Gimp: Sharpness lost with perspective edit

I brought a JPEG of a door into the Gimp, v 2.10 on Ubuntu 18.04, and it was quite sharp.
Applied the perspective shift - wire cube - tool to make its frame vertical and parallel, and it was blurred.

Accepted the slight tapering on it and sharpened it and all is fine, but I’m wondering why it blurred on adjustment of perspective.

If the perspective adjust was a lot, its going to get soft. Hard to tell without a screenshot or something.

It blurred because you are moving pixels around, stretching some while eliminating others. That means interpolation is at play. Interpolation is usually a trade off between softness and artifacts. A softer algorithm is better because it leaves the user more options to sharpen the image.

Depends a bit on if your shrink or if you stretch. When you stretch you have obviously more pixels and these are mere interpolations of existing pixels. When you shrink you also interpolate and create some blur but this is less noticeable.

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That makes sense. I’ll take to squishing one end rather than stretching the other and see how it goes.