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as "This is the same format as used by gl.lrectwrite() and the imgfile module." This implies a certain byte order in multi-byte pixel formats. However, the code was originally written on an SGI (big-endian) and *uses* the fact that bytes are stored in a particular order in ints. This means that the code uses and produces different byte order on little-endian systems. This fix adds a module-level flag "backward_compatible" (default not set, and if not set, behaves as if set to 1--i.e. backward compatible) that can be used on a little-endian system to use the same byte order as the SGI. Using this flag it is then possible to prepare SGI-compatible images on a little-endian system. This patch is the result of a (small) discussion on python-dev and was submitted to SourceForge as patch #874358.
100 lines
4 KiB
TeX
100 lines
4 KiB
TeX
\section{\module{imageop} ---
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Manipulate raw image data}
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\declaremodule{builtin}{imageop}
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\modulesynopsis{Manipulate raw image data.}
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The \module{imageop} module contains some useful operations on images.
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It operates on images consisting of 8 or 32 bit pixels stored in
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Python strings. This is the same format as used by
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\function{gl.lrectwrite()} and the \refmodule{imgfile} module.
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The module defines the following variables and functions:
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\begin{excdesc}{error}
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This exception is raised on all errors, such as unknown number of bits
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per pixel, etc.
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\end{excdesc}
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\begin{funcdesc}{crop}{image, psize, width, height, x0, y0, x1, y1}
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Return the selected part of \var{image}, which should by
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\var{width} by \var{height} in size and consist of pixels of
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\var{psize} bytes. \var{x0}, \var{y0}, \var{x1} and \var{y1} are like
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the \function{gl.lrectread()} parameters, i.e.\ the boundary is
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included in the new image. The new boundaries need not be inside the
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picture. Pixels that fall outside the old image will have their value
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set to zero. If \var{x0} is bigger than \var{x1} the new image is
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mirrored. The same holds for the y coordinates.
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\end{funcdesc}
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\begin{funcdesc}{scale}{image, psize, width, height, newwidth, newheight}
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Return \var{image} scaled to size \var{newwidth} by \var{newheight}.
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No interpolation is done, scaling is done by simple-minded pixel
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duplication or removal. Therefore, computer-generated images or
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dithered images will not look nice after scaling.
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\end{funcdesc}
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\begin{funcdesc}{tovideo}{image, psize, width, height}
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Run a vertical low-pass filter over an image. It does so by computing
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each destination pixel as the average of two vertically-aligned source
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pixels. The main use of this routine is to forestall excessive
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flicker if the image is displayed on a video device that uses
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interlacing, hence the name.
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\end{funcdesc}
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\begin{funcdesc}{grey2mono}{image, width, height, threshold}
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Convert a 8-bit deep greyscale image to a 1-bit deep image by
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thresholding all the pixels. The resulting image is tightly packed and
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is probably only useful as an argument to \function{mono2grey()}.
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\end{funcdesc}
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\begin{funcdesc}{dither2mono}{image, width, height}
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Convert an 8-bit greyscale image to a 1-bit monochrome image using a
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(simple-minded) dithering algorithm.
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\end{funcdesc}
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\begin{funcdesc}{mono2grey}{image, width, height, p0, p1}
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Convert a 1-bit monochrome image to an 8 bit greyscale or color image.
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All pixels that are zero-valued on input get value \var{p0} on output
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and all one-value input pixels get value \var{p1} on output. To
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convert a monochrome black-and-white image to greyscale pass the
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values \code{0} and \code{255} respectively.
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\end{funcdesc}
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\begin{funcdesc}{grey2grey4}{image, width, height}
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Convert an 8-bit greyscale image to a 4-bit greyscale image without
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dithering.
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\end{funcdesc}
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\begin{funcdesc}{grey2grey2}{image, width, height}
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Convert an 8-bit greyscale image to a 2-bit greyscale image without
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dithering.
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\end{funcdesc}
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\begin{funcdesc}{dither2grey2}{image, width, height}
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Convert an 8-bit greyscale image to a 2-bit greyscale image with
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dithering. As for \function{dither2mono()}, the dithering algorithm
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is currently very simple.
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\end{funcdesc}
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\begin{funcdesc}{grey42grey}{image, width, height}
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Convert a 4-bit greyscale image to an 8-bit greyscale image.
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\end{funcdesc}
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\begin{funcdesc}{grey22grey}{image, width, height}
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Convert a 2-bit greyscale image to an 8-bit greyscale image.
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\end{funcdesc}
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\begin{datadesc}{backward_compatible}
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If set to 0, the functions in this module use a non-backward
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compatible way of representing multi-byte pixels on little-endian
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systems. The SGI for which this module was originally written is a
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big-endian system, so setting this variable will have no effect.
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However, the code wasn't originally intended to run on anything else,
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so it made assumptions about byte order which are not universal.
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Setting this variable to 0 will cause the byte order to be reversed on
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little-endian systems, so that it then is the same as on big-endian
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systems.
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\end{datadesc}
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