Introduction to IDL Dealing with Data

1
Introduction to IDLDealing with Data

• Week 2
• Rachel McCrary
• 3-31-2009

2
Goals for today!

• Part 1
• How to open a file for reading and writing
• How to read/write ASCII or formatted data
• How to read/write unformatted or binary data
• How to read NetCDF data files
• Part 2
• How to plot to the display window
• How to use the plot command and modify it
• How to plot multiple data sets, multiple axes,
  and multiple plots on the same page
• How to add color to your plots

3
Opening Files

• All input/output in IDL is done with logical unit
  numbers.
• A logical unit number is a pipe or conduit that
  is connected between IDL and the data file that
  you want to read from or write to.
• There are three types of open commands
• openr - Open a file for Reading
• openw - Open a file for Writing
• openu - Open a file for Updating

4
Opening Files

• The syntax of these three commands is the same
  name of the command, logical unit number (lun),
  file
• openw, 20, filename.txt
• You can use logical a unit numbers directly (and
  manage them yourself) ... as in the above
  example.
• You can also have IDL manage logical unit numbers
  by using /GET_LUN
• openr, lun, filename.txt, /GET_LUN
• When you are finished with the logical unit
  numbers you can
• close it using the close command (in the first
  example) - close, 20
• or free the lun using FREE_LUN - FREE_LUN, lun

5
Reading/Writing Formatted Data

• IDL distinguishes between two types of formatted
  files with regard to reading and writing data
• Free File Format uses either commas or
  whitespace (tabs and spaces) to distinguish each
  element in the file. It is more informal than an
  explicit file format.
• An explicit format file is formatted according to
  rules specified in a format statement. The IDL
  format statement is similar to format statements
  you might use in FORTRAN.

6
Writing a Free Format File

• Writing a free format file in IDL is easy ... you
  just use the printf command to write the
  variables into a file. (This is the same as using
  print to write variables to the display)
• see example write_free_format.pro
• IDL puts white space between each element of the
  array variables and starts each variable on its
  own line.
• IDL uses the 80 column width default, you can use
  can change this using the width keyword in the
  openw command

7
Reading a Free Format File

• Many ASCII files are free format. In IDL you
  want to use the readf command to read free format
  data from a file.
• IDL uses 7 (fairly simple) rules to read free
  format data

8
Rule 1

• If reading into a string variable, all characters
  remaining on the current line are read into the
  variable.
• Example try to read in the first line of the
  file we just created called test_free_format.txt
  with the following

IDLgt OPENR, lun, 'test_free_format.txt', /GET_LUN
IDLgt header ''IDLgt READF, lun, headerIDLgt
print, headerTest data file.

• You can then read the first two lines (or any
  number of lines) in a similar way using the
  following

IDLgt header strarr(2)IDLgt READF, lun,
headerIDLgt print, headerTest data file. Created
Sun Mar 29 133552 2009
9
Rule 2

• Input data must be separated by commas or
  whitespace (spaces or tabs).
• This is exactly the kind of data that is in the
  test_free_format.txt file (after the header)!

10
Rules 3, 4 and 5

• Input is performed on scalar variables, Arrays
  and structures are treated as a collection of
  scalar variables.
• Translation - if the variable you are reading
  into contains 10 elements, then IDL will try to
  read 10 separate values from the data file. It
  will then use rules 4 5 to determine where
  those values are located in the file.
• If the current input line is empty, and there are
  still variables left requiring input, read
  another line.
• If the current input line is not empty, but there
  are no variables left requiring input, ignore the
  remainder of the line.

11
Rule 4 5

• Try the following

IDLgt data fltarr(8)IDLgt READF, lun, dataIDLgt
print, data0.00000 1.00000 2.00000 3.00000
4.00000 5.000006.00000 7.00000

• IDL reads 8 separate data values from the file.
  When it got to the end of the first row, it went
  to the second row (rule 4) because more data
  remained to be read. IDL read to the middle of
  the second row.
• Now rule 5 comes into play. If you read more
  data, you will start on the third data line,
  because the rest of the second line is ignored.

