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RE: SVG 2 rendering model

From: Rik Cabanier <cabanier@adobe.com>
Date: Fri, 3 Aug 2012 10:50:10 -0700
To: Leonard Rosenthol <lrosenth@adobe.com>, Nikos Andronikos <nikos.andronikos@cisra.canon.com.au>, "public-svg-wg@w3.org" <public-svg-wg@w3.org>
Message-ID: <83F37C1A4497B54589EAEDC750D03A9473B7E6A0@nambx09.corp.adobe.com>
> For PDF, there is no group opacity - except at the XObject level.  
Ah, I think that is where the disconnect comes from. We've been talking about how to handle groups.
In HTML, a group is a stacking context
In SVG, every <g> is a group
In PDF, a group is a transparency group (=XObject) .

> For SVG, I think that having a non-0 or 1 value of opacity
> on a group would very clearly force it into isolated.
This will give unexpected results since adding a <g> with 99.99999% opacity might render very different than with 100% opacity

You probably mean:
> So maybe that's the approach.  Default is NON-isolated UNLESS there is a group
> opacity or a filter, and then it shall be considered as isolated.

Group opacity should not create isolation. That is not the correct model (and it doesn't help)
I am unsure what should happen with filters, but it seems undesirable that the content changes appearance if you apply a filter.

