R. W. O'CONNELL
IDL EXERCISES II:
ARRAYS AND IMAGE DISPLAYS
Before starting these exercises, you should copy to your local
directory all the test data files (*.FITS) from the
IDL exercises data directory.
NOTE: commands intended to be typed into your terminal during your IDL
session are given in CAPITAL letters below so they will stand out.
However, IDL itself is case-insensitive (except for file names), so
you needn't follow this convention.
0) From a UNIX shell, start X-windows if it is not already running.
From the shell within an X-window, "cd" into your "IDL" directory.
Normally that would be named "~/idl".
Parts 3-7 of these exercises involve graphics displays. If you
plan to do these, first check whether your computer display
supports "true-color" graphics (for background, see the section
titled "Setting a Visual Display Mode on Your Computer" in
the IDL Guide).
From the shell within an X-window, type: "xdpyinfo"
If the response includes "TrueColor" or "24 planes" then your display
is a 24-bit, true-color device. If not, it is an 8-bit, pseudo-color
Start IDL in the X-window.
[On most UNIX systems, just type "idl".
This will start IDL running in the command-line mode using the
window from which you called it.]
If you plan to do parts 3-7 of these exercises, set the visual
class of your display to pseudo-color or emulated pseudo-color:
If you have an 8-bit display, type DEVICE,RETAIN=2,PSEUDO_COLOR=8
If you have a 24-bit display, type DEVICE,RETAIN=2,TRUE_COLOR=24,DECOMPOSED=0
Before starting the exercises, give the IDL command:
This will return control to the main program level if
an error occurs in a called routine.
1) SIMPLE ARRAY FUNCTIONS
Create a 2-D array, A, with 5 columns and 3 rows using the INDGEN
[NOTE: remember that the IDL subscripting convention for arrays
is reversed from standard matrix notation and that in computer
languages like FORTRAN: in IDL A[I,J] yields the value of the
I-th column and J-th row.]
Print the array to the screen and verify its structure and
Print TRANSPOSE(A). What are its characteristics?
Print REVERSE(A). What are its characteristics?
Now let B=[0,1,2]
Predict, then verify, the outcome of the following operations:
C = A+1
C = A*0
C = A^2
C = A*(A+3)
C = A/A
C = EXP(A)
C = A+B
C = A*B
C = A[*,2]*B
C = A[4,2]*B
C = A#B
C = B#A
C = B##A
Now replace the element in A(3,1) with the value 100
Verify the result by printing
Type the following command and interpret the result:
PRINT, WHERE(A EQ 100)
2) SIMPLE ARRAY CREATION
Create Z, a 301x301 array, using the FINDGEN function.
What are the maximum and minimum values of Z?
Using internal subscript notation, print to the terminal:
The first 50 elements of the first row of Z
The first 50 elements of the 99th column of Z
The last 10 elements of the 100th row of Z
Use the AstUseLib routine IMLIST to double-check the results of
the preceding exercise for the last 10 elements of the 100th
row of Z. To get information on the IMLIST routine, type
MAN,'IMLIST. [Note: you may have to use the optional WIDTH
keyword to increase the size of the printed display in order to
read all digits in the Z entries.]
Write an IDL expression which yields the contents of Z[I,J] as a
function of I and J.
Verify the expression against the actual contents
of Z for selected locations.
Now define B = FLTARR(101,101)+20000.
What are the maximum and minimum values of B?
Using internal subscript notation, set the 51st column of B equal
Then set the 51st row of B equal to 100000.
Now insert the array B into the original array Z, with the lower
left-hand corner of the insert beginning at Z[100,100].
Confirm the placement of the insert using IMLIST
3) GREYSCALE IMAGE DISPLAYS
Now open a graphics display window: type "WINDOW,0" or
[NOTE: To open a new window, use WINDOW,N. To expose or hide
an existing window, use WSHOW,N. To make a given window
"active"--- i.e. ready for I/O---use WSET,N.
The MOUSSE routine CHAN,N combines these three functions and is
If you are using an 8-bit display, move the cursor into the
If the other parts of your terminal screen blink out or change
color, then your X-windows system is not properly configured.
For troubleshooting the problem, see the
section on "Reserved Colors on 8-bit Monitors" in the
Load the default black and white color table with the command
On an 8-bit display, a change of color table takes effect
immediately. On a 24-bit display, you must re-load the image
before the color table takes effect.
