Using the
Precision Background Eraser
Kris Zaklika, Jasc Software, Inc.
July 2003

This a description of how to use the Precision Background Eraser to best effect.  It covers basic principles as well as how and when to use the various settings.

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bulleteraser.gif  Principles of operation
What does erasing the background mean?
Basic tool settings
Choosing the Size setting  
Choosing the Hardness setting
bulleteraser.gif  Choosing the Step setting
bulleteraser.gif  Other brush-specific settings
bulleteraser.gif  Settings specific to the Background Eraser
bulleteraser.gif  How to set Sharpness

bulleteraser.gif  How to set the Sampling
bulleteraser.gif  How to set the Limits
How to take advantage of unerasing  
How to see what you are doing  
How to deal with difficult images 
What to do when too much is erased
bulleteraser.gif  What to do when not enough is erased
bulleteraser.gif  My Background Eraser is too slow
bulleteraser.gif  What you can accomplish



image 01

 Principles of operation

The idea of the Precision Background Eraser is this. You hold the center of the brush over the background and overlap the edge of the brush over the object. As you move the brush, the region at the brush center samples the background and then the colors under the brush are analyzed to remove background and leave object behind.

To make it easier to use the Precision Background Eraser switch on Show brush outlines on the Display and Caching tab of File > Preferences > General Program Preferences.

The method of positioning the brush is shown in the image above. A red object lies on a green background. The center of the brush, shown by the crosshair lies on the background, away from the edge of the object. The brush size is sufficient to overlap the object. In the example, the green background within the brush outline will be erased, while the object shown darkened will remain unerased. The normal brush operation is the selective one just described. In addition, the brush has an unselective or unconditional mode of erasing obtained by holding down the Spacebar while using the brush. In this mode the brush will not do any color analysis and will simply erase everything under the brush in the same way as the regular Eraser. The most efficient way to use the Precision Background Eraser is to run it around the edges of your object to perform the difficult definition of object edges, leaving the object with a surrounding band of transparency. The remaining background far from the object can then be erased quickly and completely using Spacebar with the brush.

 What does erasing the background mean?

Erasing means converting some image colors to transparency. In Paint Shop Pro, only regular layers can have transparency and this transparency can vary from none through partial to complete. A background layer does not support transparency at all and a paletted image such as a GIF does not support partial transparency. Consequently, when you try to use the Precision Background Eraser on such images you may see a message from Paint Shop Pro informing you that the image needs to be promoted to a full layer and that its color depth must be increased to 16 million colors. (Whether or not you see a message depends on the settings on the Auto Action tab of File > Preferences > General Program Preferences.) However, erasing is not the only thing that the Precision Background Eraser does.

In the previous example 01 image, you can see that the red object has a soft edge where the color is some mixture of red object color and green background color. If all the Precision Background Eraser did was just to erase, the results would not be good for such an image. Either only the green would be erased to leave a red object with a dark halo, or the background and halo would be erased to leave the object with unnaturally sharp edges. Accordingly, the Precision Background Eraser does two things. First, it erases, and where there is a mixture of object and background colors it erases only partially to allow a soft edge. Second, in partly transparent areas it subtracts the color of the background from that of the object, leaving only background. This is called color unmixing and is shown in image 02.

 image 02

Because of this color unmixing, which gives you pure object colors uncontaminated by background, the colors in your image are changed. Unerasing the transparent areas of your result image using the regular Eraser will reveal colors that were never in the original image. Here is an example (image 03).

However, as long as you are in a single session of the Precision Background Eraser, the tool can keep track of the original image colors. Therefore, if you unerase while in the Background Eraser you will always get your original image colors back. Unerasing in the Precision Background Eraser is done with the right mouse, just as for the regular Eraser. Remember that holding down Spacebar while unerasing will unconditionally restore everything under the brush, just as if you were using the regular Eraser to unerase. For this reason, you should never jump back and forth between the Background Eraser and the regular Eraser.

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image 03