[Gimp-developer] Downscaling quality.

[Geert Jordaens]

David Gowers wrote:
> On 8/7/07, Geert Jordaens <geert.jordaens at telenet.be> wrote:
>   
>> I can't seem to find the associated bug. Does anybody know which is the
>> bug report?
>> I've got a test version (for scale-funcs.c) that scales down in reducing
>> the image 1/4 each step.
>> Between each step a the image is blurred before starting the next reduce
>> cycle.
>> The final step performs a bilinear interpolation.
>>
>> 1000 x 1000 => blur (3x3 gauss) => 500 x 500 => blur (3x3 gauss) =>
>> 250x250 => bilinear interpolation => 200x200
>>     
>
> I don't understand why you gauss-blur 3x3. won't this percolate
> roughly .3 of each pixel into it's neighbours (in terms of the scaled
> down result), and thus, isn't this an aesthetic choice rather than a
> technical one?
> Trying it out myself  it makes things look too blurry. It's great for
> simple antialiasing enhancement, but it tends to damage detail. Doing
> it without the blur IMO looks better, proportionate to the amount of
> detail in the original.
>
> BTW: the current algorithym you implemented has the following problems:
>  * Layers with alpha are treated improperly -- the resultant alpha
> channel tends to be completely wrong.
>  * Layers where width is not even, become skewed to 45 degrees after scaling.
>
> If you would replace the gauss+downscale with just downscaling, that
> would be good. Why do you use the gaussian blur?
>
>
>   
I did some reading on image pyramid's (googling) and there the blur 
action is described as a low pass filter.
Factoring the blur out is not a problem.  I also have to look to the 
non-uniform scaling as mentioned by Sven and odd size.
If those things get fixed would this be a viable solution?

Geert