.hgignore

syntax:glob
main
*.o

image.h

typedef int Image;

Image *make_image(int foo);
char access_pixel(Image *img, int r, int c);

image_cv.c

#include <stdlib.h>
#include <image.h>

typedef struct {
    int foo, bar;
} IplImage;

Image *make_image(int foo) {
    IplImage *img = (IplImage *) malloc(sizeof(IplImage));
    if (!img)
        return NULL;
    img->foo = foo;
    img->bar = foo + 14;
    return (Image *) img;
}

char access_pixel(Image *img, int r, int c) {
    IplImage *iplimg = (IplImage *) img;
    return iplimg->foo + r + c;
}

main.c

#include <stdio.h>
#include <image.h>

int main() {
    Image *img = make_image(42);
    if (!img) {
        fprintf(stderr, "error: querying an image for 42 failed\n");
        return 1;
    }
    struct {
        char c;
    } a = {'a'};
    printf("access_pixel(img, 0, 1) = %d\n", access_pixel(&a, 0, 0));
    return 0;
}