IDLgt vector3 fltarr(3)IDLgt READF, lun,
vector3IDLgt print, vector312.0000 13.0000
14.0000
12
Rule 6

• Make every effort to convert data into the data
  type expected by the variable.
• To see what this means, read the 4th and 5th
  lines into a string array, so that the file
  pointer is positioned at the sixth data line
  (starts 33.6000). Try this

IDLgt dummy strarr(2)IDLgt READF, lun, dummy

• Now suppose you want to read two integer values.
  IDL makes every effort to convert the data
  (floating point values in this case) into
  integers.
• See how the floating point values have simply
  been truncated.

IDLgt ints intarr(2)IDLgt READF, lun, intsIDLgt
print, ints 33 77
13
Rule 7

1. Complex data must have a real part and an
   imaginary part separated by a comma and enclosed
   in parentheses. If only a single variable is
   provided, it is considered the real value and the
   imaginary value is set to 0.

IDLgt value complexarr(2)IDLgt read, value
(3,4) (4.4,25.5)IDLgt print, value( 3.00000,
4.00000)( 4.40000, 25.5000)
14
Always make sure...

• When you are done with a file, make sure that you
  close it! To do this type

IDLgt FREE_LUN, lun
15
Reading a Free Format File

• Lets start by reading in the file we just created
  test_free_format.txt
• example read_free_format.pro
• notice that the data variable is a 5x5 array in
  this case. It was put into the file as a
  25-element vector. Reading the data this way is
  equivalent to reading a 25 element vector and
  reformatting the vector into a 5x5 array.
• Remember that data in IDL is stored in row order.

16
Writing a column-format data file

• It is not uncommon to see data stored in files in
  columns. It is a good idea to know how to
  read/write this sort of data in IDL ... but IDL
  has some surprises for you )
• Example write_column_ascii.pro
• As you can see, the first way does not give us
  what we want.
• Must use a loop to get the desired column format
  in our output file.

17
Reading ASCII

• 72469 DNR Denver Observations at 12Z 15 Mar 2009
• --------------------------------------------------
  ---------------------------
• PRES HGHT TEMP DWPT RELH MIXR DRCT
  SKNT THTA THTE THTV
• hPa m C C g/kg deg
  knot K K K
• --------------------------------------------------
  ---------------------------
• 1000.0 44 -9999 -9999 -9999 -9999 -9999
  -9999 -9999 -9999 -9999
• 925.0 705 -9999 -9999 -9999 -9999 -9999
  -9999 -9999 -9999 -9999
• 850.0 1413 -9999 -9999 -9999 -9999 -9999
  -9999 -9999 -9999 -9999
• 830.0 1625 0.6 -15.4 29 1.40 165
  4 288.7 293.1 289.0
• 824.0 1683 2.8 -16.2 23 1.32 209
  4 291.6 295.8 291.9
• 819.0 1732 6.0 -15.0 21 1.46 247
  4 295.5 300.2 295.8
• 816.0 1761 6.6 -15.4 19 1.42 269
  4 296.5 301.1 296.7
• 809.2 1829 6.8 -18.0 15 1.15 320
  4 297.4 301.2 297.6
• 804.0 1882 7.0 -20.0 13 0.98 319
  5 298.2 301.4 298.3
• 785.0 2076 6.0 -19.0 15 1.09 316
  10 299.1 302.7 299.3
• 779.3 2134 5.5 -19.0 15 1.10 315
  12 299.3 302.9 299.5
• 750.3 2438 3.1 -19.0 18 1.14 300
  15 299.9 303.7 300.1
• 726.0 2702 1.0 -19.0 21 1.18 291
  12 300.4 304.3 300.6
• 722.2 2743 0.8 -19.9 20 1.10 290
  12 300.7 304.3 300.9

filename - sounding.txt
18
sounding.txt - info

• File info
• 5 lines of header
• missing data filled with -9999
• 11 columns (floating pt. and integers)
• 117 rows of data

19
Read as a free-format file

• The quick and dirty way of reading the file
  sounding.txt into IDL is to treat it as a
  free-format data file, where every element is
  separated by a space.
• See example read_free_format_sounding.pro
• treat all the data in the file as floating point
  data
• read all of the data into an 11 x 117 floating
  point data array
• you will have to keep track of which column
  corresponds with which variable.
• This will not work if there are character flags
  in your data file

20
Reading a Column-Formatted Data file

• What if you want to read the file sounding.txt
  and maintain the variable type of each column,
  and put the information into different variables
  (not just one block)?
• You could try what his shown in
  read_column_wrong.pro
• This will not give an error, but nothing is read
  into the variables.
• Why doesnt this work? - apparently there is a
  rule in idl that you cannot read into a
  subscripted variable.
• Why not? - IDL passes subscripted variables into
  IDL procedures like readf by value and not by
  reference. Data that is passed by value cannot
  be changed inside of the called routine, since
  the routine has a copy of the data and not a
  pointer to the data itself.

