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87 lines
3.4 KiB
TeX
87 lines
3.4 KiB
TeX
\section{Built-in Module \sectcode{imageop}}
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\bimodindex{imageop}
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The \code{imageop} module contains some useful operations on images.
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It operates on images consisting of 8 or 32 bit pixels
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stored in Python strings. This is the same format as used
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by \code{gl.lrectwrite} and the \code{imgfile} module.
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The module defines the following variables and functions:
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\renewcommand{\indexsubitem}{(in module imageop)}
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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 \code{lrectread} parameters, i.e.\ the boundary is included in the
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new image. The new boundaries need not be inside the picture. Pixels
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that fall outside the old image will have their value set to zero. If
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\var{x0} is bigger than \var{x1} the new image is mirrored. The same
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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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tresholding all the pixels. The resulting image is tightly packed and
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is probably only useful as an argument to \code{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 \code{dither2mono}, the dithering algorithm is
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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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