\section{Built-in module \sectcode{imageop}}
\bimodindex{imageop}

The \code{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 \var{image}, which should by
\var{width} by \var{height} in size and consist of pixels of
\var{psize} bytes, and returns the selected part of that image. \var{x0},
\var{y0}, \var{x1} and \var{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 \var{x0} is bigger than \var{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 an \var{image} scaled to size \var{newwidth} by
\var{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
\var{p0} on output and all one-value input pixels get value \var{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}
