[Award Winner] Bitmapped Images - The technical side of things explained.

[Redrobes]

In all that talk about color we completely ignored transparency which images often have. It comes in two flavors depending on whether the image is a color index type or not. If it is then its quite simple. One of the index colors is assigned to be the transparent color type so any pixel which references that index is transparent. For the other type of image you need another channel or component. This channel is called the Alpha Channel and it usually (but not always) has as many bits as the main RGB color channels. The net effect is to have RGBA images. This means that a true color image with alpha transparency has 4 bytes instead of 3. What it also implies is that the transparency has a range of 256 shades so that it can range from completely transparent to slightly transparent through to nearly opaque and then fully opaque. Note however that color index type images with transparency have the one single color index so that these types can only do either fully opaque or fully transparent and nothing else between the two. The upside is that you only lose one color index and the number of bits stays the same so the image size does not increase.

Most virtual table top (VTT) applications use the alpha channel in full color images to provide transparency and thus allow images to take on shapes other than rectanges - e.g. characters holding weapons and shields.