This is where the important difference between the Eraser Tool and layer masks comes in. Remember when we used the Easer Tool to blend the images together by erasing away part of the left side of the top image? The Eraser Tool physically deleted that part of the image and it was forever gone at that point, and if we looked in the top layer's preview thumbnail, we could see that large chunk of the image missing on the left. This time though, we've used a layer mask to hide not just part of the left side of the image but rather the entire image, yet if we look in the layer's preview thumbnail, the image is still there, completely intact:

Where the Eraser Tool deleted the contents of the layer, the layer mask simply hides it from view! To prove that the photo on the top layer is still there, I'm going to fill the layer mask with white. To fill a layer mask with white, or do anything at all with a layer mask, you first need to select the mask so that you're working on the mask itself and not the actual layer, and to select it, all you need to do is click directly on the mask's thumbnail in the Layers palette:

You can switch between selecting the layer itself and its layer mask by clicking on the corresponding thumbnail. You can tell which one is currently selected by which thumbnail has the white highlight border around it, as we can see around the layer mask thumbnail in the image above.

To fill the mask with white, I'll go up to the Edit menu at the top of the screen and choose Fill, which brings up Photoshop's Fill command dialog box. For Contents I'll choose white:

With white chosen as my fill contents, I'll click OK in the top right to exit out of the dialog box and have Photoshop fill my layer mask with white. I can now see in the Layers palette that the mask thumbnail is filled with white:

And with the mask now filled with solid white, my photo on the top layer is completely visible in the document once again, proving that even though the image was hidden from view a moment ago when we filled the layer mask with black, it was always there, untouched and unharmed:

And that's the basics of how Photoshop's layer masks work! When the mask is filled with white, the contents of that layer are 100% visible in the document, and when the mask is filled with black, the contents of the layer are 100% transparent - hidden from view but not deleted as was the case with the Eraser Tool. Layer masks don't physically alter or affect the contents of the layer in any way. All they do is control which parts are visible and which are not. The contents of the layer are always there, even when we can't see them.

"Okay," you're wondering, "We've seen how we can hide a layer completely by adding a layer mask to it and filling it with black, and we've seen how we can show the layer completely once again by simply filling the layer mask with white. And we know that whether the contents on the layer are visible or not, they're still always there. The Eraser Tool deletes parts of the image but layer masks simply hide them. That's all great. But is this all we can do with a layer mask, either show the entire layer or hide it? How do we use a layer mask to blend these two images together like we did with the Eraser Tool?"

Excellent question, and the answer is, very easily! We'll do that next.

All Content © 2006 - 2010 Photoshop Essentials.com. All Rights Reserved. Photoshop is a trademark of Adobe Systems Inc.