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### 5.2 Filter

This menu groups operations which filter images, or which are filters in the photoshop sense.

Convolution
  This menu has several standard convolution operations (blur, sharpen, edge detect, etc.), plus the option to convolve with a custom kernel.

  Two menu items are slightly more complicated. Unsharp Mask transforms to CIE LAB colour space, then sharpens just the L band with a cored unsharp filter. The Tasks / Print menu has a version of this filter tuned for typical inkjet printers.

  Custom Blur builds and applies a square or gaussian convolution kernel for you based on a radius setting.

Rank
  A preset median filter, and a custom rank filter that lets you specify window size and rank.

  The Image Rank item does pixel-wise ranking of a set of images.

Morphology
  These menu items implement basic morphological operations. Images are zero for background and non-zero (usually 255) for object. Matricies are shown as 0, 1 and * for background, object and don’t-care.

  The Threshold item does a simple level threshold. Use the Math / Relational menu to construct more complex image binarisations. Use Math / Boolean to combine morphologies.

  The first half of the menu lists simple erode and dilate operations, 4- and 8-way connected. The second half contains several useful compound filters.

  See also Histogram / Find Profile for something that can search an image for object edges. And MathStatistics / Edges can count the number of edges across and down an image.

Fourier
  A selection of ideal, Gaussian and Butterworth Fourier space filters.

  You can make other mask shapes yourself using the Image / Make Patterns menus, then apply them using Math / Fourier. You can also use the image paintbox to directly paint out peaks in a fourier-space image before transforming back to real space.

Enhance
  A selection of simple image enhancement filters. Statistical Difference passes a window over an image and tries to match the region statistics at each point to a target mean and deviation.
Spatial Correlation
  Place a small image at every possible position in a big image and calculate the correlation at each position. Simple Difference is the much faster unnormalised version.
GREYCstoration
  VIPS includes a copy of the CImg library and you can use two useful CImg operations from this menu: denoising and enlarging.
Tilt Brightness
  A selection of tools for adjusting the brightness of an image across it’s surface. Useful for correcting lighting problems.
Blend
  Blend two objects together using either a third object to control the blend at each point, or a slider to set all points together. You can blend almost anything with anything.

  One useful version is to use a text image (see Image / Make Patterns / Text) to blend between two colours (see Colour / New).

  Along Line does a left/right or top/bottom fade between two images.

Overlay
  Make a colour overlay of two monochrome images. Useful with Image / Transform / for testing image superposition.
Colourize
  Use a colour image to tint a monochrome image. Useful in conjunction with Image / Transform / .
Browse
  Look at either the bits or the bands of an image.
Photographic Negative and friends
  A small selection of simple, faintly photoshop-style filters.

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