Raster Images in IDL

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Raster Images in IDL

• Matt Jolly
• NTSG, Univ. of MT.

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Outline

• Review of topics from last time
• Dimensioning two-dimensional data types
• Reading in raster data
• Focusing on reading binary images
• Displaying images in IDL
• Writing images as graphic files

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Review

• Data types define the type of information stored
  in a variable
• Floats store decimal values
• Integers store whole numbers
• Files are read into IDL variables using the OPEN
  and READ commands
• Procedure for reading data OPEN, READ, CLOSE

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Procedure for reading rasters into IDL

• Dimension the raster variable
• Open the file containing the image of interest
• Read the information from the file into the
  variable
• Close the file
• Use the variable for display or analysis

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What you need to know ahead of time

• The rows and columns of the image to read in
• The data type of the image
• Remember, sometimes if you dont know the data
  type you can figure it out
• Filesize rows columns number of bytes

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Dimensioning the variable

• As discussed in our last meeting, data types
  should be determined from the type of information
  being read in
• Use the appropriate array dimensioning function
• Normally intarr(x,y), bytarr(x,y), fltarr(x,y)
  or dblarr(x,y)
• MyVar intarr(573,361) (columns,rows)

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Opening the file

• Use the OPENR command to open the file
• OPENR, 1, dialog_pickfile()
• OPENR,lun,pickfile(),/get_lun

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Read the file as binary

• Read using the unformatted read command
• READU,1,MyVar
• MyVar is the variable you intialized earlier
• Once you have read from the file, close it!
• Close,1
• Free_lun, lun

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Color tables in IDL

• Before displaying a color image in IDL, you
  should set the decompose state to zero and load a
  color table
• Device, decompose0
• Loadct, ltcolor table numbergt OR
• Xloadct to select your color table

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Indexing in IDL

• Cartesian coordinates are always based from the
  lower left corner
• Images usually start from the upper left corner
• You can switch the origin in IDL
• Tvscl,MyVar,/Order
• /Order tells IDL to plot using the upper left
  corner as the origin
• Or do it for all images by setting the system
  variable
• !order1

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Displaying your newly read image

• Create a new window first
• Window, xsizeltxgt, ysizeltygt
• Where ltxgt is the x dimension of the data and ltygt
  is the y dimension of the data
• Use the TVSCL command in most cases
• This command commands byte-scaling and display
  into a single function
• Tvscl, MyVar
• Or you can combine the TV command with the bytscl
  function
• tv,bytscl(ltMyVargt,min0,max5000)

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• !order1

• !order0

(0,0)
(0,0)
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Writing a JPEG of your image

• Read the true color values into an array
• Use the Write_Jpeg function

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Reading the true color values into an array

• The display colors must be read in true color
  (I.E. R,G,B)
• Outimg tvrd(true1)

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Writing a JPEG

• Use the write_jpeg function
• Write_jpeg,ltfilenamegt,ltoutimggt, true1,quality
  100,/order
• ltfilenamegt is the output filename (you can use
  pickfile() here to select your own filename)
• ltoutimggt is the true color image variable you
  read using tvrd()
• True1 tells IDL to treat the variable as a true
  color image (RGB)
• Quality 100 tells IDL not to compress the image
  (Low loss)
• /Order is the same as earlier

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Example

• Data intarr(890,561)
• Openr,1,pickfile()
• Readu,1,data
• Close,1
• Device,decompose0
• Loadct,5
• Window,xsize890,ysize561
• Tv,bytscl(data,min0)
• Imgtvrd(true1)
• Write_jpeg,c\temp\img.jpeg,img,true1,quality1
  00,/order

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