Adobe added a new feature called Content-Aware Scaling to Photoshop CS4.  Under the right circumstances, this feature can seem like magic.  But, it has its limits.

The above photos are from the same source image.  The first image is unaltered while the second image was resized using content-aware scaling.  The results are impressive and very realistic.

1.  Start with a quick mask to define the protected areas

I started a Quick Mask by clicking the icon in the lower left and drew over the tiger.  This quick mask will ultimately be used by Photoshop CS4 to determine the areas to protect during the content-aware scaling.  Click the Quick Mask icon again to convert the Quick Mask into a selection.

2.  Invert selection and save as a Alpha Channel

With the selection still active, I inverted the selection (Control-Shift-I on PC and Command-Shift-I on a Mac).  Clicking on the Channel Panel, I clicked the Create Channel icon to save the selection as an Alpha Channel.   I then filled in any gaps in the tiger with pure white.

3. Content-Aware Scaling in the Edit menu

Be sure that the layer you are working on is NOT named Background.  Content-aware scaling will not work on the Background Layer.  Just double click on the layer name and rename it.  You will then see Content-Aware Scaling displayed in black (and accessible) in the Edit menu drop-down.

4. Select the Alpha Mask to protect the tiger and start scaling

The whole layer will now appear with Transform handles.  But, before you start, you will need to identify the area to protect.  In the top bar, you will see a little man icon.  Click the “Protect” menu and select the Alpha Channel from the previous step.  With that set, you can now scale the background while the tiger remains protected.  Just apply the transform and you are done.  (This transform is Photoshop intensive and can take a while to process depending on the file size.)

5.  Content-Aware Scaling is not very good with straight lines

Content-Aware Scaling works very well with organic backgrounds like landscapes.  However, the current version has a problem with straight lines.  The following images were my original samples for this tutorial.

Take a look at the pillars, the second floor and the skylights.  The once crisp sharp lines have been distorted instead of resized.  If I was simply resizing the image, Photoshop would not have this problem.  Hopefully, Adobe will enhance this feature with the next release.