CS4 - Using Content-Aware Scaling

CAS uses a rather complex mathematical algorithm that decides what is important in an image, and what is expendable, when it comes to altering or discarding information upon resizing.
However, it wouldn't be an Adobe product if you couldn't manipulate the function and use it to its best advantage. In this tutorial, we will explore a few ways of getting the best out of the Content-Aware Scale.

We will begin with a sample image, such as:

Before anything else, let's make a duplicate layer so that we can compare our results. Right-click the layer and choose Duplicate Layer.
Now, to use CAS, goto the Edit menu, and then click Content-Aware Scaling.

You will notice that it looks like a regular Free Transform. But go ahead and resize the image, and you should see that the girl remains relatively untouched as you resize - the algorithm correctly predicted that she was the main focus of the image:

Compared to the original image, there is still some distortion to the girl. So undo the last resize and click on the Protect Skin Tones icon () in the toolbar of the CAS tool (at the top, when selected).
Now try resizing with CAS again:

Notice that we get a much less distorted image of the girl with this option selected. It protects pixels that match the tones of skin. Another example indicating this is below:

Original:

Without Protect Skin Tones:

With Protect Skin Tones:

You may also have noticed the Protect drop-down menu next to the Skin Tones option. With this, we can set parts of the image to be more important than others. We do this by using Channels.

For this example, we will use the following image:

Now goto the Channels panel (Window > Channels), and create a new Channel. It should be named Alpha 1 by Default, but the name doesn't really matter, and it should be filled with Black. You can click the eye of the RGB channel to see the image through the new channel, like so:

Next, choose a White Brush, and making sure you have the new Channel selected, paint there areas you do not want scaled, like so:

Now we can click on the eye by the new Channel to hide it, and go back to our Layers panel. Goto Edit > Content-Aware Scale and then from the Protect menu, choose the name of the Channel you just created:

Proceed to scale the image and notice that the areas you painted with White in the Channel are left unchanged:

Original:

CAS with Alpha 1 Protected: