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