GTK3: Resize Image while retaining aspect-ratio - c

Question

When I'm resizing the GtkWindow, i want the GtkImage to resize as well while keeping the same aspect ratio as before.
I can't find any good examples on how to set this up with GTK3
This is what i've tried so far:
#include <gtk/gtk.h>
#include <gdk-pixbuf/gdk-pixbuf.h>
GtkAllocation *allocation = g_new0 (GtkAllocation, 1);
gboolean resize_image(GtkWidget *widget, GdkEvent *event, GtkWidget *window) {
GdkPixbuf *pixbuf = gtk_image_get_pixbuf(GTK_IMAGE(widget));
if (pixbuf == NULL) {
g_printerr("Failed to resize image\n");
return 1;
}
gtk_widget_get_allocation(GTK_WIDGET(widget), allocation);
pixbuf = gdk_pixbuf_scale_simple(pixbuf, widget->allocation.width, widget->allocation.height, GDK_INTERP_BILINEAR);
gtk_image_set_from_pixbuf(GTK_IMAGE(widget), pixbuf);
return FALSE;
}
int main(int argc, char **argv) {
GtkWidget *window = NULL;
GtkWidget *image = NULL;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
image = gtk_image_new_from_file("image.jpg");
g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
g_signal_connect(image, "draw", G_CALLBACK(resize_image), (gpointer)window);
gtk_container_add(GTK_CONTAINER(window), image);
gtk_widget_show_all(GTK_WIDGET(window));
gtk_main();
return 0;
}
This code should just resize the Pixbuf to the size of the parent, but it doesn't work, i get these errors:
GdkPixbuf-CRITICAL **: gdk_pixbuf_get_width: assertion 'GDK_IS_PIXBUF
(pixbuf)' failed
GLib-GObject-CRITICAL **: g_object_unref: assertion 'G_IS_OBJECT
(object)' failed
Even if this code would work, i wouldn't be able to keep the same aspect ratio, how to achieve this?
Thanks in Advance

Answer 1

Well, a trick to do this is using a GtkLayout between the image and the window.
Also, instead of using the draw signal, use the size-allocate signal.
We will load a GdkPixbuf and use it as reference for scaling, otherwise the quality would deteriorate with cumulative 'resizing'.
A simple approach would be:
#include <gtk/gtk.h>
#include <gdk-pixbuf/gdk-pixbuf.h>
struct _resize_widgets {
GtkWidget *image;
GdkPixbuf *pixbuf;
};
typedef struct _resize_widgets ResizeWidgets;
gboolean resize_image(GtkWidget *widget, GdkRectangle *allocation, gpointer user_data) {
int x,y,w,h;
GdkPixbuf *pxbscaled;
GtkWidget *image = (GtkWidget *) ((ResizeWidgets *) user_data)->image;
GdkPixbuf *pixbuf= (GdkPixbuf *) ((ResizeWidgets *) user_data)->pixbuf;
x = 0;
y = 0;
h = allocation->height;
w = (gdk_pixbuf_get_width(pixbuf) * h) / gdk_pixbuf_get_height(pixbuf);
pxbscaled = gdk_pixbuf_scale_simple(pixbuf, w, h, GDK_INTERP_BILINEAR);
if (w < allocation->width) {
x = (allocation->width - w) / 2;
gtk_layout_move(GTK_LAYOUT(widget), image, x, y);
}
gtk_image_set_from_pixbuf(GTK_IMAGE(image), pxbscaled);
g_object_unref (pxbscaled);
return FALSE;
}
int main(int argc, char **argv) {
GtkWidget *window = NULL;
GtkWidget *image = NULL;
GtkWidget *container = NULL;
GdkPixbuf *pixbuf = NULL;
ResizeWidgets *widgets;
gtk_init(&argc, &argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
container = gtk_layout_new(NULL, NULL);
image = gtk_image_new();
pixbuf = gdk_pixbuf_new_from_file ("image.png", NULL);
if (pixbuf == NULL) {
g_printerr("Failed to resize image\n");
return 1;
}
widgets = g_new0(ResizeWidgets, 1);
widgets->image = image;
widgets->pixbuf = pixbuf;
gtk_container_add(GTK_CONTAINER(window), container);
gtk_layout_put(GTK_LAYOUT(container), image, 0, 0);
gtk_widget_set_size_request (GTK_WIDGET(window), 20, 20);
g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
g_signal_connect(container, "size-allocate", G_CALLBACK(resize_image), widgets);
gtk_widget_show_all(GTK_WIDGET(window));
gtk_main();
g_object_unref (pixbuf);
g_free(widgets);
return 0;
}
Compile with gcc -o main main.cpkg-config --cflags --libs gtk+-3.0`
This will keep aspect ratio. If that is not desired, then the resize_image callback handler would be:
gboolean resize_image(GtkWidget *widget, GdkRectangle *allocation, gpointer user_data) {
int x,y,w,h;
GdkPixbuf *pxbscaled;
GtkWidget *image = (GtkWidget *) ((ResizeWidgets *) user_data)->image;
GdkPixbuf *pixbuf= (GdkPixbuf *) ((ResizeWidgets *) user_data)->pixbuf;
pxbscaled = gdk_pixbuf_scale_simple(pixbuf, allocation->width, allocation->height, GDK_INTERP_BILINEAR);
gtk_image_set_from_pixbuf(GTK_IMAGE(image), pxbscaled);
g_object_unref (pxbscaled);
return FALSE;
}
This is a simple hack, there are other options obviously, like cairo for drawing or simply checking Eye of Gnome (eog) implementation which is much more complete.