{----------------------------------------------------------------------

	graphics0.p

	Started by Jeff Ondich on 1/25/95
	Last modified 10/11/96


	This program opens a graphics window, fills it with a black
	background, and draws a red circle in the center.
  
	To compile this program, use the following command:
  	
		gpc -o myProgram myProgram.p -lgraphics

	You will need to hit the Enter key to end the program.

	Try doing these things:

		0. Compile and run the program as it is.
		1. Read the whole program to find out how it works.
		2. What happens if you remove the "readln" at the
		   end of the main program?
		3. Modify the code to draw a tall and skinny graphics
		   window.  Where does the circle get drawn when you
		   run your new version of the program?  Why?
		4. Change the background color.  Can you make it white?
		5. Make the circle green.
		6. After the readln at the end of the main program, add

			setrgbcolor( 1, 0, 0 );
			flood;
			flushgraphics;
			readln

		   What do you expect will happen?  What does happen?
 ----------------------------------------------------------------------}

#include	<gpc-graphics.h>

program OneCircle(input,output);

import graphics;

const	radius = 20;
		windowWidth = 600;
		windowHeight = 600;
		windowBottom = 100;
		windowLeft = 100;

{----------------------------------------------------------------------
	StartGraphics initializes the graphics package
	and opens a black window for drawing.
 ----------------------------------------------------------------------}
procedure StartGraphics;
begin
 { Call initializegraphics only once during the program,
   before you do any other graphics }
   
 	initializegraphics;


 { Open a window and flood it with a black background. }

	createwindow( windowLeft, windowBottom,
			windowLeft + windowWidth, windowBottom + windowHeight );
	setrgbcolor( 0.0, 0.0, 0.0 );
	flood; 
	flushgraphics
end;


{----------------------------------------------------------------------
	main program
 ----------------------------------------------------------------------}
begin

	StartGraphics;

 { "RGB" stands for red-green-blue.  The parameters for setrgbcolor
   can range in value from 0.0 to 1.0.  All zeros gives you no red,
   no green, and no blue.}

	setrgbcolor( 1.0, 0.0, 0.0 );
	circle( windowWidth div 2, windowHeight div 2, radius );
 
 { When you're done with drawing a particular object, you need to
   call flushgraphics, or you won't see anything.  When you get to
   more complicated programs, you can experiment leaving the
   flushgraphics calls out to see what happens. }

	flushgraphics;

 { This is here so the program won't end until the user hits the
   return key.  Otherwise, the program will end and the window will
   just disappear as soon as the drawing is done. }

	readln
end.