The GIMP logo, by Jakub Steiner, licensed under Creative Commons by-sa 3.0

Quick GIMP tricks

2 minutes read, 438 words
Categories: editing
Keywords: editing gimp image manipulation jpegoptim optipng

I’m not a daily GIMP user. In fact, I actually nearly never edit images.

From time to time. I crop, rotate or correct red eyes.

Nonetheless every time I sue GIMP I need to research for those functionalities.

While GIMP can do all those operation and much more complex actions, it makes doing simple things a bit more difficult. Of course using a photo-editing suite just for rotating an image is an overkill, but as it is multi-platform an easily available, I prefer it to installing many different graphical programs for those rare times I need to edit an image.

Crop image

From the keyboard, it is possible to select the crop tool with Shift+c. Otherwise (depending on the language) select "Tools", then "Transform Tools" and at least "Crop".

Once the tool has been selected, use the mouse for selecting the area to crop.

crop layer option
Figure 1. GIMP crop layer option

Rotate Image

This functionality can be found under "Layer" (not "Tools"), then "Transform" and the select "Rotate 90" or "Rotate 180" degrees.

While it does rotate the image, it does not adjust it height and width so it does not what a "naive person" would expect.

GIMP differentiates between the image and canvas.

To fix this "issue", click "Image", then "Fit Canvas to Layers".

If you want to to rotate by something different by a multiple of 90 degrees, then there is a "Rotate" under "Tools" and then "Transform Tools".

It is possible to automate the "Fit Canvas to Layers" step.

After selecting the "Rotate" Tool, on the tool options (left bar), it is possible to specify if the tool should act on the layer, on a selected area, on a path, or on the image.

rotate image option
Figure 2. GIMP rotate image option

Choosing on the image will automatically execute "Fit Canvas to Layers".

The option is sticky, so it will persist between GIMP sessions, but notice that it applies only for the "Rotate" tool not for the option under the layout menu.

Reduce image quality

This is actually an operation I do pretty often. As opening an image with GIMP just for exporting it with a different quality is really tedious, for those operation I can only recommend a CLI tool.

In particular, consider using jpegoptim (for .jpg files) and optipng (for .png files).

Both have a much simpler interface, and most important support lossless optimizations. It means they can reduce the image size without affecting the quality (yes, even for .jpg).

In case the gain is not big enough, both also support lossy optimizations.

Otherwise, when exporting (not saving) images with GIMP, it is possible to define an export quality.