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Adaptive Threshold

 

See Also: About thresholding, Simple Threshold

 

Before After

 

Adaptive Threshold uses an adaptive threshold to dynamically reduce the number of colors on an image.

 

1. Click or select Raster Effects Menu > Threshold > Adaptive.
2. In the Preview Window zoom into a difficult part of the image, for example an area that contains both faint and dark lines. Zoom in so that you will be able to clearly see the effects of the threshold.
Warning: If you display a large section of your raster image in the Preview Window this function may operate very slowly.
3. Select Window and Background values by moving the sliders in the dialog.

As you move the sliders you will see the image changing in the Preview Window. We recommend you position the sliders as follows:

Before

 

a) Move both sliders as far to the left as you can. The background will become slightly or very dirty.

b) Move the Background slider to the right until the background is as clean as possible but not so far to the right that faint lines disappear.

You may notice that areas of solid color such as arrowheads or thick lines become hollow.

       

c) Move the Window slider to the right until the areas of solid color become solid again.

If text characters start to “bleed” so that white spaces within them or between them become filled, move the Window slider back to the left a little.

d) You may find that moving the Window slider to the right causes background speckles and dirt to reappear. If so, repeat steps (b) and (c) until the lines on your image are as solid as possible but the background contains as little dirt as possible.

 

4. Zoom and pan around the image in the Preview Window to make sure the Window and Background values you have chosen are appropriate to other parts of the image.

Adjust the Window and Background values if necessary.

The Restore button resets the Window and Background values to the values they were set at when you entered the dialog.

The Simple button launches the Simple Threshold function and allows you to try a simple threshold on the image.

5. Click OK.

 

Click to undo.

 

Once you have thresholded the image you may want to …

Reduce its color depth using Palette Menu > Reduce Color Depth > 1 Bit [Mono]. This will make the image much smaller.

 This is because thresholding reduces the number of colors on the image to black and white, but it does not reduce the image’s color depth to two colors (see Reducing colors vs reducing color depth).

Smooth raster line edges (Raster Effects Menu > Smooth ).
Remove speckles (Raster Effects Menu > Clean > Remove Speckles ).
Remove holes (Raster Effects Menu > Clean > Remove Holes ).

 

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