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
• Copy the script into ~/.gimp-2.8/scripts.

How-To

Remove Moiré effect on a photo scan

Method 1 - blur and sharpen

• Reference: [1]
• Parameters below are for a 600 dpi scan.
• Apply Filter → Blur → Gaussian Blur effect, method RLE, radius 5px.
• (USELESS - HAS NO EFFECT) Apply Filers → Eg → smart separate sharpen (redux) (default options)
• Apply Image → Scale Image → 35% and Cubic.

Method 2 - Descreen plugin

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

Make a polygon selection

• Select the lasso tool.
• Click instead of dragging to make polygon edges.

Pick a color on screen

Use the color selection dialog (or dock) [4]:

• Click the button with the pipet and screen icon.
• Click anywhere on screen
• Color is set.