calculating transparency

[BillieJoe]

Hello there!

Sorry, I'm from Austria and my english isn't the best... but I have a drawing Problem in my 2D-Game.

I load the Images and store them in an Array by using the PixelGrabber class. But now I have a problem by drawing Images one upon the other.

For Example, I have these Values:

-65280 1357369291
-65280 518382559
-65280 114414291

Left ist the Background (red), right the foreground of an *.png Image with Alpha.

How to combine, calculate back- and foreground?

[crystalsquid]

Hi BillieJoe,
  You have to split the values into their integer colour channels. Off the tope of my head:

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// bgcol = BackGround Colour // fgcol = Foreground Colour int blend( int bgcol, int fgcol ) {     int bg_r = bgcol&0xff;     int bg_g = bgcol&0xff00;     int bg_b = bgcol&0xff0000;     int fg_r = fgcol&0xff;     int fg_g = fgcol&0xff00;     int fg_b = fgcol&0xff0000;     int alpha = fgcol >> 24;

Alpha blending operation is as follows:

Final  = (FG - BG) * Alpha + BG;

When alpha = 0, Final = BG, when alpha = 1, Final = FG.
In code:

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// Blend values    bg_r += ( (fg_r - bg_r) * alpha)>>8;     bg_g += ( (fg_g - bg_g) * alpha)>>8;     bg_b += ( (fg_b - bg_b) * alpha)>>8;     // Mask channels back to 8 bit    bg_r &= 0xff;     bg_g &= 0xff00;     bg_b &= 0xff0000;     // Return combined RGB values, with 255 as the alpha    return ( 0xff000000 + bg_r + bg_g + bg_b); }

This can be optimised as you can do the B and R parts simultaneously.  Hope this helps,

- Dom

[BillieJoe]

Thanks a lot, it works!