21
Reading a Column-Formatted Data file

• One way to solve this problem is to read the data
  into a temporary variables in the loop.
• See example read_column_right.pro
• The only time this might be a problem is if you
  are trying to read a whole lot of data through
  the loop (remember loops slow things down in
  IDL).
• Remember that loops slow things down in IDL, so
  it might be better to read all the data in at
  once into an 11 x 17 array and then pull the
  vectors out of the larger array using array
  processing commands. (Again, will not work if
  there are characters in your data file)
• See example read_column_other.pro

22
Explicit File Format

• To read/write an explicit file format, use the
  same readf and printf commands that were used for
  free format files, except now the file format is
  explicitly stated with the format keyword.
• Example write_format_ascii.pro

Format Specifier What do they do
I Specifies integer data
F Specifies floating point data
D Specifies double precision data
E Specifies floating point data using scientific notation
A Specifies character data
nx Skip n character spaces
23
Example - I

• Specifies integer data. Print the vector out as
  a two-digit integers, five numbers on each line,
  each number separated by two spaces

IDLgt data findgen(20)IDLgt fmt
'(5(I2,2x),/)'IDLgt print, data, formatfmt 0 1
2 3 4 5 6 7 8 910 11 12 13 1415
16 17 18 19
24
Example - F

• Specifies floating point data. Write the data
  out as floating values with two digits to the
  right of the decimal place, one value per line.
  (Be sure to include enough space in the number
  for the decimal point itself.)

IDLgt fmt '(F5.2)'IDLgt print, data, format fmt
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
9.0010.00etc.
25
Example - D

• Specifies double precision data. Write out five
  numbers per line, four spaces between each
  number, in double precision values with 10 digits
  to the right of the decimal point.

IDLgt fmt '(5(D13.10,4x))'IDLgt print, data,
formatfmt 0.0000000000 1.0000000000
2.0000000000 3.0000000000 4.0000000000
5.0000000000 6.0000000000 7.0000000000
8.0000000000 9.000000000010.0000000000
11.0000000000 12.0000000000 13.0000000000
14.000000000015.0000000000 16.0000000000
17.0000000000 18.0000000000 19.0000000000
26
Reading/Writing Unformatted Data

• Unformatted or binary data is much more compact
  than formatted data and is often used with large
  data files.
• To read/write binary data use the readu and the
  writeu commands
• Syntax for writing
• Syntax for reading

IDLgt x findgen(10000)IDLgt openw, lun,
'unformat_data.txt', /get_lunIDLgt writeu, lun,
xIDLgt free_lun, lun
IDLgt x findgen(10000)IDLgt openr, lun,
'unformat_data.txt', /get_lunIDLgt readu, lun,
xIDLgt free_lun, lun
27
Reading netCDF files

• netCDF (network Common Data Form) is designed to
  be simple and flexible.
• The basic building blocks are
• Variables are scalars or multidimensional arrays
  (supports string, byte, int, long, float and
  double)
• Attributes contain supplementary information
  about a single variable or an entire file.
  Variable attributes include units, valid range,
  scaling factor. Global attributes contain
  information about the file i.e. creation date.
  Attributes are scalars or 1-D arrays.
• Dimensions are long scalars that record the size
  of one or more variables

28
Whats in a netCDF File?

• use ncdump unix command or
• testreadnc.pro
• Try on the file air.mon.mean.nc
• testreadnc prints info to the file file_info.txt

29
Reading a Variable
open the netcdf file nc ncdf_open('/Users/rac
hel/idl_course_week2/air.mon.mean.nc') Extract
the variables of interestncdf_varget, nc, 'lat',
latncdf_varget, nc, 'lon', lonncdf_varget, nc,
'level', level close the netcdf file
ncdf_close, nc

• File is opened with the ncdf_open command,
  returning the file identifier nc
• the ncdf_varget function is used to read in the
  entire contents of the variable
• the file was then closed using ncdf_close
• See example read_netcdf.pro

