{===================================================================

	graphics3.p

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

	1. Compile it, run it, figure out how it works and
	   how to stop it.

	2. Complain about the flickering.  Try to get the ball to
	   bounce from side to side instead of up and down.  Can
	   you get the colors to change gradually from red to
	   blue instead of abruptly?

 ===================================================================}

#include	<gpc-graphics.h>

program Bounce(Input, Output);

import graphics;

const	radius = 40;
		windowWidth = 600;
		windowHeight = 600;
		windowBottom = 100;
		windowLeft = 100;

var		x, y : integer;
		red, blue, green : real;
		goingUp : boolean;

{===================================================================
	StartGraphics initializes the graphics package
	and opens a black window for drawing.
 ===================================================================}
procedure StartGraphics;
begin
 	initializegraphics;

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

procedure NewCenter( var horiz, vert : integer );
begin
	if goingUp then
		vert := vert + 1
	else
		vert := vert - 1;
	
	if vert >= windowHeight - radius then
	begin
		goingUp := false;
		red := 1.0;
		blue := 0
	end

	else if vert <= radius then
	begin
		goingUp := true;
		red := 0;
		blue := 1.0
	end

end;

{===================================================================
	main program
 ===================================================================}
begin
	StartGraphics;

	x := windowWidth div 2;
	y := radius;
	goingUp := true;
	red := 0.0;
	blue := 1.0;
	green := 0.0;

	while true do
	begin
		setrgbcolor( red, green, blue );
		circle( x, y, radius );
		flushgraphics;
		
		setrgbcolor( 0, 0, 0 );
		circle( x, y, radius );
		flushgraphics;
		NewCenter( x, y )
	end
end.