GIMP Tips

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GIMP is a free and open source image manipulation software. I only use a small subset of functions of the software. Here are some tips I keep a record for myself.

Move Pixels In An Area

Ctrl + Alt + Move Selection is the shortcut to move selected area of an image. GIMP adds a “Floating selection” temporary layer in the layer dialog. After you make the change, click the mouse in another area to confirm the move. The temporary layer merges onto the main layer.

Add White Background After Deleting An Area

Sometimes GIMP leaves a hole on an image when you delete the pixels in an area. Choose menu item Layer > Transparency > Remove Alpha Channel and the area will show white background.

Remove Background Grey Hue

Some scanned image file has a grey hue on the background. The Colors > Curves tool can be used to remove the background.

gimp curve

Resize Image to a Certain Size

If you want to resize a scanned plan image file to a certain size such as Arch D (36” x 24”), you can follow those steps (revised on 10/30/2020).

  1. Select an area on the image and use Image > Crop to Selection to crop to the selected area.
  2. Choose menu item Image > Scale Image, and lock the Width and Height ratio on the dialog box, and enter the width or height to desired width (e.g., 36”) or desired height. If you enter the width 36” in this step, the height should be less than 24”.
  3. Click menu item Image > Canvas Size, and unlock the ratio. Resize the canvas size to the final size such as 36” x 24”. Also click the Center button on the dialog box.
  4. The image will have some transparent areas on the top and bottom, or on the left and right. On the layer panel, add a new layer and choose Layer Fill Type value White. Put this new layer below the existing layer and merge the existing layer down.

Reduce File Size

When you have a grayscale image saved in RGB (color) mode, the file size would be higher than in Grayscale mode. The size of exported PNG file is reduced to 4.02 mb from 9.49 mb when the mode is changed from RGB to Grayscale. You can reduce the file size further by saving it in Indexed mode with 16 or 64 index values. You can change the mode via menu item Image > mode.

JPG File Size

I often need to mark up a photo/picture JPG file in Inkscape such as adding texts and drawing lines. Inkscape can’t export a JPG file directly. Sometime the exported PNG file from Inkscape has a much larger file size (4mb to 20mb).

GIMP can open the SVG file directly, and export to a JPG file. The JGP from GIMP could have a smaller size. The file size reduced from about 4mb to 2mb.

This article will be continuously updated (last update: 4/2/2021).