30
Reading an Attribute
open the netcdf file nc ncdf_open('/Users/rac
hel/idl_course_week2/air.mon.mean.nc') Extract
the variables of interestncdf_varget, nc, 'air',
temperature Extract the attributes of
interestncdf_attget, nc, 'air', 'add_offset',
offsetncdf_attget, nc, 'air', 'scale_factor',
scale close the netcdf file ncdf_close, nc

• In this example, we extract the attributes from
  the variable air called add_offset and
  scale_factor using the ncdf_attget command.
• See example read_netcdf.pro

31
Some netCDF routines
Name Purpose
ncdf_open() Open a netCDF file
ncdf_close() Close a netCDF file
ncdf_varid() Return a variable identifier
ncdf_varget() Read a variable
ncdf_attget() Read an attribute
ncdf_inquire() Return file information
ncdf_varinq() Return variable information
ncdf_attname() Return an attribute name
ncdf_create() Create a netCDF file
ncdf_dimdef() Create a dimension
ncdf_vardef() Create a variable
ncdf_attput Write attribute data
ncdf_control Begin or end define mode
ncdf_varput Write variable data
32
Simple Graphical Displays

• The bread and butter of scientific analysis is
  the ability to see your data as simple line
  plots, contour plots, and surface plots.
• The quickest way to visualize data graphically in
  IDL is to plot to the display window.
• This is not the way you will create publishable
  graphics, but it is a good way to start looking
  at your data and it is a good way to learn how to
  display your data using the plot command.

33
A few things you should know ....

• IDL supports a system known as Direct Graphics
  which is built to a device - oriented model.
• What does this mean?? - you select a particular
  graphics device (screen, printer, PostScript) and
  then draw graphics on that device.
• This also means that you will have to switch the
  device to change how you visualize data.

34
Graphics

• The set_plot procedure selects a named graphics
  device.
• All subsequent graphics output is sent to the
  selected graphics device until another set_plot
  call is made or the IDL session ends
• set_plot, device
• X, WIN, PS
• Default is the screen display (set_plot, x)
• The following would then display a plot to the
  screen IDLgt plot, indgen(10)

35
A few things you might notice ...

• IDLs default setting is to plot things with a
  black background using white font. This can be
  quite annoying at times and I will teach you how
  to change that in a bit.
• The window is labeled IDL 0
• We plotted 1 vector, but got an x vs. y plot.

36
Window Options

• Graphics window can be created directly using the
  window command or indirectly by issuing a
  graphics display command when no window is open.
  IDLgt window
• The title bar of the window has a number in it (0
  in this case) - this is the windows graphics
  window number index.
• You can open a new window with a graphics window
  index number of 1 by doing the following IDLgt
  window, 1
• You are allowed up to 128 different windows (but
  boy that would be confusing)
• IDL will assign window index numbers for windows
  32-127 buy using the /free keyword IDLgt window,
  /free
• If you dont explicitly open a new window, IDL
  will plot over whatever is on the current window.

37
Window options

• There are other options for setting, deleting,
  and clearing windows. Try the following
• Open two windows (indexed 1 and 2)
• Plot something in the current window (window 2)
• Plot something in window 1 (using wset)
• delete window 2
• erase the contents of window 1

IDLgt window, 1IDLgt window, 2IDLgt plot,
indgen(10)IDLgt wset, 1IDLgt plot,
sin(findgen(200))IDLgt wdelete, 2IDLgt erase, 1
Name Purpose
window Create a new window
wset Makes an existing window the current window
wdelete Delete an existing window
wshow Expose, hide or iconify an existing window
erase Erase the contents of an existing window
38
Window Options

• You can also change the position and size of a
  graphics window.
• Use xsize and ysize to change the size of the
  window (these are in pixels)
• Use xpos and ypos to relocate the position of the
  graphics window. Windows are positioned on the
  display with respect to the upper left-hand
  corner of the display in pixel or device
  coordinates.

IDLgt window, 1, xsize200, ysize 300
IDLgt window, 2, xpos75, ypos150
39
Creating Plots

• The plot procedure draws graphs of vector
  arguments. It can be used to create line graphs,
  scatter plots, barplots, and even polar plots.

• The plot command can be modified with over 50
  different keywords!

