Threshold an image with VIPS and a GTK slider UI.

threshold_scale.c

/** threshold_scale.c
 *
 * Threshold an image with VIPS and a GTK slider UI.
 *
 * COMPILE
 *
 * gcc -Wall -o threshold_scale threshold_scale.c `pkg-config vips gtk+-3.0 --cflags --libs`
 *
 * EXAMPLE
 *
 * ./threshold_scale
 *
 */

#include <vips/vips.h>
#include <gtk/gtk.h>

// The label text will match the current value of the slider.
static gchar label_text[4] = "0";

// Input and output image file paths
static gchar* input_image_path = "in.png";
static gchar* output_image_path = "out.png";

// The "value-change" signal handler
static void value_changed(GtkRange* range, gpointer window);

// The function called to update the image.
static void update_image(guint8 threshold);

// Define the image widget with a placeholder
static GtkWidget* image = NULL;

int main (int argc, char** argv) {
    // Initialize VIPS.
    if (VIPS_INIT(argv[0]))
        vips_error_exit(NULL);
    
    // Initialize GTK
    gtk_init(&argc, &argv);

    // Update the image for the first time, possibly creating it
    update_image(0);

    // Initialize the image
    image = gtk_image_new_from_file("out.png");

    // Create the window
    GtkWidget* window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_default_size(GTK_WINDOW(window), 250, 50);
    gtk_window_set_resizable (GTK_WINDOW(window), FALSE);
    gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);

    // Create the root widget, which contains all the other widgets
    GtkWidget* root = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 20);

    // Create the scale widget, a type of range widget.
    GtkWidget* scale =
            gtk_scale_new_with_range(GTK_ORIENTATION_HORIZONTAL, 0, 100, 1);
    gtk_scale_set_draw_value(GTK_SCALE(scale), FALSE);
    gtk_widget_set_size_request(scale, 150, -1);

    // Create label to show current value 
    GtkWidget* label = gtk_label_new("0");

    // Pack the image widget, scale widget, and label into the root widget
    gtk_box_pack_start(GTK_BOX(root), image, FALSE, FALSE, 20);
    gtk_box_pack_start(GTK_BOX(root), scale, FALSE, FALSE, 0);
    gtk_box_pack_start(GTK_BOX(root), label, FALSE, FALSE, 20);

    // Add the root widget to the window
    gtk_container_add(GTK_CONTAINER(window), root);

    // Connect signal handlers
    g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
    g_signal_connect(scale, "value-changed", G_CALLBACK(value_changed), label);            

    // Show the window
    gtk_widget_show_all(window);

    // Start the GTK main loop
    gtk_main();

    return EXIT_SUCCESS;
}

static void value_changed(GtkRange* range, gpointer window) {
    guint8 value = gtk_range_get_value(range);
    value = value < 0 ? 0 : value > 100 ? 100 : value;
    snprintf(label_text, 4, "%d", value);
    gtk_label_set_text(GTK_LABEL(window), label_text);
    update_image(value);
}

static void update_image(guint8 threshold) {
    // Hang images off a dummy object, `context`
    VipsObject* context = (VipsObject*) vips_image_new();
    VipsImage** t = (VipsImage**) vips_object_local_array(context, 4);

    // Threshold the image, producing a binary image with only black (#000) or
    // white (#FFF) pixels
    if (!(t[0] = vips_image_new_from_file(input_image_path,
                "access", VIPS_ACCESS_SEQUENTIAL, NULL)) ||
            vips_colourspace(t[0], &t[1], VIPS_INTERPRETATION_B_W, NULL) ||
            vips_extract_band(t[1], &t[2], 0, "n", 1, NULL) ||
            vips_moreeq_const1(t[2], &t[3], threshold, NULL) ||
            vips_image_write_to_file(t[3], output_image_path, NULL)) {
        g_object_unref(context);
        vips_error_exit(NULL);
    }

    // Clean up the dummy object and all images.
    g_object_unref(context);

    // Update the image widget
    gtk_image_set_from_file((GtkImage*) image, output_image_path);
}