eleven

1
eleven

• colors and rasters

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Color objects

• Meta represents colors with color objects
• You can create a color by saying color
  r g b
• r amount of red (0-255)
• g amount of green (0-255)
• b amount of blue (0-255)
• red c, green c, blue creturn the amount of
  red, green or blue in the color c

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Rasters (a.k.a. bitmaps)

• A bitmap is a specific kind of data object that
  represents an image
• E.g. JPEG files, GIF files, paint files
• But not like draw programs or the pictures weve
  been making so far
• Has a prespecified width and height (in pixels)
• It allows you to explicitly retrieve (or change)
  the color at each pixel

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Bitmap procedures

• new Bitmap string
• Reads in the the jpeg or gif file with name
  string
• Converts it to internal bitmap format and returns
  it
• height bitmap, width bitmap
• Returns the width/height of a bitmap, in pixels
• pixel bitmap x y
• Returns the specified pixel of the bitmap
• bitmap-from-procedure procedure width height
• Makes a bitmap of size widthheight
• Repeatedly calls procedure with the x and y
  coordinates of each pixel to get the color of the
  pixel

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Example
? bitmap-from-procedure x y ? color x
x x 256 256 ?

• The procedurex y ? color x x x
• Returns a color with equal amounts of red, green,
  and blue light
• So its grey
• Its brightness varies with its horizontal
  position

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Example 2
? bitmap-from-procedure x y ? color x
0 y 256 256 ?

• The procedurex y ? color x 0 y
• Changes red and blue independently
• So hue varies over space

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Example 3
? define sine-wave n ? 128
127 sin
/ n 20 ltProcedure sine-wavegt ?
bitmap-from-procedure x y ?
color 0 sine-wave x
sine-wave y 256
256
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Bitmap procedures

• map-bitmap procedure bitmap
• Makes a new bitmap of the same size as bitmap
• Calls procedure with each pixel of bitmap
• Procedure returns the color of the respective
  pixel for the new bitmap
• Returns the new bitmap

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Extracting color components of an image

• define cones
• new Bitmap c/documents and settings/ian/my
  documents/cones.jpg

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Extracting color components of an image

• define gray i ? color i i i
• ltProcedure graygt
• map-bitmap c ? gray
  red c cones

• map-bitmap c ? gray green c
  cones

Note the gray procedure is already built in