Writing out an XLib window to an image?

siepmann · 06-06-2007, 08:15 AM

I have very little experience in C, and none with XLib (I'm a Java user) so what I'm about to ask might not make sense, but...

I want to capture what I see in an XLib window (which I understand is a portion of the the program display) and stick it into an image file.

Is this possible?

Any help grateful received!

Many thanks,

Peter

theNbomr · 06-07-2007, 12:57 AM

xwd does precisely what you are asking for.
--- rod.

siepmann · 06-07-2007, 04:59 AM

Thanks very much for this. I had some trouble using xwd, though, probably because I'm trying to do the capture programmatically, not from the command line. Instead, though, I found the XGetImage function and so I've got

Code:

XImage* ximg = XGetImage(display, playground, 0, 0, 100, 100, ~0, XYPixmap);

where 'playground' is the Window I want to write out.

My next step is to try and write this out to an image file, so I tried:

Code:

XWriteBitmapFile(display, "name", (XImage)&ximg, 100, 100, -1, -1);

but this doesn't compile:

error: conversion to non-scalar type requested

Any more help grateful received.

Thanks so much!

Peter

siepmann · 06-07-2007, 05:22 AM

Update: now discovered the XGetPixel function, so I can loop through my Ximage getting each pixel value:

Code:

int pix = XGetPixel(ximg, xp, yp);

Now I just need to know how to make an image file from an array of pixels!...

siepmann · 06-07-2007, 05:32 AM

Problem solved. I hope this thread may be interesting to others trying to do the same thing.

Code:

printf("P3 %d %d 255\n", pwidth, pheight);

 for (xp=0; xp<pwidth; xp++) {
   for (yp=0; yp<pheight; yp++) {
     long pix = XGetPixel(ximg, xp, yp);
     printf("%ld %ld %ld\n", pix>>16, pix>>8&255, pix&255);
   }
 }

results in a nice ppm file.