Rik

> -----Original Message-----
> From: Leonard Rosenthol
> Sent: Friday, August 03, 2012 10:33 AM
> To: Rik Cabanier; Nikos Andronikos; public-svg-wg@w3.org
> Subject: RE: SVG 2 rendering model
> 
> For PDF, there is no group opacity - except at the XObject level.  And in that
> case, it's not uncommon to handle the XObject as a separate "thing" (buffer,
> display list, etc.).   For SVG, I think that having a non-0 or 1 value of opacity
> on a group would very clearly force it into isolated.  Same as has been
> discussed with filters.
> 
> So maybe that's the approach.  Default is isolated UNLESS there is a group
> opacity or a filter, and then it shall be considered as isolated.
> 
> Leonard
> 
> -----Original Message-----
> From: Rik Cabanier
> Sent: Friday, August 03, 2012 12:59 PM
> To: Leonard Rosenthol; Nikos Andronikos; public-svg-wg@w3.org
> Subject: RE: SVG 2 rendering model
> 
> How would you implement group opacity on a non-isolated group without a
> buffer?
> 
> I agree that if you have a non-isolated group with default parameters, you
> could optimize it away since it's not supposed to do anything.
> 
> Rik
> 
> > -----Original Message-----
> > From: Leonard Rosenthol
> > Sent: Friday, August 03, 2012 5:48 AM
> > To: Rik Cabanier; Nikos Andronikos; public-svg-wg@w3.org
> > Subject: RE: SVG 2 rendering model
> >
> > Based on the text you copied, there is actually NOTHING in the spec
> > that requires a specific implementation in the case of a non-isolated
> > group.  It simply requires that certain things have to take place in a
> > certain order.  I agree with that.
> >
> > Consider the "buffer" in this case to be the full page output and
> > non-isolated groups can be composited directly into that buffer.  It
> > represents both the group backdrop as well as the immediate backdrop,
> > since if you only draw to it, it is " the result of compositing all
> > elements up to but not including the first element in the group".  Right?
> >
> > The size of the group only matter for isolated (or potentially
> > knockout) groups, since in those cases, you have no choice but to use
> > a separate "buffer".
> >
> > Leonard
> >
> > -----Original Message-----
> > From: Rik Cabanier
> > Sent: Thursday, August 02, 2012 11:21 PM
> > To: Leonard Rosenthol; Nikos Andronikos; public-svg-wg@w3.org
> > Subject: RE: SVG 2 rendering model
> >
> > No, a buffer is always required.
> > From the pdf spec:
> >
> > From the perspective of a particular element in a nested group, there
> > are three different backdrops of interest:
> > *The group backdrop is the result of compositing all elements up to
> > but not including the first element in the group. (This definition is
> > altered if the parent group is a knockout group; see 11.4.6, "Knockout
> > Groups") *The initial backdrop is a backdrop that is selected for
> > compositing the group's first element. This is either the same as the
> > group backdrop (for a non-isolated
> > group) or a fully transparent backdrop (for an isolated group).
> > *The immediate backdrop is the result of compositing all elements in
> > the group up to but not including the current element.
> > When all elements in a group have been composited, the result shall be
> > treated as if the group were a single object, which shall then be
> > composited with the group backdrop
> >
> > So, according to the PDF spec, the buffer is initialized with the
> > backdrop of the group which is the " the result of compositing all
> > elements up to but not including the first element in the group "
> > In the case of HTML and Flash, the size of this buffer is calculated
> > automatically. In PDF, it is given as the bbox parameter in the
> > transparency group. From the PDF spec:
> >
> > If the transparency group is non-isolated (the value of the I entry in
> > its group attributes dictionary is false), its initial backdrop,
> > within the bounding box specified by the XObject's BBox entry, shall
> > be defined to be the accumulated colour and alpha of the parent group
> > or page-that is, the result of everything that has been painted in the
> parent up to that point.
> >
> > Also, if there is no buffer, how could you implement group opacity?
> >
> > > -----Original Message-----
> > > From: Leonard Rosenthol
> > > Sent: Thursday, August 02, 2012 5:54 PM
> > > To: Rik Cabanier; Nikos Andronikos; public-svg-wg@w3.org
> > > Subject: RE: SVG 2 rendering model
> > >
> > > In the case of a non-isolated transparency group, no buffer is required.
> > And
> > > in fact, it may actually be WRONG to use one, since you may well be
> > > blending the wrong colors in the wrong order depending on what's in
> > > the
> > group.
> > >
> > > Leonard
> > >
> > > -----Original Message-----
> > > From: Rik Cabanier
> > > Sent: Thursday, August 02, 2012 4:25 PM
> > > To: Leonard Rosenthol; Nikos Andronikos; public-svg-wg@w3.org
> > > Subject: RE: SVG 2 rendering model
> > >
> > > An implementor always has to do this anyway.
> > > The only difference is if the bitmap is filled with transparent
> > > black (= isolated) or the backdrop (=non-isolated)
> > >
> > > In PDF, this information comes from the bbox in the transparency group.
> > > Regardless of isolation, the buffer is created. (See 11.4.3 in the
> > > PDF
> > > spec)
> > >
> > > Rik
> > >
> > > > -----Original Message-----
> > > > From: Leonard Rosenthol
> > > > Sent: Thursday, August 02, 2012 1:09 PM
> > > > To: Rik Cabanier; Nikos Andronikos; public-svg-wg@w3.org
> > > > Subject: Re: SVG 2 rendering model
> > > >
> > > > On 8/2/12 3:34 PM, "Rik Cabanier" <cabanier@adobe.com> wrote:
> > > >
> > > > >Hi Leonard,
> > > > >
> > > > >> The problem with making isolated the default is that it will
> > > > >> impact performance for all of the normal cases.
> > > > >Isolated is faster since you don't have to composite all the
> > > > >group to get the background.
> > > >
> > > > But you DO have to create an "offscreen buffer", composite the entire
> > > > group into that, and then blit the entire "offscreen" back.   So you're
> > > > clearly using more memory and time than simply putting bits into
> > > > the existing buffer.
> > > >
> > > >
> > > > >The compositing spec specifies the order in which filters and
> > > > >compositing happens.
> > > > >The filters spec should say if the stacking group it creates is
> > > > >isolated or not. I believe people don't want it to be isolated
> > > > >(and can post some examples if needed)
> > > >
> > > > But how would you apply a filter to a group that has already been
> > > > blended into the background?  The result is certainly going to be
> > > > different - which may be good or bad.
> > > >
> > > > I would think that given a group with a filter applied to it, you
> > > > have to make that specific group isolated, apply the filter, and
> > > > only then blend it to the background.
> > > >
> > > > Leonard
Received on Friday, 3 August 2012 17:50:44 GMT

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