def interleave_planes(ipixels, apixels, ipsize, apsize):
    """
    Interleave (colour) planes, e.g. RGB + A = RGBA.

    Return an array of pixels consisting of the `ipsize` elements of
    data from each pixel in `ipixels` followed by the `apsize` elements
    of data from each pixel in `apixels`.  Conventionally `ipixels`
    and `apixels` are byte arrays so the sizes are bytes, but it
    actually works with any arrays of the same type.  The returned
    array is the same type as the input arrays which should be the
    same type as each other.
    """

    itotal = len(ipixels)
    atotal = len(apixels)
    newtotal = itotal + atotal
    newpsize = ipsize + apsize
    # Set up the output buffer
    # See http://www.python.org/doc/2.4.4/lib/module-array.html#l2h-1356
    out = array(ipixels.typecode)
    # It's annoying that there is no cheap way to set the array size :-(
    out.extend(ipixels)
    out.extend(apixels)
    # Interleave in the pixel data
    for i in range(ipsize):
        out[i:newtotal:newpsize] = ipixels[i:itotal:ipsize]
    for i in range(apsize):
        out[i + ipsize:newtotal:newpsize] = apixels[i:atotal:apsize]
    return out

    Interleave (colour) planes, e.g. RGB + A = RGBA.

    Return an array of pixels consisting of the `ipsize` elements of
    data from each pixel in `ipixels` followed by the `apsize` elements
    of data from each pixel in `apixels`.  Conventionally `ipixels`
    and `apixels` are byte arrays so the sizes are bytes, but it
    actually works with any arrays of the same type.  The returned
    array is the same type as the input arrays which should be the
    same type as each other.
    
png.py