[whatwg] effect of smoothing on drawImage (was: Bicubic filtering on context.drawImage)

Rik Cabanier cabanier at gmail.com
Sat Mar 22 21:09:48 PDT 2014

On Fri, Mar 21, 2014 at 10:47 PM, K. Gadd <kg at luminance.org> wrote:

> Hi, the attached screenshots and test case in
> https://bugzilla.mozilla.org/show_bug.cgi?id=782054 demonstrate how
> the issue affects 2D games that perform scaling/rotation of bitmaps.
> There are other scenarios I probably haven't considered as well. As
> far as I can tell the mechanism used to render these quads is
> rendering quads that are slightly too large (probably for coverage
> purposes or to handle subpixel coordinates?) which results in
> effectively drawing a rectangle larger than the input rectangle, so
> you sample a bit outside of it and get noise when texture atlases are
> in use.
> Interestingly, I raised this on the list previously and it was pointed
> out that Chrome's previous ('correct' for that test case) behavior was
> actually incorrect, so it was changed. If I remember correctly there
> are good reasons for this behavior when bilinear filtering is enabled,
> but it's quite unexpected to basically get 'antialiasing'
> on the edges of your bitmaps when filtering is explicitly disabled.
> Getting opted into a different filter than the filter you expect could
> probably be similarly problematic but I don't know of any direct
> examples other than the gradient fill one.

ah. I remember looking at your test case.
I made a simpler version that shows the issue:
According to the spec, there should be a faint line [1]:

If the original image data is a bitmap image, the value painted at a point
in the destination rectangle is computed by filtering the original image
data. The user agent may use any filtering algorithm (for example bilinear
interpolation or nearest-neighbor). When the filtering algorithm requires a
pixel value from outside the original image data, it must instead use the
value from the nearest edge pixel. (That is, the filter uses
'clamp-to-edge' behavior.) When the filtering algorithm requires a pixel
value from outside the source rectangle but inside the original image
data, *then
the value from the original image data must be used.*

You were told correctly that the Chrome behavior is incorrect. When doing
smoothing, Chrome is not looking outside the bounds of the source image, so
you don't get the faint line.
This is also an issue with the cairo and core graphics backends of Firefox.
Safari and IE seem to work correctly.

I will log bugs against Chrome and Firefox so we can get interoperable
behavior here.

I was not able to reproduce the issue of smoothing was disabled. If you
want smoothing but not the lines, you can do a drawimage to an intermediate
canvas with the same resolution as the source canvas. I verified that this


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