How much size upscale or size downscale before pixelation or degradation of image?

[Tetragram]

Hey guys, I have been reading around this subject, how much size upscale and size downscale can be done before significant pixelation and significant degradation of image, and haven't found many correlative answers, other than every software behaves different.

Specifically I'm thinking of an image with this qualities: - 23 megapixels camera resolution and 5520x4144 pixels .jpg - treated in Gimp and rendered in .png with alpha layers. After all the reading I did I settled on nohalo for upscale, and lohalo for downscaling. Sometimes I keep nohalo for downascaling as well. 

I'm transforming my images to 32 bit floating point precision with max resolution (1.000.000 ppi) in Gimp when first opened.

But the heart of the question is, how much upscale and downscale generally can be done to an image before it degrades or pixelates in Gimp?

[Blighty]

(01-17-2019, 01:14 AM)Tetragram Wrote: But the heart of the question is, how much upscale and downscale generally can be done to an image before it degrades or pixelates in Gimp?

Any upscaling or downscaling in any bitmap editor will degrade the image.

The real question is: When will the degrading become visible and unacceptable?
That will depend on the image itself. And it will also depend on what the image will be used for (eg viewing distance).

[Ofnuts]

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.

[Tetragram]

(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.