The PNG Guide is an eBook based on Greg Roelofs' book, originally published by O'Reilly.

Grayscale with Transparency

PNG supports two kinds of transparency with grayscale and RGB images. The first is a palette-style ``cheap transparency,'' in which a single color or gray value is marked as being fully transparent. I noted earlier that the structure of tRNS depends on the image type; for grayscale images of any pixel depth, the chunk contains a 2-byte, unscaled gray value--that is, the maximum allowed value is still 2bit depth-1, even though it is stored as a 16-bit integer. This approach is very similar to GIF-style transparency in palette images and incurs only 14 bytes overhead in file size. There is no corresponding TIFF image type, and standard JPEG does not support any transparency.

Last Update: 2010-Nov-26