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bulleteraser.gif   Choosing the Hardness setting

You should generally work with a Hardness of 100 because the brush makes all the decisions for you and automatically determines how soft transparent edges should be and what should be erased. When the Hardness is less than 100, then erasing at the edges of the brush is incomplete and you simply end up having more work to do to completely remove the background. If the erased edges do not look as you wish them to, refer to the description of the Sharpness setting below, which is the main way of controlling what erased edges look like. The Hardness setting exists primarily to help you when you are using the brush like the regular Eraser (by holding down Spacebar), for instance because in some portion of the edge the object and background are identical in color and only you know where the edge should be. This can happen in shadow regions, for example, where dark hair or clothing is indistinguishable from the shadow. A second reason for the Hardness setting arises when there is virtually no color difference between background and object. That means that image noise can randomly make the difference between a color being assigned to the object or to the background, with the result that the erased edge is irregular or noisy - and so unnatural. In this case reducing Hardness below 100 and running the edge of the bush along the edge can give a more smooth and pleasing result. Some people find it helpful to use a reduced Hardness when erasing around hair but, unless one of the special cases mentioned above exists, it is usually more effective to leave Hardness at 100 and use Sharpness to achieve a soft erased edge.

bulleteraser.gif   Choosing the Step setting

The Step setting is very important to the behavior of the brush. It determines how often the brush updates its estimate of what is background as you make a stroke. Very small values provide highly accurate sampling but make the brush slow. The default setting of 5 is a good compromise between accuracy and speed. In a number of cases you might be able to get away with a larger setting such as 10, especially if the background in your photo is more uniform because it is out of focus. If you have a very slow computer, you can make the brush faster by increasing the Step and reducing the Size of the brush. However, settings above 10 are not recommended except for extremely uniform backgrounds - those containing essentially a single color.

bulleteraser.gif   Other brush-specific settings

When the Precision Background Eraser is being used in the normal automatic mode, the remaining brush-specific settings should not be altered. However, in the manual mode using Spacebar, custom brush shapes - including changes in brush shape obtained using Thickness and Rotation - could be an advantage. Lowering Opacity may also be helpful in some cases, such as with very soft edges. Nonetheless, if you are having to resort to these settings and to manual work the chances are that you are not using the other bush settings optimally or your image is one of those rare ones completely unsuited to the background eraser, for instance because the object has no definition.

bulleteraser.gif   Settings specific to the Precision Background Eraser

There are a number of settings that are specific to the Background Eraser. These are summarized here and explained in more detail below. The most effective way to use the Precision Background Eraser is with Auto Tolerance checked. When in Auto Tolerance mode, the brush automatically examines the image as you make a stroke and determines the best tolerance for separating the background from the object. The Tolerance control will be grayed out but will still update continuously to show you the Tolerance setting the brush has automatically determined. Because of this, should you wish to set the Tolerance manually by unchecking Auto Tolerance, you already know a good starting point for your manually selected Tolerance. In practice it is very seldom that setting the Tolerance manually exceeds the selectivity and completeness of erasing provided by the Auto Tolerance mode so you should rely on automatically determined tolerances wherever possible.

The Sharpness setting determines how much the softness of an erased edge depends on the color difference between object and background. The Sampling control specifies how the estimate of background color is made. The Limits control determines whether and how edge information is to be used under the brush. These three items will be described in more detail later.

Sample merged works as follows. When Sample merged is unchecked, only the color information from the currently active layer is used by the brush. When it is checked, the color information from all the underlying layers is used by the brush as if the current layer and all those below it had been merged together. Checking Sample merged can be useful if the layer you are erasing has transparent regions and you are planning to composite the object on the active layer against the underlying layers. Note that in Sample merged mode all the erasing is still done on the active layer and none of the underlying layers are affected. These layers simply act to contribute a background color where the active layer is transparent. The Ignore lightness control is of limited usefulness but can be very helpful in some specific circumstances. These occur when you have a vividly colored object against a colorless background or the converse. An example might be a person in blue jeans and a red shirt riding down a steel escalator or standing against a gray metal door. In other words, when there is a large difference in the colorfulness of the object and the background, it is sometimes easier to separate the two by concentrating on the difference in vividness of color. Use of Ignore lightness is counterproductive when both object and background are colorful - as in most photos - or when both are unsaturated - for example as in a greyscale image. It should not be used in these cases. Additionally, when an image is heavily JPEG compressed, use of Ignore lightness may have a tendency to treat the JPEG block artifacts as object edges.

bulleteraser.gif   How to set Sharpness

The Hardness brush setting relates to the geometry of the brush - as Hardness is decreased there is less and less effect at the edges of the brush. In contrast, Sharpness is not related to brush geometry at all. Instead, it determines how the transparency of the erased image will depend on color differences in the original image. The Precision Background Eraser is designed to automatically respond to the softness of edges in the image. However, when there are untypical edges - for instance ones that form a gradient - it can help to adjust Sharpness. Where edges have a gradual color transition extending over some distance, reducing the Sharpness will give a better result. The first image below shows the results of erasing edges of increasing softness at two different Sharpness settings and illustrates that for very soft edges a low Sharpness produces superior results. Note that for a very soft edge you will need to set the brush Size larger than the width of the edge.

09_BGE_SharpnessEffect.gif
image 09

However, image 10 shows that Sharpness must be changed with caution. Real images, such as photos, have a great deal of color variation and a low Sharpness setting causes undesired erasing of parts of the object. In general, if you are using the default brush settings and correct brush positioning yet portions of the object are being erased along with the background, one of your first setting changes should be to increase Sharpness. The effect can be quite dramatic so make step-by-step modest changes, such as steps of 10 units or less. The above image has a practical demonstration of what happens at excessively low Sharpness, where the maroon portions of the red object are partly erased in an undesirable way. (Also see Limits below.)

10_BGE_IncorrectSharpness.jpg
image 10

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