Creating an Alpha Channel
Photoshop CS3 Tutorial
How to Create an Alpha Channel in Photoshop
Often times images from Photoshop are used in other applications, such as After Effects, that support alpha channels. This grayscale channel has black areas to indicate transparency and white sections that are opaque.
The alpha channel can be found in Photoshop's Channels palette and is labeled as channel #4.
Open a new 500 pixel by 500 pixel document in Photoshop (File > New).
Using the Text tool, add some text to the document.
Cmd or (PC- Ctrl) click the text icon in the Layers palette to select the active pixels.
Click the Channels palette and click the Save Selection as Channel icon.
Notice that this creates Channel #4 (Alpha 1). Click the channel icon and notice that the text area is white and the background is black.
By saving the image as a PSD or TIF file, the alpha channel is included with the image.
24 vs 32 Bit Images
24 bit images contain eight bits of red, eight bits of green and eight bits of blue information. A 32 bit image contains the RGB info plus another eight bit grayscale image that stores the alpha information.
Transparency and Alpha
Many programs use alpha channels to set the object's opacity. This includes video software, 3D software and compositing software. JPG images do not retain the alpha information.