * (255 - alpha)) / 255);
dst[2] = static_cast<Uint8>((src[2] * alpha + dst[2] * (255 - alpha)) / 255);
dst[3] = static_cast<Uint8>(alpha + dst[3] * (255 - alpha) / 255);
// Interpolate RGBA components using the alpha values of the destination and source pixels
Uint8 src_alpha = src[3];
Uint8 dst_alpha = dst[3];
Uint8 out_alpha = static_cast<Uint8>(src_alpha + dst_alpha - src_alpha * dst_alpha / 255);

dst[3] = out_alpha;

if (out_alpha)
for (int k = 0; k < 3; k++)
dst[k] = static_cast<Uint8>((src[k] * src_alpha + dst[k] * (out_alpha - src_alpha)) / out_alpha);
else
for (int k = 0; k < 3; k++)
dst[k] = src[k];
}

srcPixels += srcStride;
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