Step 1
Download a pair of images from one of the CADTutor Image Bank pages. Right-click one image at a time, and choose "Save Target As…" from the context menu. See the Download Main Page for more details. Save the two images to a convenient folder on your computer.
Step 2
Open both images in Photoshop and make the mask image (the black and white one) the current image.Step 3
In the Layers palette, shift-click and drag the mask "Background" layer and drop it onto the colour image window. Holding the Shift key down ensures that the layers are perfectly aligned, one on top of the other. Your Layers palette should now look something like the one illustrated on the right. You should have two layers, "Layer 1", the mask and "Background", the colour image.Step 4
Click on the Channels tab to display the channels palette. Right-click the Blue channel and select "Duplicate Channel…" from the context menu. Now, using the Duplicate Channel dialogue box, give the new channel a name by typing in the "As" edit box. Now click the OK button.
In fact you could duplicate either the Red, Green or Blue channels and get the same effect. Since the mask image is black-and-white, the three colour channels are identical. Your Channels palette should now look something like the one shown on the right.
The channel you have just created contains the selection information for this particular image. This type channel is often known as an "Alpha Channel".Step 5
Click the "Load channel as selection" button
at the bottom left of the Channels palette. You have now used your new channel to create a selection and you should see it displayed in the main image window.Step 6
Click the Layers tab to display the Layers palette. Click "Layer 1", the mask layer, to make it the current layer. Delete this layer by clicking the "Delete layer" button
at the bottom right of the Layers palette. Since you now have a selection saved as a channel, you no longer need the mask layer.Step 7
You now have a couple of options. If you would like to save this image with a selection (alpha channel) for future use, you should use the "Save As" option from the File pull-down and save the image as either a TIFF or a PSD file. These are the only two file formats that can save channel information. Saving alpha channel information as an integral part of a file is very useful because it means you never have to bother making that selection again. It also means that you can use these files when creating Bryce image objects to load both colour and alpha channel information simultaneously. See the Bryce Image Objects tutorial for details.Alternatively, if you have no future use for the selection, you can simply copy and paste the selection into another image so that the selected object forms a separate layer. The original image file can then be closed without saving.
Source : http://www.cadtutor.net/tutorials/photoshop/selections-from-image-masks.php
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