Modulo patterns in C (PGM and PPM)

During one my Engineering Computing classes last semester, I was writing an example C program to create a PGM image file when I accidentally produced an interesting pattern. I had been using a simple function of the pixel coordinates x and y to quickly generate a pattern and I included a modulo operator to limit the maximum pixel value to 255. What I saw was something like the following:

Here’s the complete C code I used to produce that image:

//
// PGM modulo pattern maker - Ted Burke, 13-2-2012
//

#include <stdio.h>

int main()
{
	int x, y, w=640, h=480;

	// Open output PGM file
	FILE* f = fopen("pattern.pgm", "w");

	// Print image header to file
	fprintf(f, "P2\n# batchloaf.wordpress.com\n%d %d\n255\n", w, h);

	// Generate pixel values and write to file
	for (y=0 ; y<h ; ++y)
	{
		for (x=0 ; x<w ; ++x)
		{
			fprintf(f, "%03d ", (x*x + y*y) % 255);
		}
		fprintf(f, "\n");
	}

	// Close file
	fclose(f);

	return 0;
}

Also, I used the following ImageMagick command to convert the image to PNG format before uploading it here:

convert pattern.pgm pattern.png

Over the weekend, I spent a couple of hours playing around with the same idea and produced some wild patterns. This time, I wanted to produce colourful patterns so I created PPM files instead. I was really surprised by how rich the patterns produced by such simple formulae are. Here’s an example:

Here’s the code I used to produce the image above:

//
// pattern.c - a PPM modulo pattern maker
// Written by Ted Burke, 13-2-2012
//
// To compile: gcc pattern.c -lm
//

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

int main()
{
	int r, g, b, x, y, w=640, h=480;

	// Open output PPM file
	FILE* f = fopen("pattern.ppm", "w");

	// Print image header to file
	fprintf(f, "P3\n# batchloaf.wordpress.com\n%d %d\n255\n", w, h);

	// Generate pixel values and write to file
	for (y=0 ; y<h ; ++y)
	{
		for (x=0 ; x<w ; ++x)
		{
			r = fmod(0.001*(pow(x-w/2,3) + pow(y-w/2,3)),255.0);
			g = fmod(0.001*(pow(3*w/2-x,3) + pow(3*h/2-y,3)),255.0);
			b = 255;
			fprintf(f, "%03d %03d %03d ", r, g, b);
		}
		fprintf(f, "\n");
	}

	// Close file
	fclose(f);

	return 0;
}

Finally, here are some more examples:

The next one uses a sine function rather than the modulo operator to clamp the pixel value to the allowed range (0 to 255):

r = sin(0.025*(pow(x,1.1) * pow(y,1.1))) * 255.0;
g = (255.0 * x)/w;
b = (255.0 * y)/h;