<p>Yes, the statement in the W3C doc is correct. You can tell because the background term includes premultiplication by the background alpha αb. If it were non-premultiplied, it wouldn't have an αb in it.</p>

CC BY-SA 4.0
2018-08-07T15:48:28.013Z
Yes, "does not contribute to the final color of the pixel" would be a good way to say it, or maybe "zero alpha meaning that the color has no influence on the final pixel color".
Nathan Reed
CC BY-SA 4.0
2018-08-06T18:48:36.140Z
2018-08-06T18:48:36.140Z
Nathan Reed
<p>I'm referring to this section: <a href="https://www.w3.org/TR/compositing-1/#simplealphacompositing" rel="nofollow noreferrer">https://www.w3.org/TR/compositing-1/#simplealphacompositing</a></p>

<p>Considering the formula <code>co = Cs x αs + Cb x αb x (1 - αs)</code> the docs state then, that <code>co</code> is the </p>

<blockquote>
  <p>the premultiplied pixel value after compositing</p>
</blockquote>

<p>I would argue that this is not the case, since in <a href="http://jcgt.org/published/0004/02/03/paper.pdf" rel="nofollow noreferrer">this article</a> the resulting color is depicted with a capital C and in the case of premultiplied version, the lower case is used (<code>c</code>). I'm not really sure if this is the case, it's not too obvious from both docs and the article. So, is the result of  <code>co = Cs x αs + Cb x αb x (1 - αs)</code> really a premultiplied value?</p>

<alpha>
Are w3.org docs correct?
uglycode
# Are w3.org docs correct?

(Asked by: username_0 on 2018-08-06)

I'm referring to this section: <https://www.w3.org/TR/compositing-1/#simplealphacompositing>

Considering the formula `co = Cs x αs + Cb x αb x (1 - αs)` the docs state then, that `co` is the

> the premultiplied pixel value after compositing

I would argue that this is not the case, since in [this article](http://jcgt.org/published/0004/02/03/paper.pdf) the resulting color is depicted with a capital C and in the case of premultiplied version, the lower case is used (`c`). I'm not really sure if this is the case, it's not too obvious from both docs and the article. So, is the result of `co = Cs x αs + Cb x αb x (1 - αs)` really a premultiplied value?

**Tags:** alpha


---

## 1 answers

### Answer by username_1 on 2018-08-06. Score: 0 | Accepted answer

Yes, the statement in the W3C doc is correct. You can tell because the background term includes premultiplication by the background alpha αb. If it were non-premultiplied, it wouldn't have an αb in it.

> **Comments:**
> **username_1**: Yes, "does not contribute to the final color of the pixel" would be a good way to say it, or maybe "zero alpha meaning that the color has no influence on the final pixel color".  

---

