[cairo] Extending XRender to support non-blurry edges for transformations(?)

Vladimir Vukicevic vladimir at pobox.com
Wed Jul 30 17:14:13 PDT 2008

We've had discussions about exactly this in the past; my EXTEND_CLAMP  
suggestion is essentially this.  The problem is that it breaks down in  
the most general case as was described in earlier threads... I do  
agree that the clamping/black/whatever behavior is what is wanted in  
99% of all cases, and that we should figure out a way to define it.   
Noone's been able to do so in a satisfactory manner so far, though.

    - Vlad

On Jul 30, 2008, at 4:45 PM, Bill Spitzak wrote:

> I would put it into the repeat property, maybe call it EXTEND_BLACK.
> I really believe the behavior you described is *always* wanted, and  
> that
> in fact the current default is so useless, I would just *change*
> EXTEND_NONE to do exactly this and thus remove the need for either  
> cairo
> or pixman to support the current default. But if people cannot be
> convinced of that, it should be added as a new option.
> It should look and act exactly like EXTEND_REPEAT when the clip region
> is set to the transformed quad surrounding the source picture.
> Clemens Eisserer wrote:
>> Hi,
>> I am currently implementing a Java2D-XRender backend and I am faced
>> with the problem that XRender always interpolates the edges with the
>> transparent pixels surrounding it when a scaling transformation is  
>> set
>> on the source and using billinear interpolation, as shown here:
>> http://picasaweb.google.com/linuxhippy/Transformations/photo#5226911151131301954
>> Java itself requires scaled images to have sharp edges, so in order  
>> to
>> comply with the java2d-spec I would have to fall back to software :-/
>> I've read that Cairo-1.6.4 now has support for EXTEND_REPEAT on
>> image-surfaces, which allows to work arround that problem.
>> However it still has some overhead if the user just would like to  
>> draw
>> a single transformed image -  it requires to draw the geometry of the
>> transformed image in a mask and its not supported by Xorg for now.
>> What do you think about extending XRender with support for  
>> restricting
>> filter to source picture only?
>> I guess for the drivers it should be no problem to implement it, and
>> if the driver only supports REPEAT_PAD (unfourtunatly I don't know
>> one) it still could be emulated on the server-side easily.
>> I've opened a feature-request:
>> https://bugs.freedesktop.org/show_bug.cgi?id=16847
>> It could be either done as:
>> - New attribute in PictureFormat
>> - A new Repeat property, like RepeatStop
>> - A new set of filters, like billinear_sharp
>> I would be happy to hear about suggestions and ideas.
>> Do you think this could be also benefitial for cairo?
>> Thanks, Clemens
>> _______________________________________________
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>> cairo at cairographics.org
>> http://lists.cairographics.org/mailman/listinfo/cairo
> -- 
> Bill Spitzak, Senior Software Engineer
> The Foundry, 1 Wardour Street, London, W1D 6PA, UK
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