Sampling Options

Whenever we have the Background Eraser selected, the Options Bar along the top of the screen displays various options for controlling how the tool behaves. One of the most important behaviors we can change is how Photoshop samples colors under the target symbol, or if it samples them at all.

Directly to the right of the Brush Presets picker on the left side of the Options Bar are three icons. Each of these icons selects a different sampling option. From left to right, we have Continuous, Once and Background Swatch:

Of these three, the two you'll switch between the most are Continuous (the icon on the left) and Once (the middle icon). Continuous is selected by default, and it means that as we move the Background Eraser around, Photoshop continuously samples the color under the target symbol. That's why, when I accidentally moved the target symbol over the green column or the green tree, Photoshop started erasing green pixels in the image even though I was initially erasing blue pixels.

The Continuous sampling option works great when you're trying to erase backgrounds that contain multiple colors, but if the color of your background doesn't change much, the Once option usually works better. I'll select it by clicking on the middle icon:

As you may have guessed from its name, Once tells Photoshop to sample the color under the target symbol once and that's it. Whichever color is under the target symbol the moment you click your mouse button is the color that Photoshop will erase no matter how many other colors you drag the target symbol over (while you're still holding the mouse button down). Watch what happens now when I "accidentally" move the target symbol over the green column. The Background Eraser has no effect on it this time because the target symbol was over the blue column when I clicked my mouse button:

We see the same thing happening in our photo. With Once selected as the sampling option, Photoshop is able to ignore the tree this time even though I've moved the target symbol over it, and that's because I initially clicked on the blue sky:

If you're having trouble positioning the target symbol over the color you want to erase, try the Background Swatch sampling option (the icon on the right). With Background Swatch selected, click on the Background color swatch in the Tools palette and select a color from the Color Picker that matches (as close as possible, anyway) the color in your image that you want to erase. If the color you've selected isn't quite right, adjust the Tolerance value in the Options Bar (which we'll look at in a moment) until you're able to erase the pixels:

Limits

Another important option for the Background Eraser is Limits. Once Photoshop knows which color you want to erase, Limits tells it where it can look for pixels that match the color so it can erase them. The two main choices for the Limits option are Contiguous and Discontiguous:

Contiguous, the default setting, means that Photoshop can only erase pixels in areas that are physically touching the pixel under the target symbol. In other words, it can't jump across tree branches, fence posts, or anything else in the photo that separates and isolates one area of pixels from another. This causes a problem for me as I try to erase the sky showing through the tree branches:

To get around this problem, I'll set my Limits option to Discontiguous:

Discontiguous means that Photoshop is free to erase any pixels anywhere in the image that match the sampled color we're erasing, whether they're in the same area as the target symbol or not. Once I've clicked the target symbol on an area of blue sky, I can simply drag the Background Eraser around inside the tree to easily erase the sky showing through it (I have my sampling option set to Once so Photoshop doesn't change the color being erased as I move over the tree):

Unfortunately, if we look closely, we can see some darker blue areas of the sky remaining around some of the tree branches. Even with the Limits option set to Discontiguous, the Background Eraser still needs a little more help with this image:

This brings us to the third important option for the Background Eraser, and the one that can make all the difference when it comes to using the tool successfully - Tolerance, which we'll look at next!

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