Using Alpha Channels in ArKaos VJ 3.0.1

ArKaos VJ 3.0.1 now facilitates the use of visuals containing an alpha channel, an additional graphic channel that can be added to some still images and QuickTime movies. Standard images have three channels containing the RGB (red/green/blue) information. The alpha channel represents an additional layer of graphic information. In VJ, this channel is used to specify the translucency level of the visual, providing a new way to mix the different layers together.

Using an alpha channel in your visual to define a translucency mask has many advantages. For example, your translucency mask doesn't depend on any color or luminance information, as it does in Luma keying or Chroma keying. As a result, you can still use your visual without alpha masking, and you have more precision for translucency information. Moreover, it’s faster to pre-compute alpha directly into your visual file than it is to use Luma or Chroma keying.

Creating a visual containing an alpha channel

Here’s how to create an image with an alpha channel using Adobe Photoshop 7.0:

  • In the "Layer" window, select the "Channels" tab

  • Create a "New Channel" to add an alpha channel

  • Select the newly created alpha channel, and begin creating your alpha mask. You create an alpha mask just as you would any other color mask using the Photoshop tools. Notice that this new layer is grayscale. The darker a pixel is in the alpha channel, the more translucent the corresponding RGB color pixel will be. (Solid black yields full transparency, and solid white yields full opacity.)

  • Save your image in either a bitmap (.bmp extension) or a pict (.pct or .pict extension). (Both file formats can hold an alpha channel and are supported by ArKaos VJ 3.0.1.)
If you want to create a video stream with an alpha channel, Adobe After Effects is one program that allows you to do so.
  • You can define an alpha mask in either of the following ways:
    • duplicate your video stream and apply any Keying Effect on the copy to define an alpha mask
    • use another video stream, image, or a manually shaped mask to define an alpha mask for the entire video

  • Finally, export your video with a QuickTime video codec (such as the Animation codec) that supports "Millions of Colors+" (32-bit color) to save your alpha channel as well as the color channels.
Using visuals with an alpha channel in ArKaos VJ 3.0.1
  • Import your visual in ArKaos VJ, and then drag-and-drop it on the program’s keyboard.
  • In order to specify that you want to use your alpha channel as a translucency mask in the "Visual Info" window, double-click on the key’s visual thumbnail or press [ctrl]+[I] to display it.

  • When you mix the background image (immediately below), with the new visual containing the translucency mask, it yields the second image.

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