Change color depth of [Buffered]Image

[Martin]

Hello,
I want to change the color depth of a given image (gif, jpg, png)
with the help of the Java API, so no extra libraries (if possible).
If you suggest using IndexColorModel then I would need your help
too - I don't know how to build r[] g[] b[] values which the
constructor needs - like values needed for 8b, 16b etc.
Any other suggestions appreciated.

Thanks in advance,
Martin