GRASS GIS manual: g.pnmcomp

Note: A new GRASS GIS stable version has been released: GRASS GIS 7.4, available here.
Updated manual page: here

NAME

g.pnmcomp - Overlays multiple PPM image files.

KEYWORDS

general, display

SYNOPSIS

g.pnmcomp

g.pnmcomp --help

g.pnmcomp input=name[,name,...] [mask=name[,name,...]] [opacity=float[,float,...]] output=name [output_mask=name] width=integer height=integer [bgcolor=name] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

Flags:

--overwrite: Allow output files to overwrite existing files
--help: Print usage summary
--verbose: Verbose module output
--quiet: Quiet module output
--ui: Force launching GUI dialog

Parameters:

input=name[,name,...] [required]: Name of input file(s)
mask=name[,name,...]: Name of input mask file(s)
opacity=float[,float,...]: Layer opacities
output=name [required]: Name for output file
output_mask=name: Name for output mask file
width=integer [required]: Image width
height=integer [required]: Image height
bgcolor=name: Background color; Either a standard color name or R:G:B triplet

DESCRIPTION
NOTES
SEE ALSO
AUTHOR

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.