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.
0) Start IDL.
[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. IDL V.5
also offers a widgit-based interface, the IDL Development
Environment, which is invoked by the command "idlde".
This offers convenience features to the experienced user,
but it is correspondingly more complicated and not
recommended for learning the basics.]
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
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 will 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. [NOTE: remember the IDL subscripting
convention for arrays is reversed from FORTRAN's: in
IDL A(I,J) yields the value of the I-th column and J-th row.]
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
Troubleshooting: Move the cursor into the window. If the
other parts of your terminal screen blink out or change
color, then your X-windows system is not properly
configured. To ameliorate the problem, try this:
As the very first two commands, type:
If this does not remedy the blinking, then other X-windows
applications have "reserved" too many display colors. You may
be able to reduce the color hogging using various "Preference"
or other settings on your applications (e.g. the Common
Desktop Environment). Try exiting other applications (e.g.
Netscape) before starting IDL. If this doesn't work, you can
try reducing the number of IDL color levels requested in the
command above. For further information on color levels in
IDL, see the IDL Guide..
Whether or not color blinking is a problem, verify that your
terminal is in pseudo-color mode by typing HELP,/DEV
The response should include the following lines:
"Display Depth, Size: 8 bits"
"Visual Class: PseudoColor"
"Colormap: Shared, N colors", where N
is 256 or smaller.
If this is not the case, follow the instructions above
to re-configure your color tables.
Load the default color table with the command LOADCT,0
The system variable !D.N_COLORS contains the maximum number of discrete
color levels you can display on your screen. How many are there?
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. (Click the right hand mouse
button while on the window to exit.)
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
NOTE: 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. Give it 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
many? Try different MIN,MAX pairs to adjust the display.
Optional: explore the use of the MOUSSE routine CTV.
How do CTV and CTVSCL differ? Try the following exercise:
FOR I = 1,20 DO CTV,I*PATT
Load the image with the original [0,240] scaling. Now use the
MOUSSE routine CONTRAST to adjust its appearance. CONTRAST will
place 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 righthand
mouse button. You can check the effect on the color tables by
reading them with TVLCT,RR,GG,BB,/GET and then plotting.
Reload color table 0.
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. 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 differ?
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. Try using the CONTINUOUS
Optional: As an alternative to ZOOM, you could REBIN the image
(or parts of it) to larger values 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). Histogram the pixel
values. Measure (by eye) the FWHM of the histogram. Is it
consistent with Poisson noise?
Arrange your display so you can see both windows 0 and 1.
Make window 0 active and call the MOUSSE routine
REFSCL,/OVER. This will place a small visual display of
your color table at the bottom of window 0.
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.
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).
Display the image, perhaps with power-law scaling, so that you
preserve most of its dynamic range. Load IDL color table 31.
This will yield a contoured image by virtue of the color table
Use the MOUSSE routine CROLL to "roll" the table for a nice
"Hashbury 1969" effect.
END OF IDL EXERCISES PART II
Part III continues analysis of 2-D arrays.
IDL Tutorial Home Page
Copyright © 2000-2003 Robert W. O'Connell. All rights reserved.
Last modified by RWO, 11/02/03