How to get pixel color from a Pixmap

Question

I can use XGetPixel() to get a pixel from an XImage. What do I use to get a pixel from a Pixmap?

Answer 1

It would be nice if there was at least one function to pull a pixel from the server's drawable. Sadly, there isn't one. However the following might be of some use.

Since Pixmap is a Drawable you can pass XGetImage() the Pixmap which will return a pointer to an XImage. Now that you have an XImage you can use XGetPixel().

XGetImage Parameters:

XImage *XGetImage(display, d, x, y, width, height, plane_mask, format)
        Display *display;
        Drawable d;
        int x, y;
        unsigned int width, height;
        unsigned long plane_mask;
        int format;

Better yet you could have a pre-created XImage and pass it, along with the Pixmap to XGetSubImage(). You can grab a single pixel by passing a width and height both set to 1, and then use XGetPixel() on your XImage.

XGetSubImage Parameters:

XImage *XGetSubImage(display, d, x, y, width, height, plane_mask, format, dest_image, dest_x, 
                     dest_y)
      Display *display;
      Drawable d;
      int x, y;
      unsigned int width, height;
      unsigned long plane_mask;
      int format;
      XImage *dest_image;
      int dest_x, dest_y;

Note: XGetSubImage() returns a pointer to the same XImage structure specified by dest_image.

Answer 2

Generally it's a bad idea from performance point of view: never read from screen, only push to it. If you need to get pixel state, maintain local buffer of screen. If you can't modify program you are using, then +1 to Jonny Henly's answer: do a GetImage request first do download a region containing your pixel first, then read locally. If you want to access multiple pixels in a loop it's better to grab them all in one request