Question

I have a pointer to an 32bit argb image’s pixel data and a 32bit xrgb image’s pixel data. How can I composite the argb on top of xrgb image while making use of the alpha component?

Visual Studio 2008 C++

Edit:

Is there a quicker (faster processing) way to do the compositing than this:

        float alpha = (float)Overlay[3] / 255;
        float oneLessAlpha = 1 - alpha;
        Destination[2] = (Overlay[2] * alpha + Background[2] * oneLessAlpha);
        Destination[1] = (Overlay[1] * alpha + Background[1] * oneLessAlpha);
        Destination[0] = (Overlay[0] * alpha + Background[0] * oneLessAlpha);

Answer 1

Presumably by XRGB you mean a bitmap with four bytes per pixel, but what would be the alpha channel left at some constant value.

An obvious starting point would be to draw the XRGB bitmap first, and the RGBA bitmap second. When you draw the second, enable blending (glEnable(GL_BLEND);) and set your blend function with glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);. This way the blending depends only on the alpha channel in the source (the RGBA) and ignores any in the destination (the XRGB bitmap that’s already been drawn).

Edit: Oops — somehow I thought I saw some reference to OpenGL, but rereading (and noting the comment) no such thing is there. Doing the job without OpenGL isn’t terribly difficult, just generally slower. Let’s call the pixels from the two input bitmaps S and D, and the corresponding pixel in the result C. In this case we can compute each pixel in C as:

C_r = S_r * S_a + D_r * (1-S_a)

C_g = S_g * S_a + D_g * (1-S_a)

C_b = S_b * S_a + D_b * (1-S_a)

This assumes that you normalize (at least) the A channel to the range of 0..1, and that the ARGB bitmap is S and the XRGB is D.