21.2. "imageop" — Manipulate raw image data
*******************************************

Obsolète depuis la version 2.6: The "imageop" module has been removed
in Python 3.

The "imageop" module contains some useful operations on images. It
operates on images consisting of 8 or 32 bit pixels stored in Python
strings.  This is the same format as used by "gl.lrectwrite()" and the
"imgfile" module.

The module defines the following variables and functions:

exception imageop.error

   This exception is raised on all errors, such as unknown number of
   bits per pixel, etc.

imageop.crop(image, psize, width, height, x0, y0, x1, y1)

   Return the selected part of *image*, which should be *width* by
   *height* in size and consist of pixels of *psize* bytes. *x0*,
   *y0*, *x1* and *y1* are like the "gl.lrectread()" parameters, i.e.
   the boundary is included in the new image. The new boundaries need
   not be inside the picture.  Pixels that fall outside the old image
   will have their value set to zero.  If *x0* is bigger than *x1* the
   new image is mirrored.  The same holds for the y coordinates.

imageop.scale(image, psize, width, height, newwidth, newheight)

   Return *image* scaled to size *newwidth* by *newheight*. No
   interpolation is done, scaling is done by simple-minded pixel
   duplication or removal.  Therefore, computer-generated images or
   dithered images will not look nice after scaling.

imageop.tovideo(image, psize, width, height)

   Run a vertical low-pass filter over an image.  It does so by
   computing each destination pixel as the average of two vertically-
   aligned source pixels.  The main use of this routine is to
   forestall excessive flicker if the image is displayed on a video
   device that uses interlacing, hence the name.

imageop.grey2mono(image, width, height, threshold)

   Convert an 8-bit deep greyscale image to a 1-bit deep image by
   thresholding all the pixels.  The resulting image is tightly packed
   and is probably only useful as an argument to "mono2grey()".

imageop.dither2mono(image, width, height)

   Convert an 8-bit greyscale image to a 1-bit monochrome image using
   a (simple-minded) dithering algorithm.

imageop.mono2grey(image, width, height, p0, p1)

   Convert a 1-bit monochrome image to an 8 bit greyscale or color
   image. All pixels that are zero-valued on input get value *p0* on
   output and all one-value input pixels get value *p1* on output.  To
   convert a monochrome black-and-white image to greyscale pass the
   values "0" and "255" respectively.

imageop.grey2grey4(image, width, height)

   Convert an 8-bit greyscale image to a 4-bit greyscale image without
   dithering.

imageop.grey2grey2(image, width, height)

   Convert an 8-bit greyscale image to a 2-bit greyscale image without
   dithering.

imageop.dither2grey2(image, width, height)

   Convert an 8-bit greyscale image to a 2-bit greyscale image with
   dithering.  As for "dither2mono()", the dithering algorithm is
   currently very simple.

imageop.grey42grey(image, width, height)

   Convert a 4-bit greyscale image to an 8-bit greyscale image.

imageop.grey22grey(image, width, height)

   Convert a 2-bit greyscale image to an 8-bit greyscale image.

imageop.backward_compatible

   If set to 0, the functions in this module use a non-backward
   compatible way of representing multi-byte pixels on little-endian
   systems.  The SGI for which this module was originally written is a
   big-endian system, so setting this variable will have no effect.
   However, the code wasn’t originally intended to run on anything
   else, so it made assumptions about byte order which are not
   universal.  Setting this variable to 0 will cause the byte order to
   be reversed on little-endian systems, so that it then is the same
   as on big-endian systems.
