[cairo] Concerns about using filters for downscaling

[Søren Sandmann]

Owen Taylor <otaylor at redhat.com> writes:

> On Fri, 2014-03-21 at 18:33 -0400, Owen Taylor wrote:
>
>> * For some reason, the results of downscaling images don't have
>>   left-right symmetry. The result of scaling down:
>> 
>>   000000 000000 000000 000000 
>>   000000 ffffff ffffff 000000 
>>   000000 ffffff ffffff 000000 
>>   000000 000000 000000 000000 
>> 
>>   By a factor of two is:
>>  
>>    646464 3c3c3c 
>>    3c3c3c 242424 
>> 
>>   (I'm a bit suspicious about the fact that pixman generates
>>   even length filters, and wonder if that has something to do
>>   with the asymmetry.)
>
> What's going on here is:
>
>  +-------+-------+ Destination image
>      *       *     Sampling points
>  +---+---+---+---+ Original image
>   * * * * * * * *  Subpixel filter points
>
> So each sampling point is between two subpixel filter points, and the
> picking one of them the asymmetry. When pixman has N bits of subpixel
> filters, it picks the points as:
>
>  0.5/2^N, 1.5/2^N ... (2^N-0.5)/2^N
>
> To give an equal distribution in the unit interval. I think that's
> actually wrong and they should be at:
>  
>  0, 1/2^N ... (N-1)/2^N
>
> It shouldn't make a difference when the scale/offset of the filter
> is arbitrary, but means that you avoid the half-way-between rounding
> for exact integer scale-downs.

I think I agree with this. It will just move the artefacts so that they
now happen at different offsets, but it is probably good to make sure
that the exact-integer scale-downs get an exact sampling of the filter,
and I can't think of any benefits from aligning the subsample grid the
way pixman does now.


Søren