The Filter Menu
Filters allow you to modify an image depending
on it's contents. For example using filters you can blur images or
sharpen them. There are two main types of filters 'User filters' which
can be set up and modified at will, and the filters which are built
into the program. The filter menu is shown to the right. If there is a
selected area, the filter will apply just to that, otherwise it will
apply to the entire image.
Built in filters
Invert
This replaces pixels in the image by their inverse. Black becomes
white, colours are replaced by their complements.
Histogram
This is slightly more than just a filter. When
you select it, a histogram of the image will be calculated. This shows
the number of pixels in each of a number of intensity levels or bands
(16 by default). If the image is a 24 or 32 bit image then a much
larger value may be used, e.g. 64 or 128 levels.
The Red, Green and Blue buttons can be used to choose which components are displayed. These buttons operate in a slightly different way to usual. Clicking with Select will select just one of them, more than one can be selected by clicking with Adjust. The same principle usually applies to multiple selections e.g. files.
The 'Equalise' button will attempt to spread out the pixels in the image so that there are equal intensities in each band.
Under the histogram, are two arrows which can be dragged from side to side. When Expand is clicked on, the range of intensities between the arrows will be expanded to fill the full range.
Expand and Equalise are crude tools. They may be of use with images where all the pixel intensities have been squashed into a narrow band. Finally the 'Recalc' button can be used to recalculate the histogram if the image is changed, or to use a different number of levels.
Max
This filter replaces each pixel by the maximum of itself and the surrounding 8 pixels. The maximum of 1,2,3,4,5 is 5.
Min
This filter replaces each pixel by the minimum of itself and the surrounding 8 pixels. The minimum of 1,2,3,4,5 is 1.
Median
This filter replaces each pixel by the median of itself and the surrounding 8 pixels. The median of a set of values is the value which splits the set into two equal parts. For example the median of 1,2,3,4,5 is 3. This filter is useful for removing noise from an image.
Average
This filter replaces each pixel by the average of itself and the surrounding 8 pixels. For example the average of 1,2,3,4,5 is (1+2+3+4+5)/5=3.
Add Noise
This adds random noise to the image. The amount
of noise is controlled by the slider and writable icon at the top of
the dialogue box. The button 'White noise' controls if equal amounts
of noise will be added to the colour components of the image. If
'White noise' is selected, the image will retain its colour. If it is
not selected the image will tend to develop coloured dots. Noise may
be useful in making images look more life like. For example images
which have been processed into a small number of brightness levels. By
adding noise the obvious levels can be smoothed out slightly.