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.