| 1 | \section{\module{imageop} ---
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| 2 | Manipulate raw image data}
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| 3 |
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| 4 | \declaremodule{builtin}{imageop}
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| 5 | \modulesynopsis{Manipulate raw image data.}
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| 6 |
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| 7 |
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| 8 | The \module{imageop} module contains some useful operations on images.
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| 9 | It operates on images consisting of 8 or 32 bit pixels stored in
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| 10 | Python strings. This is the same format as used by
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| 11 | \function{gl.lrectwrite()} and the \refmodule{imgfile} module.
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| 12 |
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| 13 | The module defines the following variables and functions:
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| 14 |
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| 15 | \begin{excdesc}{error}
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| 16 | This exception is raised on all errors, such as unknown number of bits
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| 17 | per pixel, etc.
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| 18 | \end{excdesc}
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| 19 |
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| 20 |
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| 21 | \begin{funcdesc}{crop}{image, psize, width, height, x0, y0, x1, y1}
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| 22 | Return the selected part of \var{image}, which should by
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| 23 | \var{width} by \var{height} in size and consist of pixels of
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| 24 | \var{psize} bytes. \var{x0}, \var{y0}, \var{x1} and \var{y1} are like
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| 25 | the \function{gl.lrectread()} parameters, i.e.\ the boundary is
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| 26 | included in the new image. The new boundaries need not be inside the
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| 27 | picture. Pixels that fall outside the old image will have their value
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| 28 | set to zero. If \var{x0} is bigger than \var{x1} the new image is
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| 29 | mirrored. The same holds for the y coordinates.
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| 30 | \end{funcdesc}
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| 31 |
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| 32 | \begin{funcdesc}{scale}{image, psize, width, height, newwidth, newheight}
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| 33 | Return \var{image} scaled to size \var{newwidth} by \var{newheight}.
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| 34 | No interpolation is done, scaling is done by simple-minded pixel
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| 35 | duplication or removal. Therefore, computer-generated images or
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| 36 | dithered images will not look nice after scaling.
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| 37 | \end{funcdesc}
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| 38 |
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| 39 | \begin{funcdesc}{tovideo}{image, psize, width, height}
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| 40 | Run a vertical low-pass filter over an image. It does so by computing
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| 41 | each destination pixel as the average of two vertically-aligned source
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| 42 | pixels. The main use of this routine is to forestall excessive
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| 43 | flicker if the image is displayed on a video device that uses
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| 44 | interlacing, hence the name.
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| 45 | \end{funcdesc}
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| 46 |
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| 47 | \begin{funcdesc}{grey2mono}{image, width, height, threshold}
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| 48 | Convert a 8-bit deep greyscale image to a 1-bit deep image by
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| 49 | thresholding all the pixels. The resulting image is tightly packed and
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| 50 | is probably only useful as an argument to \function{mono2grey()}.
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| 51 | \end{funcdesc}
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| 52 |
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| 53 | \begin{funcdesc}{dither2mono}{image, width, height}
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| 54 | Convert an 8-bit greyscale image to a 1-bit monochrome image using a
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| 55 | (simple-minded) dithering algorithm.
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| 56 | \end{funcdesc}
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| 57 |
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| 58 | \begin{funcdesc}{mono2grey}{image, width, height, p0, p1}
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| 59 | Convert a 1-bit monochrome image to an 8 bit greyscale or color image.
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| 60 | All pixels that are zero-valued on input get value \var{p0} on output
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| 61 | and all one-value input pixels get value \var{p1} on output. To
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| 62 | convert a monochrome black-and-white image to greyscale pass the
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| 63 | values \code{0} and \code{255} respectively.
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| 64 | \end{funcdesc}
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| 65 |
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| 66 | \begin{funcdesc}{grey2grey4}{image, width, height}
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| 67 | Convert an 8-bit greyscale image to a 4-bit greyscale image without
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| 68 | dithering.
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| 69 | \end{funcdesc}
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| 70 |
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| 71 | \begin{funcdesc}{grey2grey2}{image, width, height}
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| 72 | Convert an 8-bit greyscale image to a 2-bit greyscale image without
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| 73 | dithering.
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| 74 | \end{funcdesc}
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| 75 |
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| 76 | \begin{funcdesc}{dither2grey2}{image, width, height}
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| 77 | Convert an 8-bit greyscale image to a 2-bit greyscale image with
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| 78 | dithering. As for \function{dither2mono()}, the dithering algorithm
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| 79 | is currently very simple.
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| 80 | \end{funcdesc}
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| 81 |
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| 82 | \begin{funcdesc}{grey42grey}{image, width, height}
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| 83 | Convert a 4-bit greyscale image to an 8-bit greyscale image.
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| 84 | \end{funcdesc}
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| 85 |
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| 86 | \begin{funcdesc}{grey22grey}{image, width, height}
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| 87 | Convert a 2-bit greyscale image to an 8-bit greyscale image.
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| 88 | \end{funcdesc}
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| 89 |
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| 90 | \begin{datadesc}{backward_compatible}
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| 91 | If set to 0, the functions in this module use a non-backward
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| 92 | compatible way of representing multi-byte pixels on little-endian
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| 93 | systems. The SGI for which this module was originally written is a
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| 94 | big-endian system, so setting this variable will have no effect.
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| 95 | However, the code wasn't originally intended to run on anything else,
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| 96 | so it made assumptions about byte order which are not universal.
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| 97 | Setting this variable to 0 will cause the byte order to be reversed on
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| 98 | little-endian systems, so that it then is the same as on big-endian
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| 99 | systems.
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| 100 | \end{datadesc}
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