Overlaying pixels with alpha value in C/C++

Question

I'm trying to create an algorithm to overlay an image with transparencies on top of fully opaque image. On the next sample I have a back fully opaque image, and front image which is a blue frame with diffuse edges. The problem I'm having is that my implementation overlays incorrectly the semi-transparent areas producing darkish pixels.

Here is my implementation:

#define OPAQUE 0xFF
#define TRANSPARENT 0
#define ALPHA(argb)  (uint8_t)(argb >> 24)
#define RED(argb)    (uint8_t)(argb >> 16)
#define GREEN(argb)  (uint8_t)(argb >> 8)
#define BLUE(argb)   (uint8_t)(argb)
#define ARGB(a, r, g, b) (a << 24) | ((r & 0xff) << 16) | ((g & 0xff) << 8) | (b & 0xff)
#define BLEND(a, b, alpha) ((a * alpha) + (b * (255 - alpha))) / 255

void ImageUtil::overlay(const uint32_t* front, uint32_t* back, const unsigned int width, const unsigned int height)
{
    const size_t totalPixels = width * height;

    for (unsigned long index = 0; index < totalPixels; index++)
    {
        const uint32_t alpha = ALPHA(*front);

        const uint32_t R = BLEND(RED(*front), RED(*back), alpha);
        const uint32_t G = BLEND(GREEN(*front), GREEN(*back), alpha);
        const uint32_t B = BLEND(BLUE(*front), BLUE(*back), alpha);

        *backPixels++ = ARGB(OPAQUE, R , G, B);
        *frontPixels++;
    }
}

UPDATE:

Test Images files

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Answer 1

Following the tips from the comments by gman and interjay, I've investigated further and yes the data is being loaded with pre-multiplied alpha. This produces the darkening when blending. The solution has been to un-multiply the front pixels, and finally I've got the expected result.

Unmultiply formula:

((0xFF * color) / alpha)

Final code:

#define OPAQUE 0xFF;
#define TRANSPARENT 0;

#define ALPHA(rgb) (uint8_t)(rgb >> 24)
#define RED(rgb)   (uint8_t)(rgb >> 16)
#define GREEN(rgb) (uint8_t)(rgb >> 8)
#define BLUE(rgb)  (uint8_t)(rgb)

#define UNMULTIPLY(color, alpha) ((0xFF * color) / alpha)
#define BLEND(back, front, alpha) ((front * alpha) + (back * (255 - alpha))) / 255
#define ARGB(a, r, g, b) (a << 24) | ((r & 0xFF) << 16) | ((g & 0xFF) << 8) | (b & 0xFF)

void ImageUtil::overlay(const uint32_t* front, uint32_t* back, const unsigned int width, const unsigned int height)
{
    const size_t totalPixels = width * height;

    for (unsigned long index = 0; index < totalPixels; index++)
    {
        const uint32_t frontAlpha = ALPHA(*front);

        if (frontAlpha == TRANSPARENT)
        {
            *back++;
            *front++;
            continue;
        }

        if (frontAlpha == OPAQUE)
        {
            *back++ = *front++;
            continue;
        }

        const uint8_t backR = RED(*back);
        const uint8_t backG = GREEN(*back);
        const uint8_t backB = BLUE(*back);

        const uint8_t frontR = UNMULTIPLY(RED(*front), frontAlpha);
        const uint8_t frontG = UNMULTIPLY(GREEN(*front), frontAlpha);
        const uint8_t frontB = UNMULTIPLY(BLUE(*front), frontAlpha);

        const uint32_t R = BLEND(backR, frontR, frontAlpha);
        const uint32_t G = BLEND(backG, frontG, frontAlpha);
        const uint32_t B = BLEND(backB, frontB, frontAlpha);

        *back++ = ARGB(OPAQUE, R , G, B);
        *front++;
    }
}