<p>2D application users seem to be using transparency as a proxy for coverage more often than not. However they are not the same, although nearly everybody makes the assumption that they are.</p>
<p>Given that we do more coverage based assumptions than actual alpha based assumptions. Wouldnt it make more sense to formulate coverage somehow instead? Is it even possible to somehow do coverage based compositing?</p>

CC BY-SA 4.0
2020-11-02T06:07:37.687Z
"*2D application users seem to be using transparency as a proxy for coverage more often than not.*" What do you mean by "coverage" in this context? Are you talking about multisample coverage?
Nicol Bolas
CC BY-SA 4.0
2020-11-02T06:26:50.293Z
Ia am talking about the fact that people by default dont want to alpha blend but instead they want to antialias edges. Alphablending mimics transparency which is one case but in practice people expect it to blend as if two objects were laid out next to eachother instead of layer at time. Which leaves cracks showing like for example in vector graphics. But same is expected in raster graphics. More like 50% +50% being 100% than 75 alpha blending gives.
joojaa
CC BY-SA 4.0
2020-11-02T06:30:41.477Z
"*in practice people expect it to blend as if two objects were laid out next to eachother instead of layer at time*" What "people" expect this? Where have you seen this expectation? Also, a coverage mask is *mathematically identical* to alpha blending (assuming the alpha value exact matches a specific number of samples, and assuming the bottom one set all of the sample values), once you perform the antialiasing resolve. So I don't know what you're talking about.
Nicol Bolas
CC BY-SA 4.0
2020-11-02T06:42:19.227Z
@NicolBolas No its not mathematically identical. It just seems naively to be. There are thousands uppon thousands of humans manually tweaking renders and composites because of this. In practice people dont expect that the statistical average is the real value but the complementary maximum. This would solve a lot of problems.
joojaa
CC BY-SA 4.0
2020-11-02T14:18:27.580Z
"*No its not mathematically identical. It just seems naively to be.*" If you have 4 samples that are black and 12 samples that are white, multisample resolve will give you a 75% grey. If you have a black pixel, and you draw a white pixel with a 75% alpha and alpha-blend them, you get a 75% grey. These are mathematically equivalent operations. If they are not, please explain in your question how they aren't identical, with specific examples of where they are different.
Nicol Bolas
CC BY-SA 4.0
2020-11-02T14:56:12.827Z
@NicolBolas yes, i am doing that as soon as i have a computer in front of me. Anyway your right in your descriptions scope.  But not if i have more than foreground and background layers.
joojaa
CC BY-SA 4.0
2020-11-02T17:47:27.573Z
@NicolBolas Coverage in this sense: http://jcgt.org/published/0004/02/03/
Nathan Reed
<transparency><theory><compositing>
coverage vs transparency
joojaa
# coverage vs transparency

(Asked by: username_0 on 2020-11-02)

2D application users seem to be using transparency as a proxy for coverage more often than not. However they are not the same, although nearly everybody makes the assumption that they are.

Given that we do more coverage based assumptions than actual alpha based assumptions. Wouldnt it make more sense to formulate coverage somehow instead? Is it even possible to somehow do coverage based compositing?

**Tags:** transparency, theory, compositing

> **Comments:**
> **username_1**: "*2D application users seem to be using transparency as a proxy for coverage more often than not.*" What do you mean by "coverage" in this context? Are you talking about multisample coverage?  
> **username_0**: Ia am talking about the fact that people by default dont want to alpha blend but instead they want to antialias edges. Alphablending mimics transparency which is one case but in practice people expect it to blend as if two objects were laid out next to eachother instead of layer at time. Which leaves cracks showing like for example in vector graphics. But same is expected in raster graphics. More like 50% +50% being 100% than 75 alpha blending gives.  
> **username_1**: "*in practice people expect it to blend as if two objects were laid out next to eachother instead of layer at time*" What "people" expect this? Where have you seen this expectation? Also, a coverage mask is *mathematically identical* to alpha blending (assuming the alpha value exact matches a specific number of samples, and assuming the bottom one set all of the sample values), once you perform the antialiasing resolve. So I don't know what you're talking about.  
> **username_0**: @username_1 No its not mathematically identical. It just seems naively to be. There are thousands uppon thousands of humans manually tweaking renders and composites because of this. In practice people dont expect that the statistical average is the real value but the complementary maximum. This would solve a lot of problems.  
> **username_1**: "*No its not mathematically identical. It just seems naively to be.*" If you have 4 samples that are black and 12 samples that are white, multisample resolve will give you a 75% grey. If you have a black pixel, and you draw a white pixel with a 75% alpha and alpha-blend them, you get a 75% grey. These are mathematically equivalent operations. If they are not, please explain in your question how they aren't identical, with specific examples of where they are different.  
> **username_0**: @username_1 yes, i am doing that as soon as i have a computer in front of me. Anyway your right in your descriptions scope.  But not if i have more than foreground and background layers.  
> **username_2**: @username_1 Coverage in this sense: http://jcgt.org/published/0004/02/03/  

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## 0 answers

