[cairo] downscaling capabilities

[Bill Spitzak]

Owen Taylor wrote:
> On Sat, 2008-04-19 at 12:16 -0700, Bill Spitzak wrote:
>>> Owen Taylor a écrit :
>>>> I'm going to directly disagree here and suggest that for
>>>> CAIRO_FILTER_GOOD, the right algorithm is:
>>>>
>>>>  - Scale down by factors of 2 repeatedly until you are less than 2 times
>>>>    the target scale factor
>>>>  - Bilinearly sample from the result
>>>>
>>>> There are certainly disadvantages to this to this approach:
>>>>
>>>>  - Works worse with non-uniform scales (that contract more in one
>>>>    direction than others)
>> If you are going to do this each time it is quite possible to use a 
>> different power of 2 horizontally than vertically.
> 
> For pure scales, yes. But the transform could also contract the image
> at a 45 degree angle to the axes.

It would scale to the length of the transform of the vector (0,1) and 
the length of the transform of the vector(1,0). IE the scaled image 
would not be rotated, it would be transformed to be about the same size 
as the result, just not rotated or skewed.

I'm thinking this will work. The biggest question is whether it will 
actually be faster. As I see it the advantages are that the 
integer-scale pass and the bilinear pass are so much simpler that they 
could be programmed to be far faster.

Disadvantages are that it seems to require a temporary buffer for the 
scaled image, the fact that two passes are made over the data, and that 
it will think about pixels during the first pass that may be clipped off 
for the final image. Also it is not going to do a great job with skew, 
but most filtering acceleration fails at that anyway.

> Yep. Locality is a bit more of an issue, overflow will be an issue at
> some point (especially with mmx).

If the scale goes over some factor it could switch the algorithim to a 
new one that does not overflow. For instance in some code I am using a 
scale of less than 1/64 just uses the 1/64 filter with the centers 
spaced further apart than 64 (ie it loses a lot of the pixels). At this 
scale it is so tiny that this seems acceptable.

  I don't know offhand how a scale
> down of 2.7 times looks with bilinear interpolation from a 
> 2-times downscaled copy vs. a 3-time downscaled copy... if there is
> a significant improvement or not.

I'm pretty certain you want the nearer-to-1 integer scale. The current 
bilinear matches this for scales down to .5, and I don't think the 
artifacts become objectionable until .5.