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Photoshop® CS4 Bible by Dan Moughamian, Simon Abrams, Stacy Cates

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8.6. Image Stacks

An Image Stack is a Smart Object that contains two or more layers and has a Stack mode applied to it. Stack modes affect how the layers' pixels are blended together, but they blend the layers in a different way than Blending modes do.

To apply a Stack mode to the layers in a Smart Object, click on the Smart Object to activate it, then choose LayerSmart ObjectStack Mode (choose a Stack mode).

You can use the Mean Stack mode to remove image noise in several nearly identical photos, such as photos of the same building at night, taken consecutively. Mean displays the average of the pixels in the Stack mode layers, so the difference in the pixels that make up the noise is diminished because of the averaging.

If you don't take the photos with a tripod, choose EditAuto-Align Layers with the Reposition Only option to align the photos on layers before you apply the Stack mode.

Use the Standard Deviation Stack mode to reveal areas in which two images that look identical are actually different. In Figure 8.49, the photo on the left is slightly different from the photo on the right in the area of the bark of the tree, although the differences are hard to see. The Standard Deviation ...

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