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+\section{Built-in module \sectcode{imageop}}
+\bimodindex{imageop}
+
+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 \code{gl.lrectwrite} and the \code{imgfile} module.
+
+The module defines the following variables and functions:
+
+\renewcommand{\indexsubitem}{(in module imageop)}
+
+\begin{excdesc}{error}
+This exception is raised on all errors, such as unknown number of bits
+per pixel, etc.
+\end{excdesc}
+
+
+\begin{funcdesc}{crop}{image\, psize\, width\, height\, x0\, y0\, x1\, y1}
+This function takes the image in \code{image}, which should by
+\code{width} by \code{height} in size and consist of pixels of
+\code{psize} bytes, and returns the selected part of that image. \code{X0},
+\code{y0}, \code{x1} and \code{y1} are like the \code{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 \code{x0} is bigger than \code{x1} the new image is mirrored. The
+same holds for the y coordinates.
+\end{funcdesc}
+
+\begin{funcdesc}{scale}{image\, psize\, width\, height\, newwidth\, newheight}
+This function returns a \code{image} scaled to size \code{newwidth} by
+\code{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.
+\end{funcdesc}
+
+\begin{funcdesc}{tovideo}{image\, psize\, width\, height}
+This function runs 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.
+\end{funcdesc}
+
+\begin{funcdesc}{grey2mono}{image\, width\, height\, threshold}
+This function converts a 8-bit deep greyscale image to a 1-bit deep
+image by tresholding all the pixels. The resulting image is tightly
+packed and is probably only useful as an argument to \code{mono2grey}.
+\end{funcdesc}
+
+\begin{funcdesc}{dither2mono}{image\, width\, height}
+This function also converts an 8-bit greyscale image to a 1-bit
+monochrome image but it uses a (simple-minded) dithering algorithm.
+\end{funcdesc}
+
+\begin{funcdesc}{mono2grey}{image\, width\, height\, p0\, p1}
+This function converts a 1-bit monochrome image to an 8 bit greyscale
+or color image. All pixels that are zero-valued on input get value
+\code{p0} on output and all one-value input pixels get value \code{p1}
+on output. To convert a monochrome black-and-white image to greyscale
+pass the values \code{0} and \code{255} respectively.
+\end{funcdesc}
+
+\begin{funcdesc}{grey2grey4}{image\, width\, height}
+Convert an 8-bit greyscale image to a 4-bit greyscale image without
+dithering.
+\end{funcdesc}
+
+\begin{funcdesc}{grey2grey2}{image\, width\, height}
+Convert an 8-bit greyscale image to a 2-bit greyscale image without
+dithering.
+\end{funcdesc}
+
+\begin{funcdesc}{dither2grey2}{image\, width\, height}
+Convert an 8-bit greyscale image to a 2-bit greyscale image with
+dithering. As for \code{dither2mono}, the dithering algorithm is
+currently very simple.
+\end{funcdesc}
+
+\begin{funcdesc}{grey42grey}{image\, width\, height}
+Convert a 4-bit greyscale image to an 8-bit greyscale image.
+\end{funcdesc}
+
+\begin{funcdesc}{grey22grey}{image\, width\, height}
+Convert a 2-bit greyscale image to an 8-bit greyscale image.
+\end{funcdesc}