User Guide

GNU Image Manipulation Program
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Figure 15.231: Blurring with a square gradient map
The gradient map is divided into four gradient triangles: each of them has its own gradient direction. In every area of the image
corresponding to gradient triangles, only lines with the same direction as gradient are blurred.
Figure 15.232: Texture example
The With white noise option is checked. Others are default. With a vertical gradient map, texture fibres are going horizontally.
Effect Channel By selecting Hue, Saturation or Brightness (=Value), filter will use this channel to treat image
Effect Operator The ‘Derivative’ option reverses ‘Gradient ’ direction:
Figure 15.233: Derivative option example
Using a square gradient map, Effect operator is on Gradient on the left, on Derivative on the right: what was sharp is blurred and
conversely.
Filter Length When applying blur, this option controls how important blur is. When creating a texture, it controls how rough
texture is: low values result in smooth surface; high values in rough surface.
Figure 15.234: Action example of Filter Length on blur
On the left: a vertical line, one pixel wide (zoom 800%). On the right: the same line, after applying a vertical blur with a Filter
Length to 3. You can see that blur width is 6 pixels, 3 pixels on both sides.