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bulleteraser.gif How to set the Sampling

The Sampling setting controls how the brush determines the background color that should be erased. The default setting is Continuous, which means that the background color is continuously updated as the brush moves over different parts of the image during the stroke. How frequently this sampling happens depends on the Step. This is the best setting for photographs, where the background color varies. Bear in mind that the brush detects very subtle color differences so that a sky in a photo that looks uniform at a glance will actually be quite variable as far as the brush is concerned. Therefore, you should resist the temptation to change from the Continuous for Sampling just because a background looks like it has the same color everywhere. Continuous will be the best setting for almost all images. The remaining Sampling settings pertain primarily to computer generated graphics with uniform backgrounds. The Once setting samples only one time, when you first click the brush to start the stroke. The advantage of this setting is that you only need to accurately place the brush at the first click. After that, provided you hold the mouse button down, you can move the brush all over the image with any particular care and only the color you sampled at the start of the stroke will be erased no matter what the center of the brush passes over. The other two settings - ForeSwatch and BackSwatch - are provided as a convenience and take the background color from the current Foreground and Background colors respectively in the Material Palette rather than sampling it from the image. For example, if you have a graphic with thin colored stripes than can be difficult to sample, simply set the color of the stripe you want to remove in the Material Palette and start erasing using the relevant Swatch setting. With the ForeSwatch and BackSwatch settings it is also unnecessary to move the brush carefully since only the set color is erased no matter where the brush center is positioned. There is one other thing to look out for. If a graphic was initially created with a uniform background and it was saved in JPEG format, the background will no longer be exactly uniform, particularly near edges. In such a case the Continuous setting may be more successful than the other three.

bulleteraser.gif How to set the Limits

The Limits setting determines how the estimated background color is used to erase under the brush. The default setting is Contiguous, which means that background color will only be erased if it is connected to the center of the brush. In contrast, Discontiguous erases the background color no matter where it appears under the brush. Contiguous is the setting you want to use when the object contains colors similar to the background that you don't want to have erased since holes will appear in the object.

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Discontiguous is used when an object has holes through which the background shows through and which must be removed to leave just the object. An example of such an object would be lace or tree branches. The difference between these two settings is shown in the image above.

Usually, the Contiguous setting does an excellent job of restricting the erasing to the outside of an object. However, in some cases of complex and variable color distributions or of indistinct edges the brush may erase inside the object. In such a situation your first change to the default brush settings should be to set Limits to Find Edges, which uses additional edge information to prevent erasing inside the object. You can also couple this with an increase in the Sharpness setting to preserve more of the object. However, be aware that when you use the Find Edges setting for Limits the sharpness of edges tends to be greater than you would expect from the Sharpness setting.

bulleteraser.gif How to take advantage of unerasing

The Precision Background Eraser never affects transparency that existed at the time the tool was invoked. Whether the brush is erasing or unerasing, this transparency is not modified. When Spacebar (backspace in PSP X) is depressed, the Precision Background Eraser behaves like the regular Eraser. With the left mouse button everything under the brush erased, while with the right mouse button everything under the brush is unerased. However, unerasing in the normal mode of the Background Eraser is more subtle.

When right mouse is used the Precision Background Eraser works in reverse. The center of the brush samples the colors in the transparent region you are unerasing and uses them to make an estimate of the object color. Then it unerases the object without unerasing the background. Additionally, in this unerase mode the brush is biased differently from the erase mode. This means that the brush unerases a little less than it erases. You can take advantage of this when trying to deal the edges of objects that differ very little in color from the background. First, choose brush settings that erase all the background with a minimal effect on the object. Under these conditions some small parts of the object may be erased as well. Do not worry about this. Next, without changing any of the brush settings run over the erased parts of the object with the center of the brush using the right mouse button. Most likely the missing parts of the object will be unerased without any of the background reappearing. This technique is a quick and effective one since it only requires switching from the left to the right mouse button.

 

bulleteraser.gif How to see what you are doing

When you use the Precision Background Eraser it can sometimes be difficult to spot small regions that you failed to erase completely, which can be a nuisance when you try to use the image later as a picture tube or for compositing with another image. There are two techniques to help you see better whether there are leftovers in the image.

