The Background Eraser tool is an intelligent (and really quite amazing) little feature. Not only does it remove the background from around very complex shapes, but it does so in a way that leaves a natural, softened, anti-aliased edge around the foreground object left behind. This technique would be impossible to pull off with the regular Eraser tool, because it indiscriminately removes whatever pixels it touches. Because the Back ground Eraser tool erases pixels based on colors and tonal values that you control and modify, there is rarely much additional cleanup required once the background has been removed. Additionally, because the Background Eraser tool always erases to transparency, if you use it to remove the background from even a flattened layer, it automatically converts that layer to a floating, transparent one. This allows you to easily place a new background behind a foreground image, or to move it into a different photo composition altogether.
To use the Background Eraser tool
Select the Background Eraser tool from beneath the Eraser tool in the toolbox (Figure 8.75).
Figure 8.75. The Background Eraser tool.
Alternatively, you can press E to select the Eraser tool and then press E again to toggle to the Background Eraser tool.
On the options bar, select a size using the brush Size slider.
From the Limits drop-down menu, select one of the two limit modes (Figure 8.76).
Figure 8.76. The Limits drop-down menu controls which pixels beneath the brush are sampled and erased.
Contiguous mode erases any pixels within the brush area that are the same as those currently beneath the crosshairs, as long as they're touching one another.
Discontiguous mode erases all pixels within the brush area that are the same as those beneath the crosshairs, even if they're not touching one another.
Select a Tolerance value, using the Tolerance slider (Figure 8.77).
Figure 8.77. You use the Tolerance slider to increase or decrease the number of pixels sampled based on their similarity to one another.
The Tolerance value controls which pixels are erased according to how similar they are to the pixels beneath the eraser crosshairs. Higher Tolerance values increase the range of colors that are erased, and lower values limit the range of colors erased.
In the image window, position the eraser pointer on the edge where the background and foreground images meet, and then drag along the edge.
The background portion of the image is erased, leaving behind the foreground image on a transparent background (Figure 8.78). The brush erases only pixels similar to the ones directly below the crosshairs, so the entire background can be completely erased while leaving the foreground image intact.
Figure 8.78. Begin by placing the crosshairs of the brush in the background portion of the image (left), then drag the brush along the outside edge of the foreground object to erase the background (right). Continue around the edge of the foreground object until it's completely separated from the background.
It's okay if the circle (indicating the brush size) overlaps onto the foreground image, but be sure to keep the crosshairs over just the background area. The Back ground Eraser tool, of course, doesn't really know the difference between background and foreground images, and is simply erasing based on the colors selected, or sampled, beneath the crosshairs. If the crosshairs stray into the foreground image, that part of the image will be erased, too.
There's a third eraser toolthe Magic Eraser toolthat I've chosen not to cover here because, frankly, it doesn't work very well. It operates on the same principle as the Magic Wand tool (and, to some extent, the crosshairs on the Background Eraser tool) by deleting like pixels based on color or tonal value. That's all well and good, but you're not given any feedback or any opportunity at all to modify your selection. You just click, and poofa large area of color is gone. Since the erasure typically is either not quite enough or a little too much, you undo, reset the tolerance, try again, undowell, you get the idea. The other two erasers work fabulously well, so I suggest just keeping the Magic Eraser tool in the toolbox.