[cairo] Tentative patch for downscaling in Cairo

Chris Wilson chris at chris-wilson.co.uk
Thu Sep 5 08:17:22 PDT 2013

On Sun, Mar 31, 2013 at 05:03:36AM +0200, Krzysztof Kosiński wrote:
> Hello
> I had a look at how complex would it be to add correct downscaling to
> Cairo now that Pixman supports convolution filters. It turns out it
> this is rather easy. Here is an initial, minimal attempt. It uses
> convolution filters only if the image is being downscaled by more than
> half a pixel in at least one dimension.
> Some discussion:
> 1. The sampling and reconstruction kernels are picked in a way that
> gives comparable quality when upscaling and downscaling. I paired box
> sampling with bilinear reconstruction and impulse (point) sampling
> with box reconstruction. This gives the expected result for NEAREST
> filter. BEST filter uses Lanczos3 for both kernels.
> 2. Subsampling bits are always set to 1, since this doesn't seem to
> affect quality at all.
> 3. I am not sure whether this code works correctly for matrices with a
> skew component. It should be OK for any combination of scale, rotation
> and translation.
> 4. This patch causes new failures in the test suite:
> - recording-surface*: possibly an effect of improved quality.
> - surface-pattern-scale-down*, surface-pattern-big-scale-down: the
> reference images should be updated.
> - pthread-same-source: I have no idea why this is failing, since this
> test shouldn't even trigger the new code.
> - large-source-roi: this test attempts to downscale an image which is
> 30000 pixels wide down to 7 pixels. The filter parameters seem to be
> created correctly, but they might trigger an overflow somewhere in the
> convolution code; the output rectangle is white instead of red, as if
> nothing was drawn.
> - device-offset-scale: there are subtle differences which look like
> convolution-related smoothing; I'm not sure whether this is OK or not.
> Note that master currently fails many more tests, I only listed new
> failures cause by this patch.

Merged to cairo-1.13, many thanks.

Chris Wilson, Intel Open Source Technology Centre

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