Re: configure-event and double buffering

[George Brink <siberianowl yahoo com>]

Problem solved.
To prevent flashes of the black screen during configure-event we need to 
implicitly call actual drawing procedure right after the off-screen 
buffer is recreated.

static gboolean configureCanvas(GtkWidget *widget, GdkEventConfigure 
*event ) {
if (pixmap) g_object_unref(pixmap);
pixmap = gdk_pixmap_new(widget->window, widget->allocation.width, 
widget->allocation.height, -1);
drawAll(); // just add this!
return TRUE;
}

Full source of my test application is in the attachment - may be it 
would be useful for other cairo-newbies :)

On 12/13/2011 1:30 PM, George Brink wrote:
> I am trying to make an animated image:
> ----------------------------------------------
> static gboolean exposeCanvas(GtkWidget *widget, GdkEventExpose *event,
> gpointer data) {
> cairo_t *cr = gdk_cairo_create (event->window);
> gdk_cairo_set_source_pixmap (cr, pixmap, 0, 0);
> gdk_cairo_rectangle (cr, &event->area);
> cairo_fill (cr);
> cairo_destroy (cr);
> return FALSE;
> }
>
> static gboolean configureCanvas(GtkWidget *widget, GdkEventConfigure
> *event ) {
> if (pixmap) g_object_unref(pixmap);
> pixmap = gdk_pixmap_new(widget->window, widget->allocation.width,
> widget->allocation.height, -1);
> return TRUE;
> }
>
> GtkWidget *canvas = gtk_drawing_area_new();
> gtk_container_add(GTK_CONTAINER(mainWindow), canvas);
> g_signal_connect(G_OBJECT(canvas), "expose_event",
> G_CALLBACK(exposeCanvas), NULL);
> g_signal_connect(G_OBJECT(canvas), "configure_event",
> G_CALLBACK(configureCanvas), NULL);
> ----------------------------------------------
>
> It works fine on a static window, but when I actually change the size of
> the window the whole drawing area goes black for a moment.
> What am I doing wrong?

#include <glib.h>
#include <gtk/gtk.h>
#include <math.h>


static GdkPixmap *pixmap = NULL;

void drawAll() {
	static gdouble angle=0;

	// actual drawing - moving line over the grid
	#define WIDTH 20
	#define HEIGHT 20
	#define LINE_LENGTH 5

	guint width, height;
	gdk_drawable_get_size(pixmap, &width, &height);

	cairo_t *cr = gdk_cairo_create(pixmap);
	cairo_set_source_rgb (cr, 0.8, 0.8, 0.8);
	cairo_paint(cr);

	guint x = (width -2)/WIDTH;
	guint y = (height -2)/HEIGHT;
	guint8 cellSize = x<y ? x : y;
	guint leftPadding = (width - WIDTH*cellSize) / 2;
	guint topPadding = (height - HEIGHT*cellSize) / 2;

	cairo_set_line_width (cr, 1.0);
	cairo_set_line_cap (cr, CAIRO_LINE_CAP_SQUARE);
	cairo_set_source_rgb (cr, 0.3, 0.3, 0.3);

	for(x=0; x<=WIDTH; x++) {
		cairo_move_to (cr, leftPadding+x*cellSize, topPadding);
		cairo_line_to (cr, leftPadding+x*cellSize, topPadding+HEIGHT*cellSize);
	}
	for(y=0; y<=HEIGHT; y++) {
		cairo_move_to (cr, leftPadding, topPadding+y*cellSize);
		cairo_line_to (cr, leftPadding+WIDTH*cellSize, topPadding+y*cellSize);
	}
	cairo_stroke(cr);

	cairo_set_line_width (cr, 3.0);
	cairo_set_source_rgb (cr, 0.0, 0.0, 0.0);
	cairo_move_to (cr, width/2, height/2);
	cairo_line_to (cr, width/2 + cos(angle)*LINE_LENGTH*cellSize, height/2 + sin(angle)*LINE_LENGTH*cellSize);
	angle += M_PI/180.0;
	if(angle> 2*M_PI) angle=0;

	cairo_stroke(cr);
	cairo_destroy(cr);
}


static gboolean exposeCanvas(GtkWidget *widget, GdkEventExpose *event, gpointer data) {
	cairo_t *cr = gdk_cairo_create (event->window);
	gdk_cairo_set_source_pixmap (cr, pixmap, 0, 0);
	gdk_cairo_rectangle (cr, &event->area);
	cairo_fill (cr);
	cairo_destroy (cr);
	return FALSE;
}

static gboolean configureCanvas(GtkWidget *widget, GdkEventConfigure *event ) {
	if (pixmap) g_object_unref(pixmap);
	pixmap = gdk_pixmap_new(widget->window, widget->allocation.width, widget->allocation.height, -1);
	drawAll();
	return TRUE;
}

static gboolean timerTick(gpointer data) {
	drawAll();

	GDK_THREADS_ENTER ();
	gtk_widget_queue_draw (GTK_WIDGET(data));
	GDK_THREADS_LEAVE ();
	return TRUE;
}



int main(int argc, char **argv) {
	gtk_init(&argc, &argv);

	GtkWidget *window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
	g_signal_connect(window, "destroy", G_CALLBACK (gtk_main_quit), NULL);
	gtk_window_set_title(GTK_WINDOW(window), "test");

	GtkWidget *canvas = gtk_drawing_area_new();
	gtk_container_add(GTK_CONTAINER(window), canvas);
	g_signal_connect(G_OBJECT(canvas), "expose_event", G_CALLBACK(exposeCanvas), NULL);
	g_signal_connect(G_OBJECT(canvas), "configure_event", G_CALLBACK(configureCanvas), NULL);
	guint timerID = g_timeout_add(100, timerTick, canvas);

	gtk_widget_show_all(window);
	gtk_main();
	return 0;
}