How to treat alpha transparency from PNG directly in memory?

Question

I would appreciate the help of you low level programmers... My problem is this: I want to print a bitmap of format ARGB8888 directly into video memory. The form of the bitmap is alright, the problem is the alpha channel that I can't figure out how to use. I've seen code in wikipedia that overlays the pixels like this:

CompositedPixelColor = Alpha * ForegroundPixelColor + (1 - Alpha) * BackgroundPixelColor

Where a color varies from 0 - 1. This is done for each channel R G B. What I'm doing is copy each byte for each color of each pixel of my bitmap directly to the video memory using the formula above, but I'm missing something because the colors don't present theirselves alright.

I'm trying to do something like the code posted in this thread: http://www.badadev.com/create-a-photo-editing-app/ But here they don't treat transparency, and that is my problem. Thanxs!

Answer 1

In the code you posted alpha is treated as a value between 0 and 1, which of course doesn't work if you use you alpha channel as an unsigned char, use the following if you want to do it in integer space:

unsigned short background = 0x40;
unsigned short foreground = 0xe0;
unsigned short alpha = 0xc0;
unsigned short compositedcolor = (alpha * foreground + (0xff - alpha) * background) >> 8;

note that while these are shorts the values should all be 0 - 255, the short is just needed as computational space for the char * char products, you could also use intermediate casts, but I just used unsigned short types for all to make it more readable.