Making complex selections and creating the most effective channel (or mask) for compositing operations in Adobe Photoshop can be a
fine art. Frequently the best mask for a given image already exists, ready to be used. In this exercise, I’ll show you a method for creating a mask directly from an object in an image. This technique works best for objects on a dark background.

1. Here’s an image we’d like to composite against a background of another color—a photo of a clouded blue sky, for instance. It’s obvious that creating a selection of the bird will be difficult, especially while attempting to preserve the motion blur in the wings.

2. It looks as if the luminosity of the RGB composite might be a good place to start creating our mask. Load the luminosity of any group of visible layers as a selection by pressing Option/Alt+Command/Ctrl+~. Next, save the selection into a new channel by choosing Select > Save Selection. Deselect and go to the new channel.

3. Use Levels or Curves to eliminate most of the midtones in your new mask, preserving only the darkest edges. Notice the gray tones along the blurred portion of the wings. These areas will create semi-transparency during compositing. Be sure to use a paint brush to clean up your mask channel.

4. Set your target back to the RGB composite and load your finished channel as a selection. Drag the selected area onto the new background.

5. Notice that the semitransparent areas along the edges are too dark as they contain some black from the background of the original image. Finish the composite by using the dodge tool to
lighten these edges. Set it to Shadow mode, and use a soft-edged brush set to 80% or 90% opacity.

Dodge and burn tools If the hue shifts when you use the dodge or burn tools, try converting
your image to LAB mode first. The dodge and burn tools only affect Luminosity when used in LAB mode.

6. Here’s our finished composite. Nice.

Tutorial Source