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(culled from the mailing list)
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Subject: Re: [GRASS5] Re: [GRASSLIST:10403] Transparency added
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Date: Sun, 19 Feb 2006 20:17:59 +0000
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g.pnmcomp isn't meant for end users. It's an internal tool for use by
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In essence, g.pnmcomp generates a PPM image by overlaying a series of
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PPM/PGM pairs (PPM = RGB image, PGM = alpha channel).
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The intention is that d.* programs will emit PPM/PGM pairs (by way of
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the PNG-driver code being integrated into libraster). The GUI will
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manage a set of layers; each layer consists of the data necessary to
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generate a PPM/PGM pair.
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Whenever the layer "stack" changes (by adding, removing, hiding,
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showing or re-ordering layers), the GUI will render any layers for
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which it doesn't already have the PPM/PGM pair, then re-run g.pnmcomp
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to generate the final image (just redoing the composition is a lot
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faster than redrawing everything).
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A C/C++ GUI would either have g.pnmcomp's functionality (image
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composition) built-in, or would use the system's graphics API to
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perform composition (for translucent layers, you would need OpenGL or
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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 g.pnmcomp.
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<p><i>Last changed: $Date: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $</i>