40
Plotting a vector

• IDL will try to plot as nice a plot as it
  possibly can with as little information as it
  has.
• Try plotting IDLgt plot, sin(findgen(100).2)
  
• In this case the x or horizontal axis is labeled
  from 0-100, corresponding to the number of
  elements in the vector. The y or vertical axis
  is labeled with the data coordinates (this is the
  dependent data axis)
• For the following see example_line_plot.pro, and
  example_line_plot2.pro

41
Plotting x vs. y

• Most of the time a line plot displays one data
  set (the independent data) plotted against a
  second data set (the dependent data).
• For example
• x represents radian values extending from 0 - pi
  and y is the sine (x)

IDLgt x FINDGEN(21).1!piIDLgt y sin(x)IDLgt
plot, x, y
42
Customize Graphics Plots

• To label the x-axis, y-axis, and title use
  xtitle, ytitle, and title keywords, respectively.
• By default, the plot title is 1.25 times larger
  than the x and y axes labels - there are a number
  of ways to change this
• To change the size of all the plot annotations
  use the charsize keyword.
• To change the character size of each individual
  axis use XYZcharsize.

IDLgt plot, x, y, xtitle 'Radians', ytitle
'Radians', title 'Sine Wave'
IDLgt plot, x, y, xtitle 'Radians', ytitle
'Radians', title 'Sine Wave', charsize 1.5
IDLgt plot, x, y, xtitle 'Radians', ytitle
'Radians', title 'Sine Wave', xcharsize 2,
ycharsize 3
43
Modify line styles and thickness

• Use the linestyle keyword to plot the data with a
  different line style. For example to get a
  dashed line (instead of a solid line) use

Index Line Style
0 Solid
1 Dotted
2 Dashed
3 Dash Dot
4 Dash Dot Dot
5 Long Dash
IDLgt plot, x, y, linestyle 2

• Use the thick keyword to change the thickness of
  the line plots. For example if you want to plot
  displayed with a dashed line that is three times
  thicker than normal try

IDLgt plot, x, y, linestyle 2, thick 3
44
Symbols

• You can also plot your data using symbols instead
  of lines. Like the linestyle keyword, similar
  index numbers exist to allow you to choose
  different symbols.
• For example you can draw the plot with asterisks
  by setting the psym keyword to 2

Index Symbol
0 No symbol
1 Plus sign
2 Asterisk
3 Period
4 Diamond
5 Triangle
6 Square
7 X

• You can also connect your plot symbols with lines
  by using negative values for the psym keyword. To
  plot triangles connected by a solid line try

IDLgt plot, x, y, psym 2
IDLgt plot, x, y, psym -5
45
Plot Style and Range

• You can also limit the amount of data you plot
  with keywords. To just plot data that lies
  between 1 and 3 on the x axis and -0.5 and 0.5 on
  the y axis try

IDLgt plot, x, y, xrange 1,3, yrange
-0.5,0.5

• Sometimes IDL is lame and wont like your chosen
  axis range (because the chosen range is not
  aesthetically pleasing), use the XYZstyle
  keyword to make IDL listen to you!

Index Axis Property
1 Force exact axis range
2 Extended axis range
4 Suppress entire axis
8 Suppress box style axis
16 Inhibit setting the y axis starting at value 0

• You can change the way your plot looks by using
  the XYZstyle keywords. For example to force an
  exact axis range use

IDLgt plot, x, y, xstyle 1
46
Adding Lines to graphics

• Use plots to add lines to your plots.
• syntax plots, x0,x1,y0,y1,z0,z1
• Example add a line that spans the length of the
  x-axis and crosses through 0 on the y-axis.
• plot, x, y, xrange 0, 2!pi, xstyle 1
• plots, 0, 2!pi,0,0

47
Tick marks, intervals, and names

• XYZticklen - controls the length of the axis
  tick marks (expressed as a fraction of the window
  size). Default is 0.02. ticklen 1.0 produces a
  grid, negative ticklen makes marks that extend
  outside the window.
• XYZtickinterval - set to a scalar to indicate
  the interval between major tick marks
• XYZtickname - a string of up to 30 elements
  that controls the annotation of each tick mark.

48
Plotting Multiple Data Sets

• Use the oplot command to plot multiple data sets
  on the same set of axes.

IDLgt x findgen(21).1!piIDLgt y sin(x)IDLgt y2
cos(x) IDLgt plot, x, y, IDLgt oplot, x, y2,
linestyle 2

• What if you have two data sets that require
  different axes?