The system variable !D.N_COLORS contains the maximum number of discrete
color levels you can display on your screen. How many are there?
On a 24-bit display, the answer should be 16M (even though
in emulated pseudocolor mode you will only be using 256
of those at a time). On an 8-bit display, the answer should
Now use the MOUSSE routine CTVSCL to display Z in window 0 using
the following command:
Inspect the display. What values are displayed as full black?
What as full white? Is the rest of the display what you expected?
Try loading Z into the display using several other values of
MAX. Does it respond as expected?
Reload Z with the full [0,100000] range. Use the AstUseLib
routine CURVAL,Z to check the values of Z at various locations
of interest using the cursor on the display window. Note that
both the value of Z and the value at the corresponding
location on the window display ("Byte Inten") are shown;
compare these. To exit, click the right hand mouse button
while on the window.
Try the AstUseLib routine TVLIST (an interactive version of
IMLIST) to print out selected areas of the image. Do the
"crosshairs" intersect at the image center?
With Z still displayed in window 0, open window 9 to make plots.
Plot extractions of Z along columns and rows and check that
they are what you expect from the displayed version.
Open window 1. Load TRANSPOSE(Z) with the same scaling as for Z.
Is it what you expect?
Now define DIFF = Z - TRANSPOSE(Z). Load DIFF into window 2,
scaling from a MIN of -50000. to a MAX of +50000.
Examine the result visually, comparing with the other two
display windows, and with CURVAL or TVLIST. Be sure you
understand the result.
4) IMAGE HARDCOPIES
The quickest way to generate a hardcopy of an image display
on your screen is the use the AstUseLib utility TVLASER.
TVLASER will dump a bitmap copy of any window to a PostScript file
and print it out. It makes use of the intrinsic IDL routine
TVRD to copy a window buffer to an IDL variable. The basic
steps in TVLASER are described in the
IDL Guide Graphics Hardcopies section.
Make a hardcopy of any of the displays you have made so far
Optional: TVLASER can be used to make quick hardcopies of windows
containing line drawings produced by the PLOT commands. However,
these will NOT have the good resolution typical of the
SET_PLOT,'PS process described in Exercises I. Try comparing
the results of the two methods.
5) FILE INPUT, GREYSCALE IMAGE DISPLAY ADJUSTMENT
Use the AstUseLib routine FITSDIR to obtain a quick listing of the
properties of FITS files you copied to your local directory.
Use FITS_INFO to obtain additional information on the
[NOTE: your UNIX interface IS CASE SENSITIVE, even though IDL is
not. Therefore, you must give file names exactly as they
appear in your directory.]
Now use the AstUseLib FITS_READ procedure to read the image file
"testpattern.fits" into RAM. Transfer both the image data
and the header. Give the saved image the name PATT.
Use HPRINT to print the image header to your screen.
Verify that the file contained a 2-D image. What
are its dimensions?
Open Window 0; load color table 0.
Now display the image with CTVSCL and MIN,MAX set to [0,240].
There are N different image values in PATT. What is N?
Make a list of the values. [Hint: use PLOTHIST and WHERE.]
Can your eye detect N distinct values on the screen? If not, how
Try different MIN,MAX pairs to adjust the display.
Optional: explore displaying the image using the IDL intrinsic
TV routine. How do TV and CTVSCL differ?
Try the following exercise; successively load scaled versions
of PATT using the command
where N is an integer. You might try writing a simple IDL
procedure to do this where successive loads are delayed
slightly by using the WAIT,K function. Set K = 0.1 or 0.2
Copy the program icontrast.pro from the IDLexercises/data directory to
your local directory. Choose any window and reload PATT with the
original [0,240] scaling. Now enter
ICONTRAST allows you to make interactive adjustment of the color
tables in the display. It will start by placing the cursor at the
center of the image. By moving the cursor up, you increase the
contrast (slope) of the color tables; by moving the cursor right,
you increase the Y-intercept of the color tables. Exit by
clicking the right-hand mouse button. You can check the effect on
the color tables by reading them with TVLCT,RR,GG,BB,/GET and then
Reload color table 0 and PATT with its original scaling.
