Overlays and Transparency

Certain file formats are designed to support the storage of still images captured from video sources. In practice, images of this sort are often overlaid on live video sources at render time. This is a familiar feature of conventional broadcast television, where still images are routinely shown next to live readers on the evening news.

Normal images are opaque, in the sense that no provision is made to allow the manipulation and display of multiple overlaid images. To allow image overlay, some mechanism must exist for the specification of transparency on a per-image, per-strip, per-tile, or per-pixel basis. In practice, transparency is usually controlled through the addition of information to each element of the pixel data.

The simplest way to allow image overlay is the addition of an overlay bit to each pixel value. Setting the overlay bit in an area of an image allows the rendering application or output device to selectively ignore those pixel values with the bit set. An example is the 16-bit variant of the TGA format, which supports data in the format:

(15 bits) = (R,G,B) = (5 bits, 5 bits, 5 bits)

Actually, this 15-bit pixel value is stored in 16 bits; an extra bit is left over which can be used to support the overlaying of images:

(R,G,B,T) = (16 bits) = (5 bits, 5 bits, 5 bits, 1 bit overlay)

The image creator or rendering application can toggle the overlay bit, which is interpreted by the display hardware as a command to ignore that particular pixel. In this way, two images can be overlaid, and the top one adjusted to allow holes through which portions of the bottom image are visible.

This technique is in widespread, but not obvious, use. A rendering application can selectively toggle the overlay bit in pixel values of a particular color. More to the point, the application can turn off the display of any area of an image that is not a particular color. For example, if a rendering application encounters an image of a person standing in front of a contrasting, uniformly colored and lighted screen, the application can toggle the overlay bits on all the pixel values that are the color of the screen, leaving an image of the person cut out from the background. This cut-out image can then be overlaid on any other image, effectively adding the image of the person to the bottom image.

This assumes, of course, that the color of the screen is different from any colors in the person portion of the image. This is often how broadcast television weather reporters are overlaid on background maps and displays, for instance. Certain conventions inherited from traditional analog broadcasting technology are in widespread use in the broadcasting industry, including the use of a particular blue shade for background screens. When used in this way, the process is called chromakeying.

A more elaborate mechanism for specifying image overlays allows variations in transparency between bottom and overlaid images. Instead of having a single bit of overlay information, each pixel value has more (usually eight bits). An example is the 32-bit variant of the TGA format, which also supports data in the format:

(24 bits) = (R,G,B) = (8 bits, 8 bits, 8 bits)

Because this 24-bit pixel value is stored in 32 bits, an extra eight bits are left over to support transparency:

(R,G,B,T) = (8 bits, 8 bits, 8 bits, 8 bits transparency)

The eight transparency bits are sometimes called the alpha channel. Although there are some complications in the TGA format, an ideal 8-bit alpha channel can support 256 levels of transparency, from zero (indicating that the pixel is meant to be completely transparent) to 255 (indicating that the pixel is meant to be opaque).

Transparency data is usually stored as part of the pixel data, as in the example above, but it may also appear as a fourth plane, stored the same way as palette data in planar format files. It can, however, be stored as a separate block, independent of other image and palette information, and with the same dimensions as the actual image. This allows manipulation of the transparency data independent of the image pixel data.