49
Positioning

• You can also position the plot inside the window
  using the position keyword.
• position is a 4-element vector giving, in order,
  the coordinates (x0,y0),(x1,y1) of the lower
  left and upper right corners of the data window.
  Coordinates are expressed in normalized units
  ranging from 0.0 to 1.0. Position keyword is
  never specified in data units.

IDLgt x findgen(21).1!piIDLgt y sin(x) IDLgt
plot, x, y, position 0.2, 0.2, 0.8,0.8
50
Plotting with Multiple Axes

• Sometimes you have two or more data sets on the
  same line plots, but you want the data sets to
  use different y axes. It is easy to establish as
  many axes as you need with the axis command.
• The key to using the axis command is to use the
  /save keyword to save the proper plotting
  parameters.
• Try this (note the /ylog keyword is set to make
  the new axis have a log-scale

IDLgt x findgen(21).1!piIDLgt y sin(x)IDLgt y2
dindgen(21)10 IDLgt plot, x, y, position
0.2,0.2,0.8,0.8, xtitle 'x', ytitle 'y'
IDLgt axis, yaxis1, yrange
.001,5E9,/save, ytitle 'other axis',/ylog
51
Multiple plots on a page

• The first way to plot multiple plots on the same
  page is to use the position command and to make
  sure to include /noerase keyword in the second
  and all subsequent plots

x findgen(21).1!piy sin(x)y2 cos(x) plot,
x, y,title 'Plot 1',position 0.2,0.2, 0.4,
0.4plot, x, y2,title 'Plot 2',linestyle
2,/noerase, position 0.5,0.5, 0.7, 0.7
52
Multiple plots on a page

• It is much easier to use the !p.multi system
  variable to create multiple plots on a page.
  !p.multi is defined as a 5 element vector as
  follows
• !p.multi0 - number of graphics plots remaining
  to plot on the display. It is normally set to 0
  meaning there is no more graphics plots remaining
  to be output on the display, the next graphic
  command will erase the display and start with the
  first of the multiple graphics plots
• !p.multi1 - this element specifies the number
  of graphics columns on the page
• !p.mulit2 - this element specifies the number
  of graphics rows on the page
• !p. multi3 - this element specifies the number
  of graphics plots stacked in the z direction ...
  but you must establish a 3-d coordinate system
• !p.multi4 - This element specifies whether the
  graphics plots are going to be displayed by
  fillin gup the rows first 0 or the columns
  first 1

53
How to use !p.multi
x findgen(21).1!piy sin(x)y2 cos(x)
set !p.multi to create 4 plots, filling in the
columns first !p.multi 0,2,2,0,1
plot, x, y, linestyle 0, title 'Plot 1'plot,
x, y2, linestyle 1, title 'Plot 2'plot, x,
y, linestyle 2, title 'Plot 3' plot, x, y2,
linestyle 3, title 'Plot 4' set !p.multi
to create 4 plots, filling in the rows
first !p.multi 0,2,2,0,0
plot, x, y, linestyle 0, title 'Plot 1'
plot, x, y2, linestyle 1, title 'Plot
2'plot, x, y, linestyle 2, title 'Plot 3'
plot, x, y2, linestyle 3, title 'Plot 4'
54
Leaving room for Titles with Multiple Graphics
plots

• When IDL calculates the position of the graphics
  plots, it uses the entire window to determine how
  big each plot should be.
• Sometimes you would like to have extra room on
  the display for titles and annotation.
• You can leave room with multiple plots by using
  the outside margin files of the !x, !y and !z
  system variables
• Syntax !p.multi 0,2,2,0,1
• !y.omargin 2,4

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Adding text to Graphics

• The xyouts command is used to place a text string
  a a specific location on the display
• xyouts can be used to add a larger title to a
  line plot or to label data points on the graph.
• Syntax xyouts, 0.5, 0.9, /Normal, Graphics
  Plots, Alignment 0.5, Charsize 2.5.
• You can specify the x and y location of the
  string with data coordinates or normalized
  (positional) coordinates.

