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
