You're mostly right. The industry-wide accepted answer is to multiply the opacity into the colors before interpolating, so the formula would be (R1xT1 + R2xT2)/2 for the average, and then to do later transparency blending as if the opacity term was already multiplied in.

Which means that your output pixel cant be both white and low transparency. I guess its a typical graphics 'close enough and better performance' outcome (where mine is marginally more difficult to calculate and needs some more logic to avoid divide by 0)

> Which means that your output pixel cant be both white and low transparency.

Did you mean low opacity?

If so, that's not quite right. (0.1,0.1,0.1,0.1) premultiplied is the same color as (1,1,1,0.1) "normal." They're both white and low opacity, just in different representations. You don't actually lose much granularity because the graphics card has to multiply the color channels by the opacity value sooner or later.

Separately, your formula doesn't work for interpolation. It works for averaging, but in order to do texture sampling, you need interpolation, so your formula can't actually be used unless you can adjust it to deal with interpolation gracefully.

Thanks for the answer - I did not know of this premultiplacation. This makes it effectively the same or very close? Assuming output transparency/alpha is (T1+T2/2), dividing by this gives the difference

I don't quite get your point on interpolation, but I'll look up when I have the chance