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5.2 Filter

This menus 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.

`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. They operate on square images only whose sides are a power of two.

  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.

`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`
  A small series of faintly photoshop-style filters.

Next: 5.3 Format Up: 5. Image processing menus Previous: 5.1 Colour Contents