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<DIV dir=ltr align=left><SPAN class=118300323-03012011><FONT face=Arial
color=#0000ff size=2>Please see my in-line comments
below:</FONT></SPAN></DIV><BR>
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<FONT face=Tahoma size=2><B>From:</B> rocallahan@gmail.com
[mailto:rocallahan@gmail.com] <B>On Behalf Of </B>ext Robert
O'Callahan<BR><B>Sent:</B> Monday, January 03, 2011 5:56 PM<BR><B>To:</B>
Szabo Carol (Nokia-MS/Boston)<BR><B>Cc:</B> chuck@jumis.com;
whatwg@whatwg.org<BR><B>Subject:</B> Re: [whatwg] Fwd: RE: Inconsistent
behaviour of globalCompositeOperation property<BR></FONT><BR></DIV>
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<P>HTML5 - Canvas.</P>
<P>I have read this thread (from and of July 2010) and I happen to agree
that the Safari/Chromium implementation is more intuitive, and likely less
expensive to implement, therefore I offer these 2 proposals for changing the
spec's Drawing model section to match what I perceive to be the preference
of most people in that discussion.</P>
<P>Of note: According to the current Drawing model, an opaque shape's shadow
would be erased as part of step 6 when drawn with source-in composite Mode
if globalAlpha is 1, which is probably not the intended behavior.</P>
<P>Version 1:</P>
<P>4.8.11.1.13 Drawing model</P>
<P><FONT face=Arial color=#0000ff size=2></FONT> </P>
<P>When a shape or image is painted, user agents must follow these steps, in
the order given (or act as if they do):</P>
<P>1. Render the shape or image onto an infinite transparent black bitmap,
creating image M1, as described in the previous sections except that for the
purpose of this step every pixel of the image will be considered to be fully
opaque white and the current fillStyle will be considered to be solid fully
opaque white and the strokeStyle will be considered fullyOpaque white as
well <BR></P></SPAN></SPAN></DIV></DIV></BLOCKQUOTE>
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<P>2. When shadows are drawn, render the shadow from image M1, using a fully
opaque white shadow color as described in the shadows section, creating
image M2. </P></SPAN></SPAN></DIV></DIV></BLOCKQUOTE>
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<P>3. Let C1 be a region obtained by intersecting the canvas's cliping
region with the set of pixels in the canvas that correspond to pixels in M1
(by having the same coordinates) that are not fully transparent.</P>
<P>4. Let C2 be a region obtained by intersecting the canvas's cliping
region with the set of pixels in the canvas that correspond to pixels in M2
(by having the same coordinates) that are not fully
transparent.</P></SPAN></SPAN></DIV></DIV></BLOCKQUOTE>
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<P>5. Render the shape or image onto an infinite transparent black bitmap,
creating image A, as described in the previous sections. For shapes, the
current fill, stroke, and line styles must be honored, and the stroke must
itself also be subjected to the current transformation matrix.</P>
<P>6. When shadows are drawn, render the shadow from image A, using the
current shadow styles, creating image B.</P>
<P>7. When shadows are drawn, multiply the alpha component of every pixel in
B by globalAlpha.</P>
<P>8. When shadows are drawn, composite B with the current canvas bitmap,
cliping results to region C2 defined above, using the current composition
operator.</P>
<P>9. Multiply the alpha component of every pixel in A by globalAlpha.</P>
<P>10. Composite A with the current canvas bitmap, cliping results to region
C1 defined above, using the current composition operator.
</P></SPAN></SPAN></DIV></DIV></BLOCKQUOTE><FONT face=Arial color=#0000ff
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<DIV><FONT face=Arial color=#0000ff size=2></FONT><BR>Making a binary
fully-transparent/not-fully-transparent per-pixel decision to create regions
C1 and C2 seems like it can't be right in the presence of
antialiasing.<BR><BR>Suppose we have a path filled with black and operator
"copy". Any pixel on the edge of that path that gets any nonzero coverage
value from antialiasing will end up solid black with this proposal. That's
going to look very ugly. We'll want a solution where any canvas pixel which
has a very small amount of coverage by the path will be mostly unchanged in
the final result.<BR><SPAN class=118300323-03012011><FONT face=Arial
color=#0000ff size=2> </FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011>
<DIV dir=ltr align=left><SPAN class=118300323-03012011><FONT face=Arial
color=#0000ff size=2>I do not understand why pixels touched by antialiasing
are going to be solid black.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=118300323-03012011><FONT face=Arial
color=#0000ff size=2>The way I understand antialiasing (and maybe I am wrong),
pixels that are partly touched retained partly their old color and
transparency and get parly the new color and transparency. More
precisely the resulting transparency and color components an average of
the color component being painted and the previous color component weighted by
the coverage fraction of the pixel. Hence partially covered pixels are
partially transparent, thus the background behind the canvas should shine
through and the partially covered pixels won't be entirely black unless
that background is black as well. I agree with you though that there are
cases when inappropriately using globalCompositeOperation can yield ugly and
perhaps surprising results, such as in the case you described if the
canvas is completely red before the operation and it is put on a page that has
green background, thus the shape will acquire an unexpected slightly
green rim between black and red.</FONT></SPAN></DIV>
<DIV dir=ltr align=left><SPAN class=118300323-03012011><FONT face=Arial
color=#0000ff size=2>I still believe that this behavior as ugly as it may be
in some cases, is still preferable to not being able to do shadows when using
the copy operation (as results from the standard's current language), is more
intuitive and less resource intensive (to me the less resource intensive is