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Positioning Text
Embedded Command Command Action
!A Shift text above the division line
!B Shift text below the division line
!C Insert a carriable return and begin a new line of text
!D Create a first-level subscript
!E Create a first-level superscript
!I Create an index-type subscript
!L Create a second-level subscript
!N Shift back to normal level after changing level
!R Restore text position to last saved position
!S Save the current text position
!U Create an index-type superscript
!X Return to entry font
!z Display one ore more characters by their unicode value
!! Display the exclamation mark !
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Adding color

• Adding color to your plots will seem quite
  annoying at first, but once you get the hang of
  it ... it will seem easy! (even though its really
  not ( )
• Remember how I said that IDL was a device
  oriented model?
• What this means is that you have to select the
  device you want to draw your graphics on (for now
  we are using the display window)
• Next you have to configure the graphics device
  (backing store, decomposed vs indexed color,
  color display (8 bits vs 24 bits).
• This involves a whole lot of computer language
  that I dont really understand myself, so I am
  just going to tell you what to do ... and if you
  want more info I can help after class.

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When plotting to the window

• More than likely your computer will have a 24-bit
  (or better??) graphics card. This means that
  when plotting to the window you want to be using
  a true color visual class (not pseudocolor or
  direct color). When plotting to the window we
  need to configure our device to use the 24-bit
  mode device, true 24
• You also need to set up how the contents of
  graphics windows will be refreshed when windows
  are moved in front of or behind each other (this
  is called backing store). To do this you need
  to set the retain keyword, i suggest using
  device, retain 2, which means that IDL is
  responsible for maintaing backing store.

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Colors

• A color in IDL is composed of three specific
  values. We call these values a color triple and
  write the color as (red, green, blue or RGB).
  Where the red, green, and blue values represent
  the amount of red, green and blue light that
  should be assigned that color on the display.
  Each value ranges from 0 to 255, thus a color may
  be made up of 256 shades or gradients of red, 256
  shades of green, and 256 shades of blue.
• This means that there are 16.7 million colors
  that can be represented on the display by IDL.
  (color triple that represents yellow is (255,
  255, 0).
• Two different color models can be used to specify
  colors on a 24-bit graphics display, and some of
  the direct graphics commands work differently
  depending on the model currently in use.

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Color models

• The two different models are called decomposed
  color and indexed color. (decomposed on and
  decomposed off)
• When configuring the device decomposed 0 (index
  model), decomposed 1 (decomposed color model).
• By default, when you set the 24-bit Truecolor
  environment, it will be using the decomposed
  model, which means that you can set the color
  using an RGB triple directly or color
  255,255,0 (yellow)
• I am going to teach you how to use the index
  model - because this is what we will need to use
  to make PostScripts.

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Device Configuration

• To configure our device so that we can draw
  graphics to the screen and everything works out
  so that we dont get crazy flashing and
  unexpected colors do the following.
• device,true24,decompose0,retain2
• Here we are using the truecolor color class,
  using the indexed color mode, and allowing IDL to
  manage backing store.
• If you are like me ... you will put this into a
  procedure that you can run and you wont think
  about it again (until you want to make
  PostScripts)

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Back to actually colors

• So how do colors work in IDL?
• Now that we have configured our device, we want
  to load in a color table.
• We will start with the IDL pre-defined color
  tables.
• The default color table in IDL is called B- W
  linear.
• We can look at the colors in this table using the
  xpalette or cindex procedures.
• To change the color table use loadct, of CT
  loadct, 39

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Setting the background and plot colors in IDL

• IDLs default setting sets the background of the
  plot as black and the font as white. To change
  this use the background and color keywords. Set
  background and color to the desired color index
  in the selected color table.
• When using the B-W linear color table do the
  following
• plot, x, y, background 255, color 0

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Choosing Colors

• Load the color table into IDL
• Use xpalette or cindex to choose the color you
  want to use.
• Set the color to the chosen index using the color
  keyword.
• For every new line you want to add to the plot,
  us the oplot command and change the color
  keyword.
• Make sure that the first think you plot is set to
  the color you want your axes to be (black, white,
  etc)...otherwise you might end up with purple
  axes.
• Use /nodata in your plot command or oplot the
  same line again with a different color

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Examples

• Plot multiple lines on the same graph, each line
  should have its own color and its own linestyle.
• Example global_temp.pro
• Plot a scatter plot!
• Example temp_vs_sunspot.pro
• Put multiple plots on the same page. Plot the
  information on each graph with different colors.
  Title the selection of graphs.
• Example plotting_examples.pro

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Adding a Legend

• use the legend.pro procedure created by to add a
  legend to your diagram.
• legend accepts, color, psym, linestyle, position,
  /center, /bottom, /top etc.

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Questions???