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<em>g.pnmcomp</em> isn't meant for end users. It's an internal tool
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for use by <em><a href="wxGUI.html">wxGUI</a></em>.
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In essence, <em>g.pnmcomp</em> generates a PPM image by overlaying a
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series of PPM/PGM pairs (PPM = RGB image, PGM = alpha channel).
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The intention is that <em>d.*</em> modules will emit PPM/PGM pairs (by
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way of the PNG-driver code being integrated into Display Library). The
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GUI will manage a set of layers; each layer consists of the data
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necessary to generate a PPM/PGM pair.
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Whenever the layer "stack" changes (by adding, removing,
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hiding, showing or re-ordering layers), the GUI will render any layers
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for which it doesn't already have the PPM/PGM pair, then re-run
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<em>g.pnmcomp</em> to generate the final image (just redoing the
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composition is a lot faster than redrawing everything).
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A C/C++ GUI would either have <em>g.pnmcomp's</em> functionality
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(image composition) built-in, or would use the system's graphics API
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to perform composition (for translucent layers, you would need OpenGL
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or the Render extension, or something else which supports translucent
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Tk doesn't support transparent (masked) true-colour images (it does
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support transparent GIFs, but that's limited to 256 colours), and an
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image composition routine in Tcl would be unacceptably slow, hence
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the existence of <em>g.pnmcomp</em>.
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<a href="g.cairocomp.html">g.cairocomp</a>
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<p><i>Last changed: $Date: 2013-01-15 07:12:43 -0800 (Tue, 15 Jan 2013) $</i>