[olaulaja]

Sounds like using the wrong metric, the upscaled image should be compared against the original full resolution one, not the downscaled one. Obviously you can't know what the full resolution one looks like when actually upscaling (vs testing), but you can make an educated guess.

[willis936]

You can remove the guesswork. You can start with a high resolution (or even raster export of a vector image for an extreme example), downsample it (various methods and downsampling factors for completeness), then attempt to upscale it.

[OJFord]

That's exactly what GP is talking about, the 'guesswork' comes in when you upscale as a function of only the downsampled version.