Why is two times 50% opacity brush not 100% opacity ?

[Chrronos]

Hey,

in the paintbrushtool how does it work if i paint multiple times at the same spot? Painting the same spot with 50% opacity two times does not mak it 100% opacity but something inbetween, why and how much inbetween ? Does the same rules apply to flow ?

thx

[Ron P.]

I'm going to make a guess at this, since your equation does not equal 100%, in Photo anyway.

With the brush opacity set at 50%, the first stroke will yield that amount. However, the second brush stroke is then applying 50% of the already 50%, so the total would be 75%. The first is half, the second is half of a half, or 25%. Keep doing this and you will arrive at 100%. I maybe (probably) wrong, but it's what is logical.

[loukash]

Strictly mathematically it will never get fully opaque if you only shift 50 % layers – or brush strokes, that doesn't make a difference.

100 % opacity = 0 = black
50 % opacity = 127 = neutral gray
0 % opacity = 255 = white

By adding another 50% layer you are multiplying by 0.5
127 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 * 0.5 …
But effectively the result will be rounded, so about the 9th layer will be de facto black.

Keep an eye on the Info panel in particular:

[RNKLN]

50% opacity = 50% transparency. So the question could also be: why is 2x 50% transparency not 100% transparency? It simply doesn't work that way.

[NotMyFault]

You can lookup the actual  formula in WikiPedia: https://en.wikipedia.org/wiki/Alpha_compositing

A=a1 + a2 * (1 - a1)

so in your case with both a1 and a2 equal to 0.5 you get a equals  0.75 after the second strike.

the alpha channel has 8 bit resolution, so 256 different values are possible, which are normalized from [0••255] to [0••1] or [0%••100%] when used for this formula. After about 8 iterations, the increment becomes lower than 1/255