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Reduce Palette Colours

The Reduce Palette Colours function is available on the Image menu.

Reduce Palette Colours dialog

There are a few reasons for which this option is useful...

  1. If your image is very large it may exceed the amount of memory in your PC during processing. The image may have been scanned as full colour 24 or 32 bit depth, in which case 3 bytes are used for every pixel. For example, if your image is 20000 pixels wide by 10000 pixels high, then you need 600 Mb of memory just to load the image. However, if the image is reduced to 256 colours (8 bit depth) then only 1 byte is required per pixel. So the same dimension image would required only 200 Mb of memory to load. If reduced to 16 colours (4 bit depth) the same image would need only 100 Mb of memory. And if reduced to 2 colours (1 bit depth) that image would required only 25 Mb of memory to load.

  2. Often the image will have many more distinct colours than are actually needed. For example, you may have a drawing which contains items in only 6 different colours, but it was scanned in 256 colour mode, and therefore contains 256 subtle shades of your 6 colours. In some cases it is undesirable to have these small variations of the pixel colours, and what you actually want is for each item to be a uniform colour. This can aid other processing functions like colour thresholding.

  3. You want to make a pop art picture from a photo. (This has nothing to do with raster to vector conversion, but it's a nice effect. Simply load a photo and convert to about 30 colours!)

On the left side of this dialog, is the information about your image before processing. It shows the colour bit depth (number of bits per pixel) and the amount of RAM that is required to hold the image in memory.

On the right side of the dialog are the choices for reducing the number of colours. You can pick one of the standard colour depths (1 bit, 4 bit or 8 bit), or you can enter any number of required colours in the box at the bottom. In each case the amount of memory that would be required for the choice is shown.

When you press the OK button WinTopo Pro will examine the colours of every pixel on the image and compile a palette comprising only the number of colours which you have specified. These colours are optimally chosen for the image. Then each pixel as assigned one of these colours to match as closely as possible its original colour.

There is no preview for this option because the result is different when applied to only a small part of the image - the final palette can only be chosen by examining every pixel on the image. Remember, you can use the Undo function to re-try the option with a different choice.

Increasing Image Colours

This facility can also be used to increase the colour palette size (and the image bit depth), although you won't see any visible difference.

This can be useful for converting from a paletted (less than 24 bit depth) to a full colour (24 bit depth) image. You may need to do this so that you can use the Image Resize facility. To achieve this simply enter a value greater then 256 for the number of colours.


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