Contour map grapher for the TI-85.  This program graphs a contour map
of an implicit function of X and Y.




Program:   CONTOUR
Author:    John P. Powers  (jpp@cpdvax.csc.ti.com)
Date:      January 11, 1993

This program graphs a contour map of an implicit function of X and Y.

Define equation variable FXY with the implicit function before
running this program.

Modify the assignment to variable BAND in the first line of program
CONTOUR to adjust the Z axis width of each contour band.  Band is set
to .1 in the distributed version of CONTOUR.  This causes alternating
bands of black and white for a Z scale of .1

Run program CONTOUR.  Depending on the complexity of the definition
of FXY, the graph will be completed around 10 minutes.

Example:
   Graph the function F(X,Y) = exp(-sqrt((X-1)^2+Y^2)) +
                               exp(-sqrt((X+1)^2+Y^2)

   Enter on the home screen
      FXY=e^-abs(X-1,Y)+e^-abs(X+1,Y) [ENTER]

   Set the graph range to x: [-2,2], y: [-2,2] for a better view of
   this function.

   Run the program
      CONTOUR [ENTER]

   A contour map demonstrating two peaks at (-1,0) and (1,0) is then
   drawn.

This program uses a brute force test of pixels on the screen to
determine if the pixel should be turn on.  The program makes two
passes over the screen filling in a quarter of the pixels each time. 
You get a good idea of how the function looks after the first pass
and may stop the program from filling in detail on the second pass.

Included in the UUE section of this distribution is CONTOUR.85G, a
grouped TI-85 file containing CONTOUR (the contour plotting program),
FXY.85E (FXY as defined in the above example), and EXP2CON.85I (the
resulting contour map picture of FXY).