the most important part).</FONT></SPAN></DIV> </SPAN><BR></DIV>
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<DIV dir=ltr align=left>Version 2:<SPAN><SPAN lang=EN>
<P><FONT face=Arial color=#0000ff size=2></FONT> </P>
<P>4.8.11.1.13 Drawing model</P>
<P><FONT face=Arial color=#0000ff size=2></FONT> </P>
<P>When a shape or image is painted, user agents must follow these steps, in
the order given (or act as if they do):</P>
<P>1. Render the shape or image onto an infinite transparent black bitmap,
creating image A, as described in the previous sections. For shapes, the
current fill, stroke, and line styles must be honored, and the stroke must
itself also be subjected to the current transformation matrix.</P>
<P>2. When shadows are drawn, render the shadow from image A, using the
current shadow styles, creating image B.</P>
<P>3. When shadows are drawn, multiply the alpha component of every pixel in
B by globalAlpha.</P>
<P>4. When shadows are drawn, composite, using the current composition
operator, B with the current canvas bitmap, cliping results to the cliping
region of the canvas and to the pixels that would have taken the shadow's
color if the composition operator would have been source-over and the shadow
would have been fully opaque and the globalAlpha would have been 1.</P>
<P>5. Multiply the alpha component of every pixel in A by globalAlpha.</P>
<P>6. Composite, using the current composition operator, A with the current
canvas bitmap, cliping results to the cliping region of the canvas and to
the pixels that would have taken the shape's or image's pixel color if the
composition operator would have been source-over and the image would have
been fully opaque, the fillStyle and strokeStyle would have been a solid
fully opaque color, and the globalAlpha would have been
1</P></SPAN></SPAN></DIV></DIV></BLOCKQUOTE><FONT face=Arial color=#0000ff
size=2></FONT>
<DIV><FONT face=Arial color=#0000ff size=2></FONT><BR>Again, this needs to be
modified to take into account the possibility that some pixels are partially
covered.<SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2> </FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2>Again, as above, Porter-Duff does not allow for partially applying
the composition operator. For every pair of source and target pixels, the
operator is applied. The only question is what are the source pixels to be
paired with the target. The current language of the standard</FONT> <FONT
face=Arial color=#0000ff size=2>provides one or even two
(in case of shadows) source pixel(s) for every target pixel inside
the cliping region. This is expensive and leads to non-intuitive behaviors
such as loss shadows with some operations.</FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff size=2>My
proposal is to apply the composition operator only to pixels covered by the
source inside the cliping region, of course. The question is the definition of
covered. I chose for that the pixels that would be changed by a plain solid
brush/pen in the graphics system. The hope is that the whole trick with 2
infinite canvases in reality will be just a way of explaining how the result
should look like and that drawing a shape and compositing an image shall be
done using simple graphics primitives in-place avoiding the need for extra
memory and calculation.</FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff size=2>I
myself have no particular quarel with the current spec language other than the
fact that it looses shadows and that it is very expensive to implement with
graphics primitives that I know of, due to the need to alocate large
intermediary images to be composited later, rather then drawing directly on
the target bitmap using an appropriate pen and compositor.</FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff size=2>If
the current spec is to survive, then I suggest instead of steps 3
to 6 of the drawing model, to say:</FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff size=2>3.
</FONT></SPAN><SPAN class=118300323-03012011><A
href="http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#when-shadows-are-drawn">When
shadows are drawn</A>, composite <VAR title="">A over B in A</VAR>.
</SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=118300323-03012011>4. Multiply the alpha component of every
pixel in <VAR title="">A</VAR> by <CODE title=dom-context-2d-globalAlpha><A
href="http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-globalalpha">globalAlpha</A></CODE>.</SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=118300323-03012011>5. Composite <VAR title="">A</VAR> within
the <A
href="http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#clipping-region">clipping
region</A> over the current canvas bitmap using the current composition
operator. </DIV></SPAN>
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<DIV><FONT face=Arial color=#0000ff size=2></FONT><FONT face=Arial
color=#0000ff size=2></FONT><FONT face=Arial color=#0000ff
size=2></FONT><BR>Given that Microsoft have indicated they're happy with the
current spec, and are presumably implementing it, I think we should get their
explicit approval before we change the spec here. I'm still happy with either
the current spec or your proposed change (after the issues have been
addressed).<BR><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2> </FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff size=2>My
understanding from some older thread was that Microsoft favored the approach
where only pixels covered by the shape are composited, but that information
may be outdated.</FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011></SPAN><SPAN
class=118300323-03012011><FONT face=Arial color=#0000ff
size=2> </FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff size=2>In
any case, I wish that this aspect of the spec is firmed up so that
implementations can be aligned.</FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2>Thanks for the review,</FONT></SPAN></DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=118300323-03012011><FONT face=Arial color=#0000ff
size=2>Carol</FONT></SPAN></DIV>
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