This program is part of Netpbm.
xwdtopnm reads an X11 or X10 window dump file as input and produces a PNM image as output. The type of the output image depends on the input file - if it's black and white, the output is PBM. If it's grayscale, the output is PGM. Otherwise, it's PPM. The program tells you which type it is writing.
Using this program, you can convert anything you can display on an X workstation's screen into a PNM image. Just display whatever you're interested in, run the xwd program to capture the contents of the window, run it through xwdtopnm, and then use pamcut to select the part you want.
Note that a pseudocolor XWD image (typically what you get when you make a dump of a pseudocolor X window) has maxval 65535, which means the PNM file that xwdtopnm generates has maxval 65535. Many older image processing programs (that aren't part of the Netpbm package and don't use the Netpbm programming library) don't know how to handle a PNM image with maxval greater than 255 (because there are two bytes instead of one for each sample in the image). So you may want to run the output of xwdtopnm through pamdepth before feeding it to one of these old programs.
xwdtopnm can't convert every kind of XWD image (which essentially means it can't convert an XWD created from every kind of X display configuration). In particular, it cannot convert one with more than 24 bits per pixel.
xwdtopnm sometimes produces output with a maxval greater than 255, which means the maximum value of a sample (one intensity value, e.g. the red component of a pixel) is greater than 255 and therefore each sample takes 2 bytes to represent. This can be a problem because some programs expect those bytes in a different order from what the Netpbm format specs say, which is what xwdtopnm produces, which means they will see totally different colors that they should. xv is one such program.
If this is a problem (e.g. you want to look at the output of xwdtopnm with xv), there are two ways to fix it:
Often, there is no good reason to have a maxval greater than 255. It happens because in XWD, byte not PNM, each color component of a pixel can have different resolution, for example 5 bits for blue (maxval 31), 5 bits for red (maxval 31), and 6 bits for green (maxval 63), for a total of 16 bits per pixel. In order to reproduce the colors as closely as possible, xwdtopnm has to use a large maxval. In this example, it would use 31 * 63 = 1953, and use 48 bits per pixel.
Because this is a common and frustrating problem when using xwdtopnm, the program issues a warning whenever it generates output with two byte samples. You can quiet this warning with the -quiet common option. The warning was new in Netpbm 10.46 (March 2009).