Re: [css-compositing] Move CSS Compositing spec to WD

On Monday 2012-07-30 13:03 -0700, Rik Cabanier wrote:
> You can find the latest version here:
> Please let us know if you have feedback on the current spec.

Some comments on the current spec follow.  In particular, the
version I'm commenting on is

1.1. What is compositing

The title of this section should either have a question mark "What
is compositing?" or should have the final two words reversed "What
compositing is".

I find the description of shape in this section to be rather
confusing.  Is it trying to describe the same concept that the
section "5.2.3. Clip to self behavior" describes?  If so, the latter
section is clearer.

1.2. simple alpha compositing

It may be worth mentioning that the color formula given here applies
to each color component.

It seems confusing that this section tries to work with a mix of
premultiplied and non-premultiplied colors in the first part,
especially given that it doesn't explain the difference until later.
Furthermore, the formula near the end in terms of pre-multiplied
colors could be simplified from:
  co = cs + Cb x αb x (1 - αs)
  co = cs + cb x (1 - αs)
by using the premultiplied color of the backdrop as well.

4. Compositing Groups

Group invariance doesn't hold for CSS, because creating a group
changes z-ordering by establishing a new stacking context.  This is
necessary because, by default, external elements can interleave
between some of the descendants of an element, but that doesn't make
sense for things composited in interesting ways.  Therefore, what
establishes a group needs to be described.  (For example, in CSS,
having 'opacity' != 1 establishes a group; probably many of the new
properties in this spec need to do the same for all of their
non-default values.)

4.3. Knockout Groups

Not clear how knockout groups work in CSS (rather than SVG) because
it's not clear what drawing operations are atomic, though perhaps it
could be defined by saying that anything that has its own item in
Appendix E of CSS 2.1 is an atomic drawing operation.

5.2.1. Isolated groups and Porter Duff modes

What does "bottom element in a group" mean?  Does it mean the
element establishing the group, i.e., the element closest to the

7.1. Behavior specific to CSS

  #  If an element specifies non-default blending, compositing or
  #  ‘opacity’, its renderstyle and that of all of its children will
  #  revert to ‘flat’.

This is the first occurrence in the spec of "renderstyle" and
"flat"; I don't know what they are.  Is this referring to
'transform-style'?  And does this apply to all of the properties in
the section?

  #  The application of non-default blending or compositing to an
  #  element formatted with the CSS box model also establishes a
  #  pseudo-stacking-context the same way that CSS ‘opacity’ does.
  #  One of the consequences is that elements with z-index will not
  #  honor the depth of elements outside of the group. 

I think the intent here is to create a stacking context, not a
pseudo-stacking context.  A pseudo-stacking context doesn't affect
z-index.  And again, this should be clear exactly which properties
"non-default blending or compositing" refers to.

  #  Order: first filtering is applied, followed by blending and
  #  then compositing. 

I'd think you need to define the order of:
 * 'filter'
 * 'mask'
 * 'clip-path'
 * 'clip'
 * 'overflow'
 * blending
 * compositing
Though, maybe "filtering" is a term for a bunch of the above?

  # Conceptually all HTML elements are part of a group that consists
  # of the following sub groups, each containing a single element.
  # In stacking order:
  #  the box shadow
  #  the background layers
  #  the text shadow
  #  the text and nested elements 

This is incorrect.  The correct stacking order is described in
Appendix E of CSS 2.1, though that doesn't include box-shadow or
text-shadow.  It does, however, include text decorations, and
describe the interaction of nested elements with the other layers
correctly.  It also describes when (most cases) the contents of an
element interleave with the contents of other elements in z-order.

All of the properties described should have a "Computed value: as
specified" line.  They should also not be animatable in CSS terms
(since there's no way to interpolate the values).  (Perhaps we
should have called that line "Interpolable".  I suppose we still

I'm not especially happy with the large number of properties added
for different blend modes, or with adding the matched lists to
text-shadow and box-shadow.

It's not clear to me why 'foreground-blend-mode' takes a list of
values.  I also think "text or nested elements" may not a sensible
grouping of things in CSS the way it is in SVG.


𝄞   L. David Baron                  𝄂
𝄢   Mozilla                    𝄂

Received on Monday, 30 July 2012 23:31:46 UTC