When scale up using mitchell, I can seem obvious artifact on image, not sure this is as expect or a bug
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Might be a bug. Can you list the steps to reproduce with the example image so that we can report this
Drag this test image into krita layer, then scale up the image using Mitchell (setting in the tool options)
For me there is very tiny 4px X 4px image, is this the correct image?
Mitchell shows distinct artefacts not related to feature boundaries in the original.
For 4 x 4 scaled up to 1000 x 1000. Lanzos3 - Nearest Neighbour - Mitchell:
I used the 5.1.5 appimage for that.
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Yes，use this 4x4 image to test the Krita scale, You can use XnConvert or other program as a compare
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Aah okay 
I thought the image got scaled down or you posted a smaller thumbnail version. It is very tiny to click may be put a link to the image.
In any case @AhabGreybeard tested it and I have tested the image too and I think you are right. Krita adds artefacts. We should report this bug
I tested the image with imagemagick using Mitchell filter and got this
This is the same image scaled up in krita with mitchell filter
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When I try to report bug, I found this: 151408 – Mitchell scaler shows tiling artifacts (kde.org)
Seems like this is a regression.
I report the bug here: 475609 – Mitchell scale up image cause artifact on image (kde.org)
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