Little C program for making bright pixels in a .png file transparent. Useful for cleaning up a picture of a document. TODO: Maybe a fragment-deletion filter and a smoothing filter.

main.c

// Image-read library
// https://github.com/nothings/stb/blob/master/stb_image.h
#define STB_IMAGE_IMPLEMENTATION
#define STBI_ONLY_PNG
#include "stb_image.h"

// Image-write-library
// https://github.com/nothings/stb/blob/master/stb_image_write.h
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

#include <stdio.h>

const char default_fname_in[] = "signature-in.png";
const char default_fname_out[] = "signature-out.png";
const float default_light_limit = 0.5;

int main(int argc, char** argv) {
	char *fname_in;
	char *fname_out;
	float light_limit;

	// get args
	if (argc == 1) {
		fname_in = default_fname_in;
		fname_out = default_fname_out;
		light_limit = default_light_limit;
	}
	else if (argc == 4) {
		fname_in = argv[1];
		fname_out = argv[2];
		light_limit = atof(argv[3]);
	}
	else {
		printf("Usage: ./executable [in.png, out.png, lightlimit (in [0.0, 1.0])]\n");
		printf("Ex:    ./executable signature-in.png signature-out.png 0.5\n");
		exit(-1);
	}

	// print args
	printf("Input  file: %s\n", fname_in);
	printf("Output file: %s\n", fname_out);
	printf("Light limit: %f\n", light_limit);

	// load image
	int width, height, components;
	unsigned char* image = stbi_load(fname_in, &width, &height, &components, 4);

	if (image == NULL) {
		printf("Unable to open file %s.\n", fname_in);
		exit(-1);
	}

	// edit image
	for (int i = 0; i < width*height; i++) {
		// get pixel
		unsigned char* pixel = image + i * 4;

		// if pixel's too bright, make it invisible
		if ((pixel[0] + pixel[1] + pixel[2]) / (float)(255 * 3) > light_limit) {
			pixel[3] = 0;
		}
	}

	// write it to output
	stbi_write_png(fname_out, width, height, components, image, width * components);

	// free
	stbi_image_free(image);

	printf("done!\n");
	return 0;
}