Round the corners of a png

main.c

#define _CRT_SECURE_NO_WARNINGS
#define STB_IMAGE_IMPLEMENTATION
#include "../stb/stb_image.h"

#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "../stb/stb_image_write.h"

int main(int argc, char** argv)
{
	char* filename[6];
	filename[0] = "C:\\Users\\Z77Pro\\source\\repos\\eigenPlayGround\\Resources\\0A.png";
	filename[1] = "C:\\Users\\Z77Pro\\source\\repos\\eigenPlayGround\\Resources\\0B.png";
	filename[2] = "C:\\Users\\Z77Pro\\source\\repos\\eigenPlayGround\\Resources\\1A.png";
	filename[3] = "C:\\Users\\Z77Pro\\source\\repos\\eigenPlayGround\\Resources\\1B.png";
	filename[4] = "C:\\Users\\Z77Pro\\source\\repos\\eigenPlayGround\\Resources\\2A.png";
	filename[5] = "C:\\Users\\Z77Pro\\source\\repos\\eigenPlayGround\\Resources\\2B.png";

	for (int ii = 0; ii < 6; ii++) {

		int width, height, channels;
		unsigned char* img = stbi_load(filename[ii], &width, &height, &channels, 0);
		if (img == NULL) {
			fprintf(stderr, "Error in loading the image\n");
			exit(1);

		}
		if (channels != 4) {
			fprintf(stderr, "Need 4 channels");
			exit(1);
		}
		int r = 30;
		for (int i = 0; i < height; i++)
			for (int j = 0; j < width; j++)
				if (
					(i < r && j < r && (i - r) * (i - r) + (j - r) * (j - r) >= r * r) ||
					(height - 1 - i < r && j < r && (height - 1 - i - r) * (height - 1 - i - r) + (j - r) * (j - r) >= r * r) ||
					(i < r && (width - 1 - j) < r && (i - r) * (i - r) + ((width - 1 - j) - r) * ((width - 1 - j) - r) >= r * r) ||
					(height - 1 - i < r && (width - 1 - j) < r && (height - 1 - i - r) * (height - 1 - i - r) + ((width - 1 - j) - r) * ((width - 1 - j) - r) >= r * r)
					)
					img[4 * (i * width + j) + 3] = 0;

		stbi_write_png(filename[ii], width, height, channels, img, width * channels);
		stbi_image_free(img);
	}
	return 0;
}