How to produce alpha channel effect?

[dc_2000]

Say, I have an image pixel imgP = RGB(im_r, im_g, im_b); it's alpha channel
value achP = RGB(ach_r, ach_g, ach_b); and the background pixel bkgP =
RGB(bg_r, bg_g, bg_b)

How do I combine them to produce alpha channel blending?

[CBasicNet]

The alpha algorithm is written in MSDN,

SCA is alpha value of range from 0.0 to 255.0. 0 means source is fully transparent and 255 means the source is fully opaque.

Dst.Red = Src.Red * (SCA/255.0) + Dst.Red * (1.0 - (SCA/255.0))
Dst.Green = Src.Green * (SCA/255.0) + Dst.Green * (1.0 - (SCA/255.0))
Dst.Blue = Src.Blue * (SCA/255.0) + Dst.Blue * (1.0 - (SCA/255.0))

You need not write your own function, you can use the GDI function, called AlphaBlend().

[CBasicNet]

You can rearrange the formula algebraically, to remove 1 division and remove all the float and int conversion by using int only.

Result = (SrcRed * Alpha + DstRed * (255 - Alpha))/255

I just show the red component as the formula is the same for other 2 color components.

[dc_2000]

Hey, thanks a lot. That's exactly what I needed. I was trying to find it in MSDN without any luck. Do you by any chance know the link there?

[CBasicNet]

Here it is,

BLENDFUNCTION struct (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/gdi/bitmaps_3b3m.asp)

[golanshahar]

This thread (http://www.codeguru.com/forum/showthread.php?t=327701) can also help.

Cheers

[dc_2000]

AWSOME! Thanks again.