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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Built-in Module \sectcode{imageop}}
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2 \bimodindex{imageop}
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3
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4 The \code{imageop} module contains some useful operations on images.
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5 It operates on images consisting of 8 or 32 bit pixels
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6 stored in Python strings. This is the same format as used
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7 by \code{gl.lrectwrite} and the \code{imgfile} module.
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8
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9 The module defines the following variables and functions:
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10
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11 \renewcommand{\indexsubitem}{(in module imageop)}
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12
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13 \begin{excdesc}{error}
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14 This exception is raised on all errors, such as unknown number of bits
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15 per pixel, etc.
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16 \end{excdesc}
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17
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18
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19 \begin{funcdesc}{crop}{image\, psize\, width\, height\, x0\, y0\, x1\, y1}
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20 Return the selected part of \var{image}, which should by
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21 \var{width} by \var{height} in size and consist of pixels of
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22 \var{psize} bytes. \var{x0}, \var{y0}, \var{x1} and \var{y1} are like
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23 the \code{lrectread} parameters, i.e.\ the boundary is included in the
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24 new image. The new boundaries need not be inside the picture. Pixels
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25 that fall outside the old image will have their value set to zero. If
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26 \var{x0} is bigger than \var{x1} the new image is mirrored. The same
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27 holds for the y coordinates.
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28 \end{funcdesc}
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29
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30 \begin{funcdesc}{scale}{image\, psize\, width\, height\, newwidth\, newheight}
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31 Return \var{image} scaled to size \var{newwidth} by \var{newheight}.
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32 No interpolation is done, scaling is done by simple-minded pixel
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33 duplication or removal. Therefore, computer-generated images or
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34 dithered images will not look nice after scaling.
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35 \end{funcdesc}
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36
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37 \begin{funcdesc}{tovideo}{image\, psize\, width\, height}
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38 Run a vertical low-pass filter over an image. It does so by computing
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39 each destination pixel as the average of two vertically-aligned source
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40 pixels. The main use of this routine is to forestall excessive
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41 flicker if the image is displayed on a video device that uses
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42 interlacing, hence the name.
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43 \end{funcdesc}
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44
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45 \begin{funcdesc}{grey2mono}{image\, width\, height\, threshold}
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46 Convert a 8-bit deep greyscale image to a 1-bit deep image by
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47 tresholding all the pixels. The resulting image is tightly packed and
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48 is probably only useful as an argument to \code{mono2grey}.
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49 \end{funcdesc}
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50
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51 \begin{funcdesc}{dither2mono}{image\, width\, height}
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52 Convert an 8-bit greyscale image to a 1-bit monochrome image using a
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53 (simple-minded) dithering algorithm.
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54 \end{funcdesc}
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55
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56 \begin{funcdesc}{mono2grey}{image\, width\, height\, p0\, p1}
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57 Convert a 1-bit monochrome image to an 8 bit greyscale or color image.
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58 All pixels that are zero-valued on input get value \var{p0} on output
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59 and all one-value input pixels get value \var{p1} on output. To
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60 convert a monochrome black-and-white image to greyscale pass the
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61 values \code{0} and \code{255} respectively.
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62 \end{funcdesc}
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63
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64 \begin{funcdesc}{grey2grey4}{image\, width\, height}
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65 Convert an 8-bit greyscale image to a 4-bit greyscale image without
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66 dithering.
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67 \end{funcdesc}
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68
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69 \begin{funcdesc}{grey2grey2}{image\, width\, height}
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70 Convert an 8-bit greyscale image to a 2-bit greyscale image without
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71 dithering.
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72 \end{funcdesc}
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73
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74 \begin{funcdesc}{dither2grey2}{image\, width\, height}
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75 Convert an 8-bit greyscale image to a 2-bit greyscale image with
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76 dithering. As for \code{dither2mono}, the dithering algorithm is
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77 currently very simple.
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78 \end{funcdesc}
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79
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80 \begin{funcdesc}{grey42grey}{image\, width\, height}
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81 Convert a 4-bit greyscale image to an 8-bit greyscale image.
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82 \end{funcdesc}
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83
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84 \begin{funcdesc}{grey22grey}{image\, width\, height}
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85 Convert a 2-bit greyscale image to an 8-bit greyscale image.
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86 \end{funcdesc}