Photoshop: Layer Mask.

Rather than using the eraser tool to delete portions of an image (because once pixels are gone they are gone) why not use a layer mask.

Masks are a way to hide portions of an image on lower layers based on areas of black or white, including levels of grey.

Once you understand what the layer mask is doing and how it works you can achieve much more than just using the eraser tool.
The example below shows a layer mask that has been added to the top layer.
Click on the images to get a larger view.

The layer mask is the black and white thumbnail to the right of the image in the layers palette. It is important to notice when the layer mask is selected by the frame around it's thumbnail.

The black ink on the mask thumbnail can thought of as a hole that is cut through the image revealing the layer below.

This also works with values of grey. In the example above the gradient runs from black to white through grey values. So painting with grey ink will make the underlying image semi-opaque.

The mask becomes really useful once you realise that you removed something from an image you thought you didn't need. You can alway go back and paint with white to restore the image.

Look at this link for a fuller explanation of using the layer mask.

What is a layer mask?

Photoshop CS4: Layer Masks in Depth | by Jan Kabili

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