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+#!/usr/bin/env python
+# $URL: http://pypng.googlecode.com/svn/trunk/code/pipstack $
+# $Rev: 190 $
+
+# pipstack
+# Combine input PNG files into a multi-channel output PNG.
+
+"""
+pipstack file1.png [file2.png ...]
+
+pipstack can be used to combine 3 greyscale PNG files into a colour, RGB,
+PNG file.  In fact it is slightly more general than that.  The number of
+channels in the output PNG is equal to the sum of the numbers of
+channels in the input images.  It is an error if this sum exceeds 4 (the
+maximum number of channels in a PNG image is 4, for an RGBA image).  The
+output colour model corresponds to the number of channels: 1 -
+greyscale; 2 - greyscale+alpha; 3 - RGB; 4 - RGB+alpha.
+
+In this way it is possible to combine 3 greyscale PNG files into an RGB
+PNG (a common expected use) as well as more esoteric options: rgb.png +
+grey.png = rgba.png; grey.png + grey.png = greyalpha.png.
+
+Color Profile, Gamma, and so on.
+
+[This is not implemented yet]
+
+If an input has an ICC Profile (``iCCP`` chunk) then the output will
+have an ICC Profile, but only if it is possible to combine all the input
+ICC Profiles.  It is possible to combine all the input ICC Profiles
+only when: they all use the same Profile Connection Space; the PCS white
+point is the same (specified in the header; should always be D50);
+possibly some other things I haven't thought of yet.
+
+If some of the inputs have a ``gAMA`` chunk (specifying gamma) and
+an output ICC Profile is being generated, then the gamma information
+will be incorporated into the ICC Profile.
+
+When the output is an RGB colour type and the output ICC Profile is
+synthesized, it is necessary to supply colorant tags (``rXYZ`` and so
+on).  These are taken from ``sRGB``.
+
+If the input images have ``gAMA`` chunks and no input image has an ICC
+Profile then the output image will have a ``gAMA`` chunk, but only if
+all the ``gAMA`` chunks specify the same value.  Otherwise a warning
+will be emitted and no ``gAMA`` chunk.  It is possible to add or replace
+a ``gAMA`` chunk using the ``pipchunk`` tool.
+
+gAMA, pHYs, iCCP, sRGB, tIME, any other chunks.
+"""
+
+class Error(Exception):
+    pass
+
+def stack(out, inp):
+    """Stack the input PNG files into a single output PNG."""
+
+    from array import array
+    import itertools
+    # Local module
+    import png
+
+    if len(inp) < 1:
+        raise Error("Required input is missing.")
+
+    l = map(png.Reader, inp)
+    # Let data be a list of (pixel,info) pairs.
+    data = map(lambda p: p.asDirect()[2:], l)
+    totalchannels = sum(map(lambda x: x[1]['planes'], data))
+
+    if not (0 < totalchannels <= 4):
+        raise Error("Too many channels in input.")
+    alpha = totalchannels in (2,4)
+    greyscale = totalchannels in (1,2)
+    bitdepth = []
+    for b in map(lambda x: x[1]['bitdepth'], data):
+        try:
+            if b == int(b):
+                bitdepth.append(b)
+                continue
+        except (TypeError, ValueError):
+            pass
+        # Assume a tuple.
+        bitdepth += b
+    # Currently, fail unless all bitdepths equal.
+    if len(set(bitdepth)) > 1:
+        raise NotImplemented("Cannot cope when bitdepths differ - sorry!")
+    bitdepth = bitdepth[0]
+    arraytype = 'BH'[bitdepth > 8]
+    size = map(lambda x: x[1]['size'], data)
+    # Currently, fail unless all images same size.
+    if len(set(size)) > 1:
+        raise NotImplemented("Cannot cope when sizes differ - sorry!")
+    size = size[0]
+    # Values per row
+    vpr = totalchannels * size[0]
+    def iterstack():
+        # the izip call creates an iterator that yields the next row
+        # from all the input images combined into a tuple.
+        for irow in itertools.izip(*map(lambda x: x[0], data)):
+            row = array(arraytype, [0]*vpr)
+            # output channel
+            och = 0
+            for i,arow in enumerate(irow):
+                # ensure incoming row is an array
+                arow = array(arraytype, arow)
+                n = data[i][1]['planes']
+                for j in range(n):
+                    row[och::totalchannels] = arow[j::n]
+                    och += 1
+            yield row
+    w = png.Writer(size[0], size[1],
+      greyscale=greyscale, alpha=alpha, bitdepth=bitdepth)
+    w.write(out, iterstack())
+
+
+def main(argv=None):
+    import sys
+
+    if argv is None:
+        argv = sys.argv
+    argv = argv[1:]
+    arg = argv[:]
+    return stack(sys.stdout, arg)
+
+
+if __name__ == '__main__':
+    main()