'Draw a GtkImage with Cairo

I tried the solutions proposed in similary notes (How to create a gtkimage from a cairo context, How do I create a gtkimage from a cairo surface) but they didn't work for me.

In my actual program, a background task calculates successive states of a physics simulation, and each iteration should add a point to a diagram. If I used a GtkDrawingArea, I would have to store all these points (not knowing in advance how many there will be), and draw everything upon each draw signal. So my idea is to use a GtkImage, where I have to draw only the one new point in each iteration.

In the following test program something should be drawn upon a button click, but nothing happens. Can somebody please indicate the error?

/* Test for drawing an image with Cairo. To be compiled with:
   gcc -Wall -g gtk_img.c -o gtk_img `pkg-config --cflags gtk+-3.0` `pkg-config --libs gtk+-3.0`
*/

#include <stdio.h>
#include <gdk/gdk.h>
#include <cairo.h>
#include <gtk/gtk.h>

#define BORDER      6
#define WIDTH       100
#define HEIGHT      100

cairo_surface_t *surf;
cairo_t         *cr;
cairo_pattern_t *back, *fore;


/*  Event Handlers
    ==============
*/

static gboolean Delete_Event
       (GtkWidget *widget, GdkEvent *event, gpointer data)
{
    return FALSE;
}

static void Destroy (GtkWidget *widget, gpointer data)
{
    gtk_main_quit ();
}

static void Draw_Now (GtkWidget *widget, gpointer data)
{
    printf("Draw button clicked\n");
    cr = cairo_create (surf);
    cairo_set_source (cr, back);
    cairo_paint (cr);
    cairo_set_source (cr, fore);
    cairo_move_to (cr, 0.0, 0.0);
    cairo_line_to (cr, 20.0, 20.0);
    cairo_stroke (cr);
}


/*  Main Program
    ============
*/

int main (int argc, char *argv[])
{
    GtkWidget   *window;
    GtkImage    *img;
    GtkBox      *vbox;
    GtkButton   *button;
    
    // init GTK, make window with vbox
    gtk_init (&argc, &argv);
    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    g_signal_connect (G_OBJECT (window), "delete_event",
              G_CALLBACK (Delete_Event), NULL);
    g_signal_connect (G_OBJECT (window), "destroy",
              G_CALLBACK (Destroy), NULL);
    gtk_container_set_border_width (GTK_CONTAINER (window), BORDER);
    vbox = (GtkBox*)gtk_box_new (GTK_ORIENTATION_VERTICAL, BORDER);
    gtk_container_add (GTK_CONTAINER(window), (GtkWidget*)vbox);

    // make image
    surf = cairo_image_surface_create (CAIRO_FORMAT_RGB24, WIDTH, HEIGHT);
    img = (GtkImage*)gtk_image_new_from_surface (surf);
    gtk_image_clear (img);
    back = cairo_pattern_create_rgb (0.9, 0.9, 0.9);
    fore = cairo_pattern_create_rgb (0.1, 0.1, 0.1);
    gtk_box_pack_start (vbox, (GtkWidget*)img, TRUE, TRUE, FALSE);

    // button
    button = (GtkButton*)gtk_button_new_with_label ("Draw");
    g_signal_connect
       (G_OBJECT (button), "clicked", G_CALLBACK (Draw_Now), NULL);
    gtk_box_pack_start (vbox, (GtkWidget*)button, FALSE, FALSE, FALSE);

    // start GTK
    gtk_widget_show_all (window);
    gtk_main ();
    
    return 0;
}

c gtk3 cairo

Solution 1:^[1]

You are drawing to the cairo surface, but "nothing knows", so no redraw is triggered.

A random idea to trigger a redraw would be to call gtk_image_clear() and then gtk_image_set_from_surface(). I bet there are better ways to do that, but I don't really know GTK, sorry.

Sources

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Source: Stack Overflow

Solution	Source
Solution 1	Uli Schlachter

'Draw a GtkImage with Cairo

Solution 1:[1]

Sources

Related Questions

Solution 1:^[1]