Now that our images are ready to be combined, we can start the actual processing. This is where the 3D effect begins to form.

Anaglyph stereo images work by fooling the eyes. Because the left lens of our stereoscopic glasses is red and thus does not allow blue light to pass, the left eye cannot see any images that are blue-green in color. Because the right image is blue-green, it is effectively hidden from the left eye. The converse is true for the other lens: its blue color will not allow red light to pass from the left image. The end result is that each eye sees only the image meant for it, and thus the illusion of three dimensions on a flat display screen is created.

Look at the two images below using a pair of 3D glasses. Cover each eye in turn. You'll notice that the left eye only sees a black square (or a very dim image) where the right image is, and the right eye can't see the left image.

To create the stereo pair:

  1. Make sure the left-hand image is the active window.
  2. Open the Adjust Levels dialog box by selecting Image > Adjust > Levels.
  3. Select the Green component in the drop-down menu (which is initally set to "RGB" when the dialog box opens.)
  4. Change the second Output Level to 0 (it is probably set to 255 at this point.) This causes the contents of the channel to be completely blacked out. Notice that the image instantly gains a severe color cast - this is what you want!
  5. Select the Blue component in the menu, and set its second Output level to zero.
  6. The image is now pure red (that is, the green and blue channels - remember the 'trichrome process' described in the procedure description? - are now empty.)
  7. The process needs to be carried out again on the second image, which will become the right image in the stereo pair. In order to create a cyan (blue-green) image, black out only the Red channel.
Select the proper color channel... ... and set both output levels to zero.
The left image is now red... ... and the right one is blue-green (cyan).
Red channel only Green and blue channels combined

Now, we're ready to create our anaglyph.

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