Gimp

Basic actions

• Create squares, ellipses, circles
  • This is done by stroking paths. Eg. to create a rectangle:
    1. If needed, create a new layer.
    2. Create a rectangle using the Rectangle Select Tool.
    3. Transform selection into path with Select → To Path.
    4. Stroke the path with Edit → Stroke Path.
  • Instead of stroking the path, the selection can be stroked as well with Edit → Stroke Selection. 3rdd step above can be skipped in that case.
  • The GIMP keeps track of all used path, so converting to path is handy to reuse previous paths. Check the dialog box Layers, Channels, Paths, Undos.

Plugins and scripts

Fourier plugin

• Download the fourier plugin.
• Install a few packages:

sudo apt-get install libfftw3-dev libgimp2.0-dev

• Build and install the plugin:

make
make install

Descreen scriptfu

• Download the Descreen script.
• Copy the script into ~/.gimp-2.8/scripts.
• Restart gimp or select Scriptfu → refresh scripts. The script appears in Enhance → Descreen.

resynthesizer

Install the plugin with:

sudo apt-get install gimp-plugin-registry

Smart Separate Sharpen

• Download [Smart Separate Sharpen http://registry.gimp.org/node/24876]
• Copy the script into ~/.gimp-2.8/scripts.

How-To

Remove Moiré effect on a photo scan

Method 1 - blur and sharpen

• Reference: [1]
• Apply Filter → Blur → Gaussian Blur effect.

• Pick method RLE, and adapt radius until the moiré disappear (5 or 4 px, possibly more).

• Apply Filters → Eg → smart separate sharpen (redux) effect

• Leave options as is.

• Scale down the photo (like a factor 2), using Cubic algorithm.

Descreen plugin

• Reference: [2] [3]
• But this produces some strange and ugly artifacts (like jpeg parasites).