How to resize an image to fit the window in GTK3

[Martin Guy <martinwguy gmail com>]

Hi
   I'm trying to write a simple app in various toolkits on the way to
choosing one for the real thing. The test piece should display an
image which gets scaled to fit a resizable window. It works by
catching resize events and rescaling from the source image to the
displayed one. In GTK2 there was the deprecated
gtk_window_set_policy(GTK_WINDOW(window), 1, 1, 1);
whereas the replacements only let you enlarge the window but I'm
having difficulty making it allow you to make it any smaller again.
   Someone suggested putting the image in a gtk_scrolled_window and
resizing the image when the scrolled window is resized. That -er- sort
of works but the scrollbars pop up when you reduce the window size and
then don't go away again, as well as making the image jiggle about as
the window is resized, sometimes leaving the image with a white border
of a few pixels, apparently at random.
   Is there a better way to achieve the desired effect? My current code is:

/*
 * image1-gtk3.c: GUI toolkit test piece to display an image file.
 *
 * The image file is given as a command-line argument (default: image.jpg).
 * The window should open to exactly fit the image at one-pixel-per-pixel size.
 * The user can then resize the window in which case the image scales to fit
 * the window without keeping its aspect ratio.
 * If they hit Control-Q or poke the [X] icon in the window's titlebar,
 * the application should quit.
 *
 * It seems that in GTK3 you cannot resize a window smaller than the image
 * it currently contains, which means you can enlarge the window, and
 * the image grows too, but you cannot then reduce the window size again!
 *   Someone suggested putting the image in a GtkScrolledWindow and resizing
 * the image to exactly the size of scrolled window so that the scrollbars
 * disappear. It sort of works - see the bugs - and is gross.
 *
 * Bugs:
 *    - When you shrink the window, the viewport's scrollbars appear and
 *      then don't go away when you release the mouse button.
 *    - If you shrink the window size quickly, the image slobbers down and right
 *      in the viewport, leaving a variable-width white border top and left.
 *      Sometimes, when you stop shrinking and release the mouse, the white
 *      border does not disappear and tweaking the scrollbars reveals that the
 *      image is visualised the correct size for the window but centered on a
 *      slightly larger white canvas tens of pixels larger than the image.
 *    - The window has an arbitrary minimum size of 42x42.
 *
 *      Martin Guy <martinwguy gmail com>, October 2016.
 */

#include <gtk/gtk.h>
#include <gdk/gdkkeysyms.h>

/* Event callbacks */
static gboolean keyPress(GtkWidget *widget, gpointer data);
static gboolean sizeChanged(GtkWidget *widget, GtkAllocation
*allocation, gpointer data);

static    GtkWidget *image;             /* As displayed on the screen */

int
main(int argc, char **argv)
{
    GtkWidget *window;
    GtkWidget *viewport;
    GdkPixbuf *sourcePixbuf = NULL;     /* As read from a file */
    char *filename =  (argc > 1) ? argv[1] : "image.jpg";

    gtk_init(&argc, &argv);

    /* Make pixbuf, then make image from pixbuf because
     * gtk_image_new_from_file() doesn't flag errors */
    {
        GError *error = NULL;
        sourcePixbuf = gdk_pixbuf_new_from_file(filename, &error);
        if (sourcePixbuf == NULL) {
            g_message("%s", error->message);
            return 1; /* exit() */
        }
    }

    /* On expose/resize, the image's pixbuf will be overwritten
     * but we will still need the original image so take a copy of it */
    image = gtk_image_new_from_pixbuf(gdk_pixbuf_copy(sourcePixbuf));

    viewport = gtk_scrolled_window_new(NULL, NULL);
    /* Saying "1x1" reduces the window's minumum size from 55x55 to 42x42. */
    gtk_scrolled_window_set_min_content_width(GTK_SCROLLED_WINDOW(viewport), 1);
    gtk_scrolled_window_set_min_content_height(GTK_SCROLLED_WINDOW(viewport),
1);

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title(GTK_WINDOW(window), "image1-gtk3");

    /* Quit if they ask the window manager to close the window */
    g_signal_connect(G_OBJECT(window), "destroy",
                     G_CALLBACK(gtk_main_quit), NULL);
    /* Quit on control-Q. */
    g_signal_connect(window, "key-press-event", G_CALLBACK(keyPress), NULL);

    /* When the window is resized, scale the image to fit */
    g_signal_connect(viewport, "size-allocate",
G_CALLBACK(sizeChanged), sourcePixbuf);

    /* The image is in a scrolled window container so that the main window
     * can be resized smaller than the current image. */
    gtk_container_add(GTK_CONTAINER(viewport), image);
    gtk_container_add(GTK_CONTAINER(window), viewport);

    //gtk_window_set_resizable(GTK_WINDOW(window), 1);
    /* Open the window the same size as the image */
    gtk_window_set_default_size(GTK_WINDOW(window),
        gdk_pixbuf_get_width(sourcePixbuf),
        gdk_pixbuf_get_height(sourcePixbuf));

    gtk_widget_show_all(window);

    gtk_main();

    return 0;
}

/* Callback functions */

/* Check for Control-Q and quit if it was pressed */
static gboolean
keyPress(GtkWidget *widget, gpointer data)
{
    GdkEventKey *event = (GdkEventKey *) data;

    if (event->keyval == GDK_KEY_q && (event->state & GDK_CONTROL_MASK)) {
        gtk_main_quit();
        return FALSE;
    } else
        return TRUE;
}

/* If the window has been resized, that resizes the scrolledwindow,
 * and we scale the image to the dimensions of the scrolledwindow so that
 * the scrollbars disappear again. Yuk! */
static gboolean
sizeChanged(GtkWidget *widget, GtkAllocation *allocation, gpointer data)
{
    GdkPixbuf *sourcePixbuf = data;     /* As read from a file */
    GdkPixbuf *imagePixbuf;     /* pixbuf of the on-screen image */

    imagePixbuf = gtk_image_get_pixbuf(GTK_IMAGE(image));
    if (imagePixbuf == NULL) {
        g_message("Can't get on-screen pixbuf");
        return TRUE;
    }
    /* Recreate the displayed image if the image size has changed. */
    if (allocation->width != gdk_pixbuf_get_width(imagePixbuf) ||
        allocation->height != gdk_pixbuf_get_height(imagePixbuf)) {

        gtk_image_set_from_pixbuf(
            GTK_IMAGE(image),
            gdk_pixbuf_scale_simple(sourcePixbuf,
                                    allocation->width,
                                    allocation->height,
                                    GDK_INTERP_BILINEAR)
        );
        g_object_unref(imagePixbuf); /* Free the old one */
    }

    return FALSE;
}

Thanks!

    M