24 bit modes
------------

The main new feature in zgv v2.0 is 15/16/24-bit colour support. The way it
works is by providing either the old 8-bit or a new 24-bit mode of
operation for JPEG files. (GIF files are always done as before, since they
can't ever do more than 256 colours.)

The default is to use 24-bit if possible. You can change this to always use
8-bit by putting 'jpeg24bit off' in /etc/system.zgvrc or $HOME/.zgvrc - see
'sample.zgvrc' and the man page for more details.


Here are the advantages of each mode;

Advantages of 8-bit mode

* Works on 8-bit displays :)
* Rotation/mirroring/zoom mode faster
* Brightness and contrast can be changed


Advantages of 24-bit mode

* Shows JPEG files in all their glory
* Faster (sometimes much faster) loading of JPEGs


There are, however, some problems using 24-bit mode with 15 or 16-bit screen
modes;

* The image is converted to the 2-byte-per-pixel format on the fly
* These modes actually have less colour depth than 256-colour modes

A short explanation of the last point is probably in order. 8-bit screen
modes will show 256 from 262,144 different colours. A 15 or 16-bit screen
mode will display either 32,768 or 65,536 colours (respectively) at once,
but these colours are unchangeable.

A good way to illustrate this point is by considering a 256-greyscale JPEG
image. In 256 colour mode, only 64 greys can be shown, since
2^(log2(262144)/3)=64. With 15-bit mode, even less can be shown -
2^(log2(32768)/3)=32. Because of the way that 16-bit modes work, which is by
providing the green channel with twice as much colour depth (i.e. an extra
bit), they can also only show 32 greys. Put another way,
2^int(log2(65536)/3)=32. (Of course, 24-bit mode will show all 256.)

Despite this, my advice is - if you've got it, flaunt it.
