Alpha Compositing and shyts

Assuming you want to blend. That is, alpha is added, and then a percent of a and a percent of b is gained:

Code:

double tot = a1 + a2;
newImage[offset + j*4 + 3] = (byte)a0;    //alpha result
newImage[offset + j*4] = (byte)(r1 * a1/tot + r2 * a2/tot);
newImage[offset + j*4] = (byte)(g1 * a1/tot + g2 * a2/tot);
newImage[offset + j*4] = (byte)(b1 * a1/tot + b2 * a2/tot);

This blows up if both alphas are zero, btw. Shouldn't matter, as zeroes shouldn't be added anyway (if ((a1 == 0) && (a2 == 0)) continue

What are you really trying to do though? There are a ton of ways to blend them, so uh.

With the method I'm using, when you put one image on top of another, the alphas are NOT given equal weight. Here's a simplistic example:

Code:

RR GG BB AA
Top layer:    FF 00 00 80
Bottom layer: 00 FF 00 80
Result layer: AA 55 00 C0

This is how it turned out at least in paint.NET.

((r1*a1 + r2*(255-a1)*(a2/(double)255))/a0 + .5)

((r1*a1/(double)255/a0 + r2*(255-a1)*(a2/(double)255))/a0 + .5)

Hope that makes sense, you need to scale back r1, g1, b1 with respect to their overall alpha (a0), or you're multiplying by 11 twice.


At least that's how I understand the formula to work. I might be off by a factor of 255 because I'm not exactly sure how that works.


Anyway, conceptually if a1 + a2 ~= a0, then you have a1/a0 + a2/a0 ~= 1 -> multiply 1 by original colours, weighted by their relative alpha.

Uhhh I'm not sure I quite understand what you're doing here o.O

a1 + a2 != a0. Otherwise we get illegal values for alpha.

As for that second formula you provided, I could swear that you divided r1 by a0 twice. For practical matters, you and I would agree that if the second color is completely transparent, then only the first color would show unmodified, right? Given that, and a2 = 0, it should be fairly easy to see that you just greatly reduced the value of r1. And since a2 = 0, r1 = r0 (red result). That doesn't look right.