[css-color-5] For color-mix(), what should happen if you provide an adjuster without a paired percent?

[weinig]

In CSS Color 5's color-mix() function, https://drafts.csswg.org/css-color-5/#color-mix, what should happen if an adjuster is specified without a percentage. For example:

color-mix(lch, red lightness, blue);

The grammar for color-adjuster allows this:

<color-adjuster> = [ [
      <srgb-adjuster> | <hsl-adjuster> | <hwb-adjuster>
    | <xyz-adjuster> | <lab-adjuster> | <lch-adjuster>
  ] | alpha ] <percentage>?

due to the "?" after , but I am unclear what the semantics are of that.

In my investigative implementation, I treat an adjuster without a percentage as equivalent to specifying 0% for that adjuster, so for the example above, it would be equivalent to:

color-mix(lch, red lightness 0%, blue);

but I am not convinced this is the intention of the spec authors.

[Myndex]

Hmmm.... IMO it should be 100% for the specified adjuster, and then ALL OTHER adjusters are 0%.

Consider in an image editing application where you have a transfer mode for one layer to another, such as "lightness" where only the lightness attribute of the layer is applied to he underlying layer.

[samweinig]

I guess I should mention my preference as well.

I don't see any reason for this to be valid at all. Seems like we should change the <percentage>? to be just <percentage>.

[svgeesus]

The handling of percentages is described in the list following "Arguments are normalized as follows:"

1. When a percentage is specified without any color-adjusters, it is expanded to all adjusters available for the given colorspace plus alpha.

2. If a percentage is omitted and there are no adjusters present on any color arguments, it defaults to 50% (an equal mix of the two colors).

3. Adjusters that are not present on a color default to 0% (i.e. the corresponding components are taken from the other color).

4. Adjusters that are only present on one of the colors default to 100% - p on the second color, where p is the percentage of the corresponding adjuster in the other color.

5. If the sum of corresponding adjusters across all colors is not 100%, percentages are scaled proportionally so that the sum is exactly 100%

[svgeesus]

The case you are mentioning (adjuster is present, it has no percentage) seems to be item 2 in that list. But perhaps that wasn't clear?

The intent is that you can do color-mix(peru, lightgoldenrod) and get color-mix(peru 50%, lightgoldenrod); a percent is only needed if you want an unequal mixture.

I'm fine requiring an explicit 50%, if we decide on that; or simply adding what I typed above as the first, simplest, example.

[samweinig]

The case I was referencing was a percentage after an adjuster, which I don't think item 2 refers to as item to states:

If a percentage is omitted and there are no adjusters present on any color arguments, it defaults to 50% (an equal mix of the two colors).

That said, since adjusters have been removed from color-mix(), the concern is no longer relevant. (Though, if color-adjusters are going to be used for color-adjust(), I think requiring the percentage probably does make sense. The spec is kind of in an odd state right now where color-adjust() references the color-adjuster production but it is no longer defined anywhere).

I think we are good to close this issue. Thanks for reviewing it.

[LeaVerou]

Since per editor resolution we removed adjusters from color-mix(), I'm going to go ahead and close this issue.