My shader system operates on many types of maps: color(RGB), alpha,

bump, lumma, specular, nomral(UVW), damage, team color ... That are
all stored in different formats of packed and unpacked ways.
Not all objects have all of the maps.  If an object has color and spec
map i can easy pack it into red,green,blue,spec texture.
If a object has color,lumma,specular,normal map again i can pack it
most effective in 2 textures (rgbl + suvw).

I needed a way to repack them easily.  At first i coded the function
in python and it was pretty slow.  So I rewrote it in c++.

panda/src/pnmimage/pnmImage.cxx

@@ -110,6 +110,39 @@ copy_from(const PNMImage &copy) {
   }
 }
 
+////////////////////////////////////////////////////////////////////
+//     Function: PNMImage::copy_channel
+//       Access: Published
+//  Description: Copies a channel from one image into another.
+//               Images must be the same size
+////////////////////////////////////////////////////////////////////
+void PNMImage::
+copy_channel(const PNMImage &copy, int src_channel, int dest_channel) {
+  // Make sure the channels are in range
+  nassertv(src_channel >= 0 && src_channel <= 3);
+  nassertv(dest_channel >= 0 && dest_channel <= 3);
+  // Make sure that images are valid
+  if (!copy.is_valid() || !is_valid()) {
+    pnmimage_cat.error() << "One of the images is invalid!\n";
+    return;
+  }
+  // Make sure the images are the same size
+  if (_x_size != copy.get_x_size() || _y_size != copy.get_y_size()){
+    pnmimage_cat.error() << "Image size doesn't match!\n";
+    return;
+  }
+  // Do the actual copying
+  for (int x = 0; x < _x_size; x++) {
+    for (int y = 0; y < _y_size; y++) {
+      LVecBase4d t = get_xel_a(x, y);
+      LVecBase4d o = copy.get_xel_a(x, y);
+      t.set_cell(dest_channel,o.get_cell(src_channel));
+      set_xel_a(x, y, t);
+    }
+  }
+}
+
+
 ////////////////////////////////////////////////////////////////////
 //     Function: PNMImage::copy_header_from
 //       Access: Published

panda/src/pnmimage/pnmImage.h

@@ -73,6 +73,7 @@ PUBLISHED:
              xelval maxval = 255, PNMFileType *type = NULL);
 
   void copy_from(const PNMImage &copy);
+  void copy_channel(const PNMImage &copy, int src_channel, int dest_channel);
   void copy_header_from(const PNMImageHeader &header);
   void take_from(PNMImage &orig);