Dear GRX

I read with interest the problems of Gernot Kleiter in mail #406 concering animation. As I newcomer to GRX I submit a very short double-buffering example: a rotating circle. Note by commenting out all the context & bitblt statements we arrive at a single buffered animation with flicker. I hope that this is useful. I am interested in the comments of fellow GRXites to improve this simple program. I run it under MinGW as I cannot get DJGPP to function on a W2000 laptop.

/* This program animates a filled circle with double buffering */ /* The circle is red on a black background. The circle is of /* radius r and centre (x,y) which moves in a circular track of radius R */ /* centered in the middle of the screen */

#include <stdio.h> #include <math.h> #include <grx20.h>

int main() { int x, y; /* centre coordinates */ int xMax,yMax; /* extent of the screen */ int Radius=100,r = 10; double Theta, dTheta,pi; /* Angle of rotation, increment and PI */ GrContext *memory,*screen; /* memory and screen contexts */ GrColor Red,Black; /* circle & background colours */

pi = 3.14159; dTheta = 0.5*pi/180.0; /* one half a degree increment */ Theta = 0.0; /* starting postion */ GrSetMode(GR_default_graphics ); xMax = GrMaxX(); yMax = GrMaxY(); Red = GrAllocColor(255,0,0); Black = GrBlack();

memory = GrCreateContext(xMax,yMax,NULL,NULL); screen = GrScreenContext();

while(1) /* run for ever! */ { x = xMax/2 + Radius*cos(Theta); /* circle centre */ y = yMax/2 + Radius*sin(Theta); Theta += dTheta; /* increment angle */ GrSetContext(memory); /* Set memory context */ GrClearContext(Black); /* Set context black */ GrFilledCircle(x,y,r,Red); /* draw red circle */ GrBitBlt(screen,0,0,memory,0,0,xMax-1,yMax-1,GrWRITE); /* copy to screen */ GrSetContext(screen); /* make visible */ }

return 0; }