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

Justin Novosad junov at chromium.org
Mon Sep 10 14:28:18 PDT 2012

On Mon, Sep 10, 2012 at 4:49 PM, Jeff Muizelaar <jmuizelaar at mozilla.com>wrote:

> On 2012-09-10, at 3:43 PM, Vladimir Vukicevic wrote:
> FWIW, there are also negative performance implications to clamping samples
> to the source rect. Many graphics APIs do not support this kind sampling,
> and supporting this behaviour on top of those apis requires a temporary
> copy of the subimage to be made.
You are referring to the fact that clamping modes only apply to texture
borders in OpenGL and DirectX?  That is not that big of a big deal, you can
implement the clamp in a shader.
I think Vladimir makes a good point that there are valid use cases for both
ways. To summarize the issues:
* clamping to source rect can cause filtering artefacts at boundaries,
especially visible when tiling.
* clamping to source image bounds can cause color bleeding artefacts when
rendering a sprite from a spritemap.

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