File:Illusion movie.ogv

//License: Public Domain
//Written by Nasca Octavian PAUL, Tg. Mures, Romania
//in 20060901
/*
Steps to obtain the illusion image:

1) Compile this program and run it

2) Convert the .pgm images to .png images with the bash script:
>>>>>cut<<<<<
for file in *.pgm
do
    echo $file
    pnmtopng $file > "$file.png"
    rm -f $file
done
>>>>>cut<<<<<

3) Convert png images to "output.avi"
mencoder "mf://*.png" -mf fps=20:type=png -ovc lavc -oac copy -o output.avi

4) Convert "output.avi" to ogg theora with:
ffmpeg2theora output.avi

5) Run the animation (full screen) and look and stare in the center of it
After 1 minute, look away and you'll see a very interesting illusion (looks like objects/faces/etc changes their shapes) which last few seconds.
*/

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

static const int sizex=640;
static const int sizey=480;
static const float freq=80;

static unsigned char data[sizex*sizey];

static unsigned char f(float x,float y,float phase){
    x=fabs(x);
    y=fabs(y);
    float m=sin(phase+(x+y)*freq)*127.0+128;
    return (m);
}

static void make_frame(float t){
    for (int j=0;j<sizey;j++){
	for (int i=0;i<sizex;i++){
	   float x=(i*2-sizex)/(float)sizex;
	   float y=(j*2-sizey)/(float)sizey;
	   float y2=y/sizex*sizey;
	   if ((x*x+y2*y2)<0.1) data[i+j*sizex]=f(x,y,t);
		else data[i+j*sizex]=f(x,y,-t);
	}
    }
}

static void save_frame(char *filename) {
    FILE *f=fopen(filename,"wb");
    if (!f) return;
    fprintf(f,"P5\n%d %d\n255\n\n",sizex,sizey);
    fwrite(data,sizex,sizey,f);
    fclose(f);
}

int main(void) {
    char tmp[100];tmp[99]=0;
    for (int k=0;k<1200;k++){
        make_frame(k*0.5);
	snprintf(tmp,99,"frame%05d.pgm",k);
	printf("%s\n",tmp);

	save_frame(tmp);
    }
    return 0;
}