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Re: Comments on SVG Compositing

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Fri, 8 Apr 2011 17:45:51 -0700
Message-ID: <BANLkTimqs8-syp4wsoNJYu1s284KS-xm2g@mail.gmail.com>
To: Rik Cabanier <cabanier@gmail.com>
Cc: www-svg <www-svg@w3.org>
On Fri, Apr 8, 2011 at 4:41 PM, Rik Cabanier <cabanier@gmail.com> wrote:
> I'm not familiar with the "clip-to-self" concept so it's better if someone
> else responds to this.

I believe clip-to-self:object (or comp-clip-to:src) is the default
behavior in Photoshop.  I suspect that this is the most intuitive
behavior for most people, so it would probably be good to just make
that the default.


>> >> I don't have strong graphics experience, so there may be something I'm
>> >> missing here, but 'enable-background' and 'knock-out' appear to be
>> >> *exactly* identical in operation, just applying to different things:
>> >> 'knock-out' transforms "A op B" to "A op (A dst_out B)", while
>> >> enable-background transforms "[group image] over [background]" to
>> >> "[group image] over ([group image] dst_out [background]".  Can these
>> >> two properties be unified in some way?
>> >>
>> > knock-out = how objects within the container blend with each other
>> > enable-background = how objects within the container blend with the
>> > background
>> > The programming logic between the 2 modes is very different so I think
>> > that
>> > this is enough for a separate attribute.
>>
>> Your description makes them seem even more similar.  ^_^  From the POV
>> of an author with relatively little graphics experience, there's no
>> important difference between these two for me.  The fact that
>> implementations might implement the two in different fashions is
>> irrelevant to me, because I'm not an implementor.
>>
>>
>> > If you unify them into 1 property, it would also result in many states:
>> > knock-out (= 3 states) * enable-background (= 3 states) = 9 different
>> > names
>> > which is more confusing.
>>
>> I'm not sure I understand.  'knock-out' and 'enable-background' have
>> only two states each.
>
> They also have separate 'inherit' states.

Ah, yeah, I naturally ignore that, since *all* properties have an
'inherit' value (and 'initial', too).


>> Further, the syntax seems like it can be very
>> simple; something like:
>>
>> comp-over: none | [ group || [ background | rect(x,y,w,h)] ]
>>
>> ...with 'comp-over: group background;' being the default.
>>
>
> Does 'group' correspond with 'knockout = false'?

Yes.


>> The only thing that would let me justify this being split into two
>> property would be if it seems like it's useful to have these cascade
>> separately.  I don't have enough experience with using these to
>> understand if that's something important or not.
>
> Adobe applications have the ability to control the 2
> properties independently including the 'inherit' state.
> I believe this was done because they are conceptually different for
> designers.
> ie see these articles:
> http://layersmagazine.com/the-joys-of-isolation-blending.html (enable-background
> is the same property as isolate)
> http://www.creativepro.com/article/illustrator-how-this-technique-a-real-knockout
> In our apps, the default state of 'knockout' is 'inherit' while
> 'enable-background'/'isolate' is 'true'/'false' by default.
> (In our imaging model, the 2 values are simple booleans. The application is
> expected to resolve the 'inherit' value before exporting to PDF)

Hm, okay.  If that's a reasonable default, then it does make sense to
keep them split.  (By the way, thanks for the links!  I now understand
the use of knock-out really well - it lets you paint with transparent
colors the same way you do with opaque, where you can fill a big area
with color and then draw the details on top of it.)  I can definitely
see how inheriting 'knock-out' by default is a good idea.  This also
means that 'knock-out' should probably be an inherited property.

In that case, try this on for size:

1. Rename the property/values of clip-to-self in the way I suggested.
Change the lacuna value to 'src', as this se

2. Keep knock-out's current name and values.  Make it an inherited property.

3. Rename 'enable-background' to 'comp-children-over', with values of
"transparent | background | rect(x,y,w,h)".  This matches my mental
model a bit better, where the property affects what I composite the
children of the group over.

It would also be *awesome* to use that post as an example for
enable-background/comp-children-over, and put in some note about why
knock-out is useful.

~TJ
Received on Saturday, 9 April 2011 00:46:39 GMT

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