C script which prints x, y, and y_offset fields of sprite coordinates

get_sprite_coords.c

#include <stdio.h>
#include <png.h>
#include <stdlib.h>

#define FATAL_ERROR(...) ({fprintf(stderr, __VA_ARGS__);exit(1);})

#define min(a, b) ((a) < (b) ? (a) : (b))

struct Image
{
    unsigned width;
    unsigned height;
    unsigned char * pixels;
    unsigned char bitDepth;
    unsigned char pixelsPerByte;
    unsigned char mask;
};

static int image_get_pixel(struct Image const * image, int x, int y) {
    int pixelOffset = y * image->width + x;
    int pixel = image->pixels[pixelOffset / image->pixelsPerByte];
    pixel >>= image->bitDepth * (pixelOffset % image->pixelsPerByte);
    return pixel & image->mask;
}

static int get_left_margin(struct Image const * image) {
    int maxw = image->width / 2;
    for (int x = 0; x < maxw; x++) {
        for (int y = 0; y < image->height; y++) {
            if (image_get_pixel(image, x, y)) return x;
        }
    }
    return maxw;
}

static int get_right_margin(struct Image const * image) {
    int maxw = image->width / 2;
    for (int x = 0; x < maxw; x++) {
        for (int y = 0; y < image->height; y++) {
            if (image_get_pixel(image, image->width - x - 1, y)) return x;
        }
    }
    return maxw;
}

static int get_top_margin(struct Image const * image) {
    int maxh = image->height / 2;
    for (int y = 0; y < maxh; y++) {
        for (int x = 0; x < image->width; x++) {
            if (image_get_pixel(image, x, y)) return y;
        }
    }
    return maxh;
}

static int get_bottom_margin(struct Image const * image) {
    int maxh = image->height / 2;
    for (int y = 0; y < maxh; y++) {
        for (int x = 0; x < image->width; x++) {
            if (image_get_pixel(image, x, image->height - y - 1)) return y;
        }
    }
    return maxh;
}

static void read_png(const char * path, struct Image * image) {
    png_structp png_ptr;
    png_infop info_ptr;
    FILE *fp = fopen(path, "rb");
    unsigned char sig[8];
    if (fp == NULL)
        FATAL_ERROR("Failed to open \"%s\" for reading.\n", path);
    if (fread(sig, sizeof(unsigned char), 8, fp) != 8)
        FATAL_ERROR("Failed to read PNG signature from \"%s\".\n", path);
    if (png_sig_cmp(sig, 0, 8))
        FATAL_ERROR("\"%s\" does not have a valid PNG signature.\n", path);
    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (!png_ptr)
        FATAL_ERROR("Failed to create PNG read struct.\n");
    info_ptr = png_create_info_struct(png_ptr);
    if (!info_ptr)
        FATAL_ERROR("Failed to create PNG info struct.\n");
    if (setjmp(png_jmpbuf(png_ptr)))
        FATAL_ERROR("Failed to init I/O for reading \"%s\".\n", path);
    png_init_io(png_ptr, fp);
    png_set_sig_bytes(png_ptr, 8);
    png_read_info(png_ptr, info_ptr);
    image->width = png_get_image_width(png_ptr, info_ptr);
    image->height = png_get_image_height(png_ptr, info_ptr);
    image->bitDepth = png_get_bit_depth(png_ptr, info_ptr);
    if (image->bitDepth != 1 && image->bitDepth != 2 && image->bitDepth != 4 && image->bitDepth != 8)
        FATAL_ERROR("Bit depth of image must be 1, 2, 4, or 8.");
    unsigned rowbytes = png_get_rowbytes(png_ptr, info_ptr);
    image->mask = (1 << image->bitDepth) - 1;
    image->pixelsPerByte = 8 / image->bitDepth;
    image->pixels = malloc(image->height * rowbytes);
    if (image->pixels == NULL)
        FATAL_ERROR("Failed to allocate pixel buffer.\n");
    png_bytepp row_pointers = malloc(image->height * sizeof(png_bytep));
    if (row_pointers == NULL)
        FATAL_ERROR("Failed to allocate row pointers.\n");
    for (int i = 0; i < image->height; i++)
        row_pointers[i] = (png_bytep)(image->pixels + (i * rowbytes));
    if (setjmp(png_jmpbuf(png_ptr)))
        FATAL_ERROR("Error reading from \"%s\".\n", path);
    png_read_image(png_ptr, row_pointers);
    png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
    fclose(fp);
    free(row_pointers);
}

static void get_sprite_coords(const char * path, int *width, int *height, int *bottom) {
    struct Image image = {};
    read_png(path, &image);
    int left = get_left_margin(&image);
    int right = get_right_margin(&image);
    int top = get_top_margin(&image);
    *bottom = get_bottom_margin(&image);
    *width = image.width / 8 - min(left, right) / 4;
    *height = image.height / 8 - min(top, *bottom) / 4;
    free(image.pixels);
}

int main(int argc, char ** argv) {
    for (int i = 1; i < argc; i++) {
        int width, height, bottom;
        get_sprite_coords(argv[i], &width, &height, &bottom);
        printf("%s %d %d %d\n", argv[i], width, height, bottom);
    }
    return 0;
}