How can I output a colored or gray raster image without deleting the existing content?

From	markus.mail@kdwelt.de
Newsgroups	comp.lang.postscript
Subject	How can I output a colored or gray raster image without deleting the existing content?
Date	2012-08-23 13:05 -0700
Organization	http://groups.google.com
Message-ID	<eb9d9d41-e6de-4156-b5ab-be8726b3d0be@googlegroups.com> (permalink)

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Hello,

I want to display a raster image on a specified area but the existing drawing in this area should be preserved (as much as it possible and makes sense). 

With monochrome images this is no problem if you use "imagemask" (not "image"). Look at PLRM 4.10.6 (Masks): "Any portion of an image, whether black, white, color, or gray, completely obscures any marks that previously existed in the same place on the page. There is a special variant of a binary image, called a mask, whose properties are quite different. Whereas an image is opaque, a mask is partially transparent. The imagemask operator applies masks."

How can I do the same with "colorimage"? Analogous with above paragraph it would be something like "colorimagemask". Unfortunately this operator seems not to be available.

Any hints?

[P.S. On Postscript, imagemask is a somewhat special operator. It includes both what to draw _and_ where to draw. On X11/GTK this is defined separately: GDK_GC_SET_CLIP_MASK defines where to draw and GDK_DRAW_GRAY_IMAGE or GDK_DRAW_RGB_IMAGE define what to draw. For monochrome images you use GDK_DRAW_RECTANGLE after setting a color. This "fills" the image area, but effects only places where the clipping mask is true.]

Markus

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How can I output a colored or gray raster image without deleting the existing content? markus.mail@kdwelt.de - 2012-08-23 13:05 -0700
  Re: How can I output a colored or gray raster image without deleting the existing content? Helge Blischke <h.blischke@acm.org> - 2012-08-23 22:38 +0200
  Re: How can I output a colored or gray raster image without deleting the existing content? luser- -droog <mijoryx@yahoo.com> - 2012-08-23 19:29 -0700
    Re: How can I output a colored or gray raster image without deleting the existing content? markus.mail@kdwelt.de - 2012-08-24 10:46 -0700

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