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88 lines
3.5 KiB
TeX
88 lines
3.5 KiB
TeX
\section{Built-in module \sectcode{imageop}}
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\bimodindex{imageop}
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The 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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This function takes the image in \code{image}, which should by
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\code{width} by \code{height} in size and consist of pixels of
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\code{psize} bytes, and returns the selected part of that image. \code{x0},
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\code{y0}, \code{x1} and \code{y1} are like the \code{lrectread}
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parameters, i.e. the boundary is included in the new image.
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The new boundaries need not be inside the picture. Pixels that fall
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outside the old image will have their value set to zero.
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If \code{x0} is bigger than \code{x1} the new image is mirrored. The
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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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This function returns a \code{image} scaled to size \code{newwidth} by
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\code{newheight}. No interpolation is done, scaling is done by
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simple-minded pixel duplication or removal. Therefore, computer-generated
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images or 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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This function runs a vertical low-pass filter over an image. It does
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so by computing each destination pixel as the average of two
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vertically-aligned source pixels. The main use of this routine is to
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forestall excessive flicker if the image is displayed on a video
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device that uses 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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This function converts a 8-bit deep greyscale image to a 1-bit deep
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image by tresholding all the pixels. The resulting image is tightly
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packed and 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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This function also converts an 8-bit greyscale image to a 1-bit
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monochrome image but it uses a (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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This function converts a 1-bit monochrome image to an 8 bit greyscale
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or color image. All pixels that are zero-valued on input get value
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\code{p0} on output and all one-value input pixels get value \code{p1}
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on output. To convert a monochrome black-and-white image to greyscale
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pass the 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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