What are the main things that decreases the quality in GIMP?

Question

I am relatively new to using GIMP.

A few days ago I learned that rotating an image might downgrade the quality because of the anti-aliasing, and that surprised me, for I thought rotating an image is harmless and I have been using it a lot.

and my questions are:

1. What tools, techniques, filters or effects that may seem safe for a beginner but in reality lower the quality of the image when used wrong?

In my work I use a lot of effects (curves, hue-brightness, contrast...) also rotations, cropping, blurring etc

2. Will it help If I do these operations while using GIMP's native XCF format?

3 - Does changing the blending mode hurt the quality?

Answer 1

All edits which are destructive (except for layer masking which is non-destructive) can potentially impact image quality in GIMP. By destructive I mean any physical changes to actual pixels. Obviously it would depend on what exactly you are doing.

However, fear not. If you work on a copy of the original image, then it doesn't matter what you do to it. You will always have the original image file to fall back on. Working on a copy will also help to prevent you from accidentally overwriting the original image file.

XCF is GIMP's native file format. Use this format if you want to save your work so you can edit it at some point in the future. The format is lossless, and preserves layer masks, layer groups, layers, and editable text layers for future editing.

Answer 2

1) Dangerous techniques and tools

Off the top of my head:

Color tools

All the "Color" tools (Curves, Levels, Contrast) entail some color loss (which translates into visible banding in the bad cases)(this not Gimp's fault, this is math). This can be mitigated by:

• using high precision images in Gimp 2.10
• avoiding accumulating changes. Ideally everything can be done with one single application of Curves

Transform tools

All the "Transform" tools (except the trivial flips and N*90° rotations) require pixel interpolation that adds some amount blur. This can be mitigated by:

• doing everything in one single operation (see the Unified transform tool in Gimp 2.10)
• working with paths and rendering the path (stroke/fill) once the transform has been applied (especially if working with text)

Selection tools

Fuzzy select and Color select do "binary" selection (a pixel is fully selected or not at all). Used alone without care, they are the biggest enemies of smooth edges in CGI because they remove the anti-aliasing pixels. See Colors>Color-to-alpha and Color erase modes in paint tools and layers.

2) XCF format

XCF is a storage format, but when it memory the image is the same if loaded from XCF or PNG or GIF... (though GIF is color-indexed, and editing GIF should not be attempted by untrained personnel). XCF helps maintaining quality because

1. It is a lossless format, no pixels are altered when saving
2. If you use high-precision images in 2.10, the XCF keeps the high precision data, there is no conversion to 8-bit as you would have if saving to JPG or PNG (some more recent format also support 16-bit data)
3. It saves everything (layers, channels, paths...) so you don't have to redo things (that would rarely be redone with as much care...)

3) Blend mode

You use a blend mode because you need it. This said, in low-precision images piling layers with various blend modes can lead to round-off errors and artifacts such as banding. Again, using high-precision images reduces that risk.