[css-paint-api] Box concrete object size

[asajeffrey]

When drawing a paint image (https://drafts.css-houdini.org/css-paint-api/#draw-a-paint-image), the image size is defined to be the concrete object size of the box (https://drafts.csswg.org/css-images-3/#concrete-object-size).

Experimentally, chrome uses the size in px, including the padding but not the border or margin. If this is the intended behaviour, the spec should probably say so.

[tabatkins]

It is indeed supposed to be the size in px.

Padding/border/margin is irrelevant here; you're not painting an element, you're painting an <image>. Every place that accepts an <image> defines how it gets sized. The spec's language is a little confusing/confused here; boxes don't have "concrete object sizes" (well, not by default at least; it's possible to invoke that algorithm to size a box).

[asajeffrey]

Not sure that padding/border/margin is irrelevant here, given a box we need to know which size to use, the only way I could see to calculate it involved border + margin.

[tabatkins]

Yeah, but that's all encapsulated in the "figure out the size of the background-image"/whatever algorithm. It's not directly relevant to this specification at all; it just needs to know the concrete object size.

[asajeffrey]

OK, but in might be nice to link to https://drafts.csswg.org/css-images-3/#object-sizing-examples, and perhaps put a note to the effect that for background images the default sizing algorithm is the box size including padding, but excluding border and margin.

[tabatkins]

Agree that linking there would be useful.

[bfgeek]

I've added a link to the object-sizing-examples just below where {{PaintSize}} is defined. I didn't add the note about how it excludes border & margin out of risk that css-images will change the definition someday :).