Photoshop/photoshopcs3/autoalign

=> Photoshop Tutorials => New Features In Photoshop Cs3 => Auto-Align Layers

=> Photoshop Tutorials => Photoshop 101 => 
The Basics of Workspace => Basics of The Menu Bar => The Edit Menu => Auto-Align Layers

Auto-Align Layers

If you have two or more layers that are very similar and you want to align them to overlap, you can use the Auto Align Layers option. Select the layers that you want to align using shift+click, ctrl+click or by linking them, Then go to Edit>>Auto Align Layers... You will have 4 options in the dialog box:

1- Auto, which will automatically detect which is better Perspective alignment or cylindrical alignment.

2- Perspective will assign on of the layers as the source and will try to align the other layers to that layer by stretching, skewing and repositioning them.

3- Cylindrical will align the source layer in the middle and overlap the other layers while trying to reduce the bow-tie effect.

4- Reposition Only will work the same was as the Perspective alignment but without stretching, skewing or repositioning.

After you have finished aligning the layers, use masking or blending options to merge between layers.