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### 53.3 Functions and Variables for pictures

Function: get_pixel (pic,x,y)

  Returns pixel from picture. Coordinates x and y range from 0 to `width-1` and `height-1`, respectively.

  Categories:Package draw ·

Function: make_level_picture
`make_level_picture` (data)
`make_level_picture` (data,width,height)

  Returns a levels picture object. `make_level_picture (data)` builds the picture object from matrix data. `make_level_picture (data,width,height)` builds the object from a list of numbers; in this case, both the width and the height must be given.

  The returned picture object contains the following four parts:

  1. symbol `level`
  2. image width
  3. image height
  4. an integer array with pixel data ranging from 0 to 255. Argument data must contain only numbers ranged from 0 to 255; negative numbers are substituted by 0, and those which are greater than 255 are set to 255.

  Example:

  Level picture from matrix.

      (%i1) make_level_picture(matrix([3,2,5],[7,-9,3000]));
      (%o1)         picture(level, 3, 2, {Array:  #(3 2 5 7 0 255)})

  Level picture from numeric list.

      (%i1) make_level_picture([-2,0,54,%pi],2,2);
      (%o1)            picture(level, 2, 2, {Array:  #(0 0 54 3)})

  Categories:Package draw ·

Function: make_rgb_picture (redlevel,greenlevel,bluelevel)

  Returns an rgb-coloured picture object. All three arguments must be levels picture; with red, green and blue levels.

  The returned picture object contains the following four parts:

  1. symbol `rgb`
  2. image width
  3. image height
  4. an integer array of length 3*width*height with pixel data ranging from 0 to 255. Each pixel is represented by three consecutive numbers (red, green, blue).

  Example:

      (%i1) red: make_level_picture(matrix([3,2],[7,260]));
      (%o1)           picture(level, 2, 2, {Array:  #(3 2 7 255)})
      (%i2) green: make_level_picture(matrix([54,23],[73,-9]));
      (%o2)           picture(level, 2, 2, {Array:  #(54 23 73 0)})
      (%i3) blue: make_level_picture(matrix([123,82],[45,32.5698]));
      (%o3)          picture(level, 2, 2, {Array:  #(123 82 45 33)})
      (%i4) make_rgb_picture(red,green,blue);
      (%o4) picture(rgb, 2, 2,
                    {Array:  #(3 54 123 2 23 82 7 73 45 255 0 33)})

  Categories:Package draw ·

Function: negative_picture (pic)

  Returns the negative of a (level or rgb) picture.

  Categories:Package draw ·

Function: picture_equalp (x,y)

  Returns `true` in case of equal pictures, and `false` otherwise.

  Categories:Package draw ·Predicate functions ·

Function: picturep (x)

  Returns `true` if the argument is a well formed image, and `false` otherwise.

  Categories:Package draw ·Predicate functions ·

Function: read_xpm (xpm_file)

  Reads a file in xpm and returns a picture object.

  Categories:Package draw ·

Function: rgb2level (pic)

  Transforms an rgb picture into a level one by averaging the red, green and blue channels.

  Categories:Package draw ·

Function: take_channel (im,color)

  If argument color is `red`, `green` or `blue`, function `take_channel` returns the corresponding color channel of picture im. Example:

      (%i1) red: make_level_picture(matrix([3,2],[7,260]));
      (%o1)           picture(level, 2, 2, {Array:  #(3 2 7 255)})
      (%i2) green: make_level_picture(matrix([54,23],[73,-9]));
      (%o2)           picture(level, 2, 2, {Array:  #(54 23 73 0)})
      (%i3) blue: make_level_picture(matrix([123,82],[45,32.5698]));
      (%o3)          picture(level, 2, 2, {Array:  #(123 82 45 33)})
      (%i4) make_rgb_picture(red,green,blue);
      (%o4) picture(rgb, 2, 2,
                    {Array:  #(3 54 123 2 23 82 7 73 45 255 0 33)})
      (%i5) take_channel(%,'green);  /* simple quote!!! */
      (%o5)           picture(level, 2, 2, {Array:  #(54 23 73 0)})

  Categories:Package draw ·

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