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Re: 256x256 px icons


On 8/15/2011 10:33 AM, Warren Young wrote:
> The hard edges in the original art are causing stair-stepping when doing
> a direct downsample, though.  (Look at the pointy bits.)  By blurring
> the high-res version and then downsampling by a non-integral amount, you
> can get a much smoother result.

This is only the case if the "downsample" operation used by GIMP, when
d/s by an integral amount, is to simply pick every Nth pixel.  That's
very fast -- but is not the correct operation (I'd posit a GIMP bug, in
fact).

Sampling theory says a downsample SHOULD be preceded, automatically, by
a low-pass filter (blurring) operation of a specific type and, er,
"radius" for lack of a better word.  (IOW, GIMP /should/ be doing this
blur FOR you, automatically).  There's lots of theory behind this, to
select the proper kind of filter (gaussian is not correct -- but is
probably a good enough approximation) and its 'radius' (which should
scale with the downsampling factor).

Since GIMP is apparently not doing that, then yes -- you need to apply a
blurring filter yourself, before using GIMP's braindead 'pick every Nth
pixel' version of "downsampling".

--
Chuck


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