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[whatwg] Fwd: RE: Inconsistent behaviour of globalCompositeOperation property - Drawing model discussion

From: Robert O'Callahan <robert@ocallahan.org>
Date: Wed, 5 Jan 2011 10:39:53 +1300
Message-ID: <AANLkTikUPFTz7B5A6c1fad-HPP0+nAQnh5MM-ORwNGtf@mail.gmail.com>
On Wed, Jan 5, 2011 at 8:12 AM, <carol.szabo at nokia.com> wrote:

>  Please see my in-line comments below:
>
>  ------------------------------
> Version 1:
>
>> 4.8.11.1.13 Drawing model
>>
>>
>>
>> When a shape or image is painted, user agents must follow these steps, in
>> the order given (or act as if they do):
>>
>> 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
>>
>   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.
>>
>   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.
>>
>> 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.
>>
>   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.
>>
>> 6. When shadows are drawn, render the shadow from image A, using the
>> current shadow styles, creating image B.
>>
>> 7. When shadows are drawn, multiply the alpha component of every pixel in
>> B by globalAlpha.
>>
>> 8. When shadows are drawn, composite B with the current canvas bitmap,
>> cliping results to region C2 defined above, using the current composition
>> operator.
>>
>> 9. Multiply the alpha component of every pixel in A by globalAlpha.
>>
>> 10. Composite A with the current canvas bitmap, cliping results to region
>> C1 defined above, using the current composition operator.
>>
>
> 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.
>
> 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.
>
>  I do not understand why pixels touched by antialiasing are going to be
> solid black.
>
> Yes, I made a mistake. The actual result will be mostly-transparent black,
but that is equally unacceptable.

In step 1, every pixel which is very slightly covered by the path will be
filled with mostly-transparent white.
In step 3, all such pixels will be added to C1.
In step 5, those pixels will be set to mostly-transparent black in image A.
In step 10, for those pixels we'll composite mostly-transparent black onto
the canvas with operator "copy", setting the canvas pixels to
mostly-transparent black.

The core problem is steps 3 and 4. Making a binary decision for each pixel
whether it's "in" or "out" of the shape simply can't work well  when
coverage-based antialiasing is being used.

If you generalize C1 and C2 to be alpha masks, or rephrase the approach so
that it permits such generalization, then this could work. But you have to
be careful about how you use partial alpha values in step 10.

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.
>
>
That's right, but your proposal interacts with this process.

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.
>
>
Yes, it's easy to produce ugliness. With great power comes great
responsibility :-).


>
>>  Version 2:
>>
>>
>>
>> 4.8.11.1.13 Drawing model
>>
>>
>>
>> When a shape or image is painted, user agents must follow these steps, in
>> the order given (or act as if they do):
>>
>> 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.
>>
>> 2. When shadows are drawn, render the shadow from image A, using the
>> current shadow styles, creating image B.
>>
>> 3. When shadows are drawn, multiply the alpha component of every pixel in
>> B by globalAlpha.
>>
>> 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.
>>
>> 5. Multiply the alpha component of every pixel in A by globalAlpha.
>>
>> 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
>>
>
> Again, this needs to be modified to take into account the possibility that
> some pixels are partially covered.
>
> 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.
>
>
Right. In practice implementations go beyond Porter-Duff to take some kind
of "mask alpha" into account to support coverage-based antialiasing.


> 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.
>
>
This depends on what your underlying implementation can do and what the
workload is. In practice usage of operators other than "over" is quite rare.
Copying one canvas to another is probably the main use. You can optimize the
case where the source fills the destination clip region --- in those cases,
which I think are most cases in practice, of course it doesn't matter
whether the operation is bounded by the source shape or not.

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).
>
> 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.
>
>
Yes, they seem to have changed their minds.

Rob
-- 
"Now the Bereans were of more noble character than the Thessalonians, for
they received the message with great eagerness and examined the Scriptures
every day to see if what Paul said was true." [Acts 17:11]
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