01-17-2019, 12:09 PM
(01-17-2019, 09:37 AM)Ofnuts Wrote: In theory, as soon as you rescale the picture, in any direction and in any factor, you interpolate pixels and this blurs the picture, how acceptable this is depends on the picture, on the medium, and on the eyesight of the viewer.
When downscaling, thing can be kept quite clean if you:
- pre-blur the picture before downscaling (if downscaling by a factor N, Gaussian blur by N pixels: 50%->2px)
- apply some sharpening (Unsharp mask or else) once this is done
Up-scaling is another matter. What really counts is the true definition (pixels in the original image per inch of output): if you have a 300px image (1"x1" at 300 Pixels Per Inch (aka PPI)) and you upscale it to 600px to print it as a 2"x2" image (still at 300PPI for the printer), your image has actually got a definition of 150PPI, because you are just spreading pixels... If you look at it from high above, the only question is to know who/what does the upscaling, you in Gimp or the print/display driver...
With this in mind:
- in print, text and sharp images should be 300PPI(*)
- on a regular screen, you can get away with 100-150PPI
(*) For text, things are even worse: your display is around 120 PPI, but each pixel is really made of thee color bands so the horizontal definition is around 360PPI if you accept some color blur. Most OS take advantage of this when displaying text (this is called sub-pixel rendering) but this requires to know where you text falls with a sub-pixel accuracy, something which cannot be done for a scaled image.
Thanks guys for the clear answers with examples. This surely helps me understand how and when pixelating and blur occurs, and how to keep it in check. It's of significant help.