DESCRIPTION
g.pnmcomp isn't meant for end users. It's an internal tool
for use by wxGUI.
In essence, g.pnmcomp generates a PPM image by overlaying a
series of PPM/PGM pairs (PPM = RGB image, PGM = alpha channel).
NOTES
The intention is that d.* modules will emit PPM/PGM pairs (by
way of the PNG-driver code being integrated into Display Library). The
GUI will manage a set of layers; each layer consists of the data
necessary to generate a PPM/PGM pair.
Whenever the layer "stack" changes (by adding, removing,
hiding, showing or re-ordering layers), the GUI will render any layers
for which it doesn't already have the PPM/PGM pair, then re-run
g.pnmcomp to generate the final image (just redoing the
composition is a lot faster than redrawing everything).
A C/C++ GUI would either have g.pnmcomp's functionality
(image composition) built-in, or would use the system's graphics API
to perform composition (for translucent layers, you would need OpenGL
or the Render extension, or something else which supports translucent
rendering).
Tk doesn't support transparent (masked) true-colour images (it does
support transparent GIFs, but that's limited to 256 colours), and an
image composition routine in Tcl would be unacceptably slow, hence
the existence of g.pnmcomp.
SEE ALSO
g.cairocomp
AUTHOR
Glynn Clements
Last changed: $Date$