Geografic-Information-System/general/g.pnmcomp/g.pnmcomp.html

<h2>DESCRIPTION</h2>

<em>g.pnmcomp</em> isn't meant for end users. It's an internal tool
for use by <em><a href="wxGUI.html">wxGUI</a></em>.

<p>
In essence, <em>g.pnmcomp</em> generates a PPM image by overlaying a
series of PPM/PGM pairs (PPM = RGB image, PGM = alpha channel).

<h2>NOTES</h2>

The intention is that <em>d.*</em> 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 &quot;stack&quot; 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
<em>g.pnmcomp</em> to generate the final image (just redoing the
composition is a lot faster than redrawing everything).

<p>
A C/C++ GUI would either have <em>g.pnmcomp's</em> 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).

<p>
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 <em>g.pnmcomp</em>.

<h2>SEE ALSO</h2>

<em>
<a href="g.cairocomp.html">g.cairocomp</a>
</em>

<h2>AUTHOR</h2>

Glynn Clements