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Gtk+/Gnome Application Development
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How do I read the pixels out of a GdkPixmap?

First and foremost: remember that a pixmap is a server-side resource, i.e. possibly across a network and definitely across some kind of socket. Therefore, you do not want to request its pixels one by one. Iterating over a pixmap that way could easily take many seconds.

GDK wraps an Xlib object called XImage. The wrapper is called GdkImage. A GdkImage is essentially a local copy of the data in a pixmap. You can copy a region of a pixmap or window into a GdkImage with the gdk_image_get() routine, then get and set pixels with gdk_image_get_pixel() and gdk_image_put_pixel(). You can also access the image's data structures directly, but this is quite complicated (due to visuals, depths, differences between host and network byte order, and so on). If you modify the image, you use gdk_draw_image() to copy it back to a server-side drawable.

Copying a pixmap to a GdkImage, or copying a GdkImage to a pixmap, still involves moving quite a bit of data over the network; however, since it's all in one burst the speed can be tolerable in many cases. Also, if the client and the server are on the same machine, and the X shared memory extension is available, GDK will automatikcally set up a shared memory segment to copy the data.

Most of the time, if you plan to do a lot of image manipulation, you are better off using RGB buffers as your primary data structure (see the section called RGB Buffers in the chapter called GDK Basics). The functions in gdk/gdkrgb.h allow you to copy an RGB buffer to a drawable. These functions use GdkImage internally, but they are tuned to be very fast and handle all the complexities for you.

Gtk+/Gnome Application Development
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