[whatwg] I believe source rectangles for HTML5 Canvas drawImage are specified incorrectly

[Justin Novosad]

On Sun, Dec 16, 2012 at 11:52 PM, Rik Cabanier <cabanier at gmail.com> wrote:

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> On Wed, Dec 12, 2012 at 10:24 AM, Justin Novosad <junov at chromium.org>wrote:
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>> On Wed, Dec 12, 2012 at 12:39 PM, Rik Cabanier <cabanier at gmail.com>wrote:
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>>> What would be the next step? Should we define a new version of drawImage
>>> with the extra parameter?
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>> That is one option, but I think a context attribute (like
>> imageSmoothingEnabled) is also worth considering.
>> Perhaps image smoothing could be an enum rather than boolean with an
>> additional mode that prevents color bleeding.
>>
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> It seems a bit too expensive to add a variable to the graphics context
> that is checked for just this call.
> Maybe just a new drawImage call (like drawNonSmoothedImage?) is all that
> is needed.
>

Yes. That sounds quite reasonable to me, but we can find a better name. The
name "drawNonSmoothedImage" suggests that the image won't be smoothed at
all, which is not the case.  It's hard to find a name that correctly
describes the right behavior without getting too technical.  I am thinking
"drawSubImage", in the sense that the sub region delimited by the source
rectangle is treated as if it were a whole image.  This gives me another
idea: we could just have a new Image constructor that creates a new image
element that is a subregion of another:
var mySprite = new Image(spriteMap, x, y, w, h);
This can be implemented in a lightweight way that just references the data
of the source image.


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>>> If so, we probably want the description of the existing drawImage call
>>> to change so it leaves the resampling issue undefined unless WebKit is OK
>>> with changing their current behavior.
>>>
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>> IMHO: Undifined behavior is a spec bug. If we have a problem with the
>> spec, we fix the spec, we don't just each do our own thing.
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