Comparing Documents for Differences

Sometimes you need to compare two files that might look identical when opened but that you suspect differ in some minor detail. You can do this in Photoshop without having to rely on visual feedback. To do so, take the following steps:

1. Open the two files that you want to compare.

2. Select the Move tool.

Figure 5-4: Duplicating a document in ImageReady by dragging a tab.

3. Working in the document windows, drag the contents of one file into the other by pressing Shift (holding down the Shift key will ensure that the content of the two files "pin registers"). For best results, the two documents need to have the same pixel dimensions.

4. Change the blend mode of the top layer (the one you just dragged) to Difference.

5. Merge the two layers either by using the shortcut keys of Ctrl+E (Windows), (Mac OS) or choosing Merge Visible or Flatten from the Layers palette menu.

6. Close the original.

At this stage, you can simply hover the mouse cursor over the image and see whether the color readings change in the Info palette. However, to be absolutely sure, you can let Photoshop do the work by choosing Image ^ Adjustments ^ Equalize. If the two images are exactly the same, Photoshop will warn that it cannot equalize because the image contains only one brightness value. If there is a difference, it will be accentuated and you should be able to see it (Figure 5-5).

Figure 5-5: Comparing two documents for dissimilarity by using the Difference layer mode followed by the Equalize command.
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