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c++ - How does photoshop blend two images together? - Stack Overflow

Photoshop blends two images together by performing a blend operation on each pixel in image A against its corresponding pixel in image B. Each pixel is a color consisting of multiple channels. Assuming we are working with RGB pixels, the channels in each pixel would be red, green and blue. To blend two pixels we blend their respective channels.

The blend operation that occurs for each blend mode in Photoshop can be summed up in the following macros:

code:c++

#define ChannelBlend_Normal(A,B) ((uint8)(A))

#define ChannelBlend_Lighten(A,B) ((uint8)((B > A) ? B:A))

#define ChannelBlend_Darken(A,B) ((uint8)((B > A) ? A:B))

#define ChannelBlend_Multiply(A,B) ((uint8)((A * B) / 255))

#define ChannelBlend_Average(A,B) ((uint8)((A + B) / 2))

#define ChannelBlend_Add(A,B) ((uint8)(min(255, (A + B))))

#define ChannelBlend_Subtract(A,B) ((uint8)((A + B < 255) ? 0:(A + B - 255)))

#define ChannelBlend_Difference(A,B) ((uint8)(abs(A - B)))

#define ChannelBlend_Negation(A,B) ((uint8)(255 - abs(255 - A - B)))

#define ChannelBlend_Screen(A,B) ((uint8)(255 - (((255 - A) * (255 - B)) >> 8)))

#define ChannelBlend_Exclusion(A,B) ((uint8)(A + B - 2 * A * B / 255))

#define ChannelBlend_Overlay(A,B) ((uint8)((B < 128) ? (2 * A * B / 255):(255 - 2 * (255 - A) * (255 - B) / 255)))

#define ChannelBlend_SoftLight(A,B) ((uint8)((B < 128)?(2*((A>>1)+64))*((float)B/255):(255-(2*(255-((A>>1)+64))*(float)(255-B)/255))))

#define ChannelBlend_HardLight(A,B) (ChannelBlend_Overlay(B,A))

#define ChannelBlend_ColorDodge(A,B) ((uint8)((B == 255) ? B:min(255, ((A << 8 ) / (255 - B)))))

#define ChannelBlend_ColorBurn(A,B) ((uint8)((B == 0) ? B:max(0, (255 - ((255 - A) << 8 ) / B))))

#define ChannelBlend_LinearDodge(A,B)(ChannelBlend_Add(A,B))

#define ChannelBlend_LinearBurn(A,B) (ChannelBlend_Subtract(A,B))

#define ChannelBlend_LinearLight(A,B)((uint8)(B < 128)?ChannelBlend_LinearBurn(A,(2 * B)):ChannelBlend_LinearDodge(A,(2 * (B - 128))))

#define ChannelBlend_VividLight(A,B) ((uint8)(B < 128)?ChannelBlend_ColorBurn(A,(2 * B)):ChannelBlend_ColorDodge(A,(2 * (B - 128))))

#define ChannelBlend_PinLight(A,B) ((uint8)(B < 128)?ChannelBlend_Darken(A,(2 * B)):ChannelBlend_Lighten(A,(2 * (B - 128))))

#define ChannelBlend_HardMix(A,B) ((uint8)((ChannelBlend_VividLight(A,B) < 128) ? 0:255))

#define ChannelBlend_Reflect(A,B) ((uint8)((B == 255) ? B:min(255, (A * A / (255 - B)))))

#define ChannelBlend_Glow(A,B) (ChannelBlend_Reflect(B,A))

#define ChannelBlend_Phoenix(A,B) ((uint8)(min(A,B) - max(A,B) + 255))

#define ChannelBlend_Alpha(A,B,O) ((uint8)(O * A + (1 - O) * B))

#define ChannelBlend_AlphaF(A,B,F,O) (ChannelBlend_Alpha(F(A,B),A,O))

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