Photoshop Opaque-to-Transparent Shadow Trick.
Have you ever found yourself with a Photoshop image that has an object's shadow on an opaque white background, when what you need is that same shadow on a transparent layer. Read the full version of this tip for a simple technique to accomplish just that!
Before: Our Original Image.
Our example image shows a piece of medical equipment photographed on a flat white background.
Step 1: Mask Your Object on its Own Layer.
For this technique, we need the shadow isolated on it's own layer. In this example, the object has been isolated on its own layer using a Layer Mask. The background shadow had been partially recreated, keeping a close resemblance to the cash shadow in the original photo.
Step 2: Hide the Object, and Command-Click on the RGB Channel
If you Command-Click (Ctrl-Click in Windows) on any channel, Photoshop will automatically create a selection based on that channel. This is the key to our trick.
Turn off all layers except for your shadow layer. Command-Click on the RGB channel (or CMYK) in the Channels pane. This loads a selection based on the white areas in your channel (which will be the opposite of your shadow).
Note: If your image has a light gray shadow, you may not see a selection. Don't worry it should still work.
Step 3: Inverse the Selection, and Fill with Black on new Layer.
Now that we've loaded our shadow as a selection, we're almost done. First invert your selection (Select > Inverse). Create a new blank layer, and fill that selection with black (or whichever color you want your shadow to be).
Step 4: Finish Up.
That should be it. You should now have a full transparent shadow in place of your opaque white background. Clean up and name your layers, and you should be ready to use this image for whatever projects you have ahead.
Source: This tip inspired by a great production question last week from Jim Mannino at Landreth Studios in Seattle, regarding a project for the in-house creative team at Philips Ultrasound in Bothell.