The first one is this. Before starting to use the Precision Background Eraser, create a new layer under the one you wish to erase and flood fill it with some color that strongly contrasts with your image. The best color to use will depend on the content of what you are erasing but consider colors like red, blue, green, white, gray, black, deep bluish-green, pale yellow or vivid pink. You must make the choice before you start using the Precision Background Eraser tool, not while you are using the tool. The reason is the following. When you switch layers (e.g. to flood fill the underlayer with another color) the Background Eraser tool thinks you have finished working on the previously active layer and discards its transient data. Consequently, when you switch back to the original layer to continue work, the original image colors are no longer available and unerasing no longer works.

The second technique does not suffer from this problem and can be applied at any time while the tool is being used. Normally, the transparency of a layer is represented as a light gray and white checkerboard. The appearance of this checkerboard can be changed in File > Preferences > General Program Preferences on the Transparency tab. The checkerboard pattern distracts the eye and hampers spotting leftovers so in the Scheme droplist try selecting one of the schemes denoted as Solid.

 

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If none of the preset colors schemes work for your image content you can define up to nine custom schemes of your own. The advantage of using different colors for the transparency checkerboard is that you can switch between them as you use the Precision Background Eraser ensuring that, whatever the colors in the portion of the image you are working on, they contrast against the background.

 

bulleteraser.gif How to deal with difficult images

Problems with images usually fall into two types. At the settings being used, either the Precision Background Eraser removes too much, biting into the object, or it doesn't remove enough, leaving specks or a halo behind in the transparent areas. Methods for dealing with both problems will be described below. However, it will help you to keep two things in mind.

First, you should not try to make the automatic mode of the Precision Background Eraser do everything. Concentrate on getting clean edges around the object since this is usually the hard part of any erasing job. Bear in mind that in this regard the Precision Background Eraser can do things that are impossible to accomplish with the regular Eraser, since the former tool unmixes the background color from the object color and the latter can not. Once you have taken some care to get the edges of the object right, the rest of the background can be erased away quickly by holding down Spacebar (backspace in PSP X). With Spacebar (backspace in PSP X), the brush is also much faster since it no longer has to do complex calculations.

The second consideration is this. As a human with knowledge of the world you make judgments about the correct shape of objects. In contrast, the Precision Background Eraser does not understand the image the way that you do. All it can do is work with the colors that are there in the image. For example, if a person with a jet black punk hairstyle is photographed against a black and white striped background, the Precision Background Eraser has no way of judging what are black stripes and what is spiky hair in the way that you can. Consequently, in the hair portion of the image the Background Eraser cannot be expected to do a good job, even though it can be very successful elsewhere in the same image. Another situation where there is no difference in color between object and background arises in blown out highlights or deep shadows. If you are having trouble with such regions you should check whether there is really any color difference for the Precision Background Eraser to work with. Open a copy of your image and do Adjust > Brightness And Contrast > Gamma Correction. Check the Link checkbox and set Gamma greater than one for shadows and less than one for highlights. If a clear color difference appears in the preview you have a chance of success with the Precision Background Eraser; otherwise, you will have to manually define the edge of the object. In JPEG compressed images you may find that what were originally differences in color between the object and background have been consolidated into a single nearly uniform JPEG block that straddles the true object edge. When you erase, such a block will cause an unnatural bump or dent in the edge, which you will have to smooth out manually. For this reason it is advisable not to use heavily compressed JPEG images with the Precision Background Eraser. Save your image in a lossless format (such as .PspImage) instead.

When you use the correct settings for the Precision Background Eraser, the chances are that you will encounter problems defining the edge of your object only in very limited parts of the perimeter of the object, if at all. You will be able to quickly define almost all of the object edge, leaving one (or a few) small troublesome portions. On the one hand, it can be very educational and useful for the future to experiment with the tool settings until you can separate object and background. On the other hand, if you are in a hurry it may be simpler to manually define that part of the object edge. You will still be ahead of the game since the Precision Background Eraser will have done the majority of the work for you. One approach that works well for manually defining the edge is to set Hardness to 50, hold down Spacebar (backspace in PSP X) and brush back and forth along the edge, progressively moving the brush inwards towards the edge. For abrupt edges increase the Hardness and reduce it for soft edges.

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