Simple Threshold

 

See Also: About thresholding, Adaptive Threshold

 

Before After

 

Simple Threshold uses a simple threshold to dynamically reduce the number of colors on an image.

 

1. Click or select Raster Effects Menu > Threshold > Simple.
2. In the Preview Window, zoom into the image 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 a threshold by moving the slider in the dialog. As you move the slider you will see the image changing.

Move the slider to the left to try to remove speckles and dirt from the image background. If you move the slider too far to the left, lines may start to break up or may disappear altogether.

Move the slider to the right to try to make thin or broken lines on the image more solid. If you move the slider too far to the right, text characters may “bleed” so that white spaces within them or between them become filled, speckles and dirt may appear in the background or the image may become completely black.

The aim is to move the slider to a position where the lines on your image are as solid as possible but where the background contains as little dirt as possible.

Before

 

 

 

 

After

 

 

Slider too far to left

Too far to right

Optimally positioned

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

Adjust the threshold if necessary.

The Restore button resets the threshold to the value it was set at when you entered the dialog.

If you can’t find a threshold that works over the entire image you may want to click on the Adaptive button. This launches the Adaptive Threshold function and allows you to try an adaptive 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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