[Webkit-unassigned] [Bug 120346] [cairo] ShadowBlur is extremely slow when canvas's scale is very small.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=120346


Noam Rosenthal <noam at webkit.org> changed:

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--- Comment #12 from Noam Rosenthal <noam at webkit.org>  2013-10-16 09:10:05 PST ---
(From update of attachment 211396)
View in context: https://bugs.webkit.org/attachment.cgi?id=211396&action=review

This is a good catch, but the patch is hard to understand and needs to be a lot clearer.

> Source/WebCore/ChangeLog:11
> +        ShadowBlur drawing is too slow if scale of canvas is set to very small value such as 0.01.
> +        The main reason is blurRadius gets huge value if scale is small.
> +        If shadow value is 100 with 0.01 scale value, blurRadius will be 10000.
> +        Almost time is consumed to make blurred-layer.

This ChangeLog is written with poor English. Please improve.
Also though you're describing the problem, you're not at all describing the solution.

> Source/WebCore/platform/graphics/ShadowBlur.cpp:402
> +    const AffineTransform transform = context->getCTM();

No need for const

> Source/WebCore/platform/graphics/ShadowBlur.cpp:403
> +    if (m_shadowsIgnoreTransforms && !transform.isIdentity() && transform.xScale() < 1 && transform.yScale() < 1)

We should exit early on m_shadowsIgnoreTransforms before we get the CTM from the context.

> Source/WebCore/platform/graphics/ShadowBlur.cpp:420
> +        if (scaledBlurLayer)
> +            transform.scale(1 / transform.xScale(), 1 / transform.yScale());

What does this do? maybe a comment is in place? at least the changelog should describe this.

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