Nonlinear transformations of images allow expansion/compression of
the dynamic range of displays:
A fractional power law is a quick way to improve discrimination
for fainter values in an image while holding the brighter
values. Define NEW1=PATT^0.3. Deterine the maximum and
minimum values of NEW1. Open a new window and use CTVSCL to
display NEW1 between its minimum and maximum values. Compare
it to the linear display. How does the power-law display
Logarithmic displays have a similar effect. Try displaying
NEW2=ALOG10(PATT > 1) between its minimum and maximum
6) IMAGE STRUCTURE, ZOOMS, HISTOGRAMS, COLOR DISPLAYS
Now read in the file "noisytestpattern.fits". This is the same
test array as before except that it has had Poisson (photon)
noise added corresponding to the mean photon count values at
each of the N levels. [Where the photon count was zero, we
have substituted the noise for a count of 1.]
Display this image in window 1 with the same [0,240] scaling.
How obvious is the photon noise in the display?
Try plotting (in a third window) cuts across the two images:
e.g.: PLOT,PATT[*,150] & OPLOT,NOISY[*,150]
Use the ZOOM routine to enlarge the display of the noisy image.
The default zoom factor is 4. Change to a factor of 10 for
a better look at pixel structure. ZOOM allows you to
inspect different regions of the image by moving and
clicking the mouse. Alternatively, try using the /CONTINUOUS
keyword in the call to ZOOM.
Optional: As an alternative to ZOOM, you could REBIN the image
(or parts of it) to a larger size and redisplay. Use the
keyword /SAMPLE to preserve the pixel structure.
Extract a 60x60 subarray from the part of NOISY which has a mean
value of 112 (refer to the original PATT and use CURVAL to
locate this region). Histogram the pixel values (use
PLOTHIST). Measure (by eye) the FWHM of the histogram. Is it
consistent with Poisson noise?
Now experiment with loading any or all of the 40 built-in IDL color
tables using the LOADCT,N routine. Explore how the different
tables emphasize different brightness levels in the images and
affect the contrast of the noise fluctuations. (Remember that
on a 24-bit display you must reload the image after you change
the color table.)
Optional: write a simple procedure that will sequentially load each
of the available 40 IDL color tables and the image, waiting (say)
0.2 seconds between tables.
The David Fanning (Coyote) program CONTRASTZOOM combines the "contrast"
and "zoom" functions in a single demonstration window. It's worth
trying, but you will have to download the Coyote library
to access all of the subsidiary routines.
7) CONTOUR PLOTS
Read in the file "gaussimage.fits". This contains the image of
a 2-D circular Gaussian.
Using any of the tools you have tried so far:
Find the [X,Y] location of the maximum of the image and
Determine the full-width-half-maximum of the image (accuracy
of a few pixels OK).
Open a plotting window. Use the SQUARE routine (from MOUSSE) to make
the axes of the next plot equal.
Now make a contour plot of the image using the CONTOUR built-in routine.
Select about 10 levels spanning the range of image values for the
contours. Make a hardcopy (note: you will need to use the SQUARE
routine again after you give the SET_PLOT,'PS command).
Optional: Copy the program icroll.pro from the IDLexercises/data directory
to your local directory and try "rolling" the color table for
the Gaussian image using the following command...
for a nice "Hashbury 1969" effect. Here, G is the variable that
contains the Gaussian image. Inspect the ICROLL program to see
how this effect was produced [e.g. use ".RUN -T ICROLL"]. Color
table 31 is specifically designed to produce an automatic
8) Using ATV
The GUI-based program ATV, written by Aaron Barth, provides a
convenient way of accessing most of the functionality we just
illustrated for the "direct graphics" mode of IDL. If ATV is not
already in your IDL path, you can download it from
its home page at UCI. In order to display an image in ATV,
simply give the command
Consult the ATV instructions for help. You can adjust
contrast, change zoom level, apply different color tables (though
limited in number), and so forth. ATV offers a nice set of other fast
diagnostic tools in the "IMEXAM" mode that you should explore.
END OF IDL EXERCISES PART II
Part I scalars, vectors, plotting.
Part III image processing.
IDL Tutorial Home Page
Copyright © 2000-2013 Robert W. O'Connell. All rights reserved.
Last modified by RWO, August 2013