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why is default scaling algorithm in GIMP so bad??

Posted by notKlaatu , 19 May 2009 · 741 views

I was making a little tutorial on a really cool audio app called Qtractor the other day, and for this I was doing quite a few screenshots. Some of the screenshots were pretty big so I went into GIMP to scale them down. To my surprise, the images looked terrible when scaled down. I understand about interpolation and I understand that an image in anything but its original form is technically compromised, but the text that looked great on my real screen looked embarrassingly bad in the scaled-down screenshot, and I'd only scaled it down by 10 or 15 percent. Something was definitely wrong.So I opened the original image in Krita and scaled it down to the same size...and it looked great. Looked like it hadn't been scaled down at all.So then I tried scaling the original image down with ImageMagick's Mogrify. This was better than GIMP but not as good as Krita.So, back to GIMP, and I messed around with the default scaling algorithm being used (to be fair, I could probably do this with ImageMagick as well -- there's surely an option to change the algorithm). Turns out that the default for GIMP is Linear Interpolation, otherwise known as /p/tarded interpolation, and looks terrible. Why this is the default, I cannot say; maybe it really does have its uses for pictures done, say, in 8-bit graphics.... Anyway, I changed it to "Sinc (Lanczos3)" and tried my experiment again. Miserably failed again -- sort of. For some reason, even though I'd just set the default Interpolation to Sinc (Lanczos3), the Scale dialog box was set to use Linear and I still had to manually change the algorithm being used. It seems to retain that setting, though, and I have not had to re-set the interpolation since.So, if you're using GIMP, set your Interpolation to Sinc (Lanczos3) for best results. Frankly it's still not as good as Krita's results, but it's certainly better than the default.I'm attaching screenshots of each example, with the exception of ImageMagick because I am pretty sure with some more complex commands (like quality control and such) I could get ImageMagick's results to be as good as GIMP+Sinc or maybe even Krita.

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