Threshold an image with VIPS in C, revised version of threshold.c. Uses a built-in VIPS relational operator instead of thresholding by hand.

threshold1.c

/** threshold1.c
 *
 * COMPILE
 *
 * gcc -o threshold1 threshold1.c `pkg-config vips --cflags --libs`
 *
 * EXAMPLE
 *
 * ./threshold1 128 in.png out.png
 */

#include <vips/vips.h>

int main (int argc, char** argv) {
    if (argc != 4) {
        printf("USAGE: %s <THRESHOLD (0-255) <INPUT_IMAGE>", argv[0]);
        vips_error_exit(NULL);
    }

    if (VIPS_INIT(argv[0]))
        vips_error_exit(NULL);

    VipsImage *in, *out;

    double threshold = strtod(argv[1], NULL);

    if (!(in = vips_image_new_from_file(argv[2],
        "access", VIPS_ACCESS_SEQUENTIAL,
        NULL)))
        vips_error_exit(NULL);
    
    if (vips_colourspace(in, &out, VIPS_INTERPRETATION_B_W, NULL))
        vips_error_exit(NULL);
    g_object_unref(in);
    in = out;

    if (vips_extract_band(in, &out, 0, "n", 1, NULL))
        vips_error_exit(NULL);
    g_object_unref(in);
    in = out;

    if (vips_moreeq_const1(in, &out, threshold, NULL))
        vips_error_exit(NULL);
    g_object_unref(in);
    in = out;

    if (vips_image_write_to_file(in, argv[3], NULL))
        vips_error_exit(NULL);
    g_object_unref(in);
}