[ft] How to draw FT_Bitmap to the ARGB image?

[John Found]

I need to draw text on a transparent image with format ARGB, pre-multiplied 
alpha. 

The rendered FT_Bitmap has format FT_PIXEL_MODE_LCD and I want the glyph to be 
displayed in some color with transparency.

So, for every pixel I have 3 numbers:

Da, Dr, Dg, Db - destination pixel ARGB (the background image).
    Fr, Fg, Fb - FreeType rendered pixel (FT_Bitmap rendered with 
FT_RENDER_MODE_LCD)
Ca, Cr, Cg, Cb - The color I want the glyph to be drawn. 

So, the question: How to properly combine these 3 numbers in order to get the 
result bitmap pixel.

A theory and math are welcome. If I understand the theory, I can write the code.

Best Regards.
-- 
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http://asm32.info
John Found <address@hidden>