diff --git a/data/sprites/breadball.png b/data/sprites/breadball.png
new file mode 100644
index 0000000..da1cba7
Binary files /dev/null and b/data/sprites/breadball.png differ
diff --git a/data/sprites/bricks.png b/data/sprites/bricks.png
new file mode 100644
index 0000000..fe10e29
Binary files /dev/null and b/data/sprites/bricks.png differ
diff --git a/helianthus/animation.c b/helianthus/animation.c
index acccf36..068d7c6 100644
--- a/helianthus/animation.c
+++ b/helianthus/animation.c
@@ -20,10 +20,10 @@ static cairo_surface_t* loadFrame(const char *path) {
 		cairo_surface_t *frame = cairo_image_surface_create_from_png_stream(&read, f);
 		fclose(f);
 		if (!frame)
-			fprintf(stderr, "helianthus: cannot load PNG content form file: %s", path);
+			fprintf(stderr, "helianthus: cannot load PNG content form file: %s\n", path);
 		return frame;
 	}
-	fprintf(stderr, "helianthus: cannot open image file: %s", path);
+	fprintf(stderr, "helianthus: cannot open image file: %s\n", path);
 	return NULL;
 }
 
@@ -46,7 +46,7 @@ static HeliAnimation* load(const char *path) {
 		if (frame) {
 			heliArrayInsert(&a->frames, -1, frame, (HeliFreeCallback)&cairo_surface_destroy);
 		} else {
-			fprintf(stderr, "helianthus: cannot load animation by path: %s", path);
+			fprintf(stderr, "helianthus: cannot load animation by path: %s\n", path);
 		}
 	}
 	
diff --git a/helianthus/array.c b/helianthus/array.c
index 707e313..6e87b14 100644
--- a/helianthus/array.c
+++ b/helianthus/array.c
@@ -60,7 +60,7 @@ HeliPair* heliArrayInsertPair(HeliArray *a, int i, void *k, void *v, HeliFreeCal
 }
 
 HeliPair* heliArrayInsert(HeliArray *a, int i, void *v, HeliFreeCallback fv)
-	{ return heliArrayInsertPair(a, i, NULL, NULL, v, fv); }
+	{ return heliArrayInsertPair(a, i, NULL, v, NULL, fv); }
 
 void heliArrayRemove(HeliArray *a, int i) {
 	if (i < 0 || i >= a->count) return;
diff --git a/helianthus/common.c b/helianthus/common.c
index 7020b40..c183e83 100644
--- a/helianthus/common.c
+++ b/helianthus/common.c
@@ -42,7 +42,7 @@ int heliStringEndsWithLowcase(const char *s, const char *tail) {
 }
 
 void heliLowercase(char *x)
-	{ while(*x) *x = tolower(*x); }
+	{ while(*x) { *x = tolower(*x); ++x; } }
 
 void heliParseColor(const char *x, double *color) {
 	#warning "TODO: heliParseColor: not implemented yet"
diff --git a/helianthus/main.c b/helianthus/main.c
index dc75f8a..b0c0a49 100644
--- a/helianthus/main.c
+++ b/helianthus/main.c
@@ -1,10 +1,38 @@
 
+#include <stdio.h>
+
 #include "helianthus.h"
 
+Sprite ball, brick1, brick2;
+
 void init() {
+	ball = createSpriteEx(200, 200, 64, 64);
+	spriteSetAnimation(ball, "data/sprites/breadball.png");
+	spriteSetDebug(ball, TRUE);
+	
+	brick1 = createSpriteEx(200-32, 200+64, 64, 64);
+	spriteSetAnimation(brick1, "data/sprites/bricks.png");
+	spriteSetDebug(brick1, TRUE);
+	
+	brick2 = createSpriteEx(200+32, 200+64, 64, 64);
+	spriteSetAnimation(brick2, "data/sprites/bricks.png");
+	spriteSetDebug(brick2, TRUE);
 }
 
 void draw() {
+	double dt = worldGetTimeStep();
+	double speed = 100;
+	
+	spritePointTo(ball, mouseX(), mouseY());
+	
+	double x = spriteGetX(ball);
+	double y = spriteGetY(ball);
+	if (keyDown("left"))  spriteSetX(ball, x - speed*dt);
+	if (keyDown("right")) spriteSetX(ball, x + speed*dt);
+	if (keyDown("up"))    spriteSetY(ball, y - speed*dt);
+	if (keyDown("down"))  spriteSetY(ball, y + speed*dt);
+	
+	drawSprites();
 }
 
 int main() {
diff --git a/helianthus/sprite.c b/helianthus/sprite.c
index 69c579a..57fb407 100644
--- a/helianthus/sprite.c
+++ b/helianthus/sprite.c
@@ -204,7 +204,7 @@ void spriteSetCollider(Sprite sprite, Collider type, double xOffset, double yOff
 void spriteSetAnimation(Sprite sprite, const char *path) {
 	HeliAnimation *prev = sprite->animation;
 	sprite->animation = heliCreateAnimation(path);
-	heliAnimationUnref(prev);
+	if (prev) heliAnimationUnref(prev);
 	spriteSetFrame(sprite, sprite->frame);
 }
 
@@ -241,7 +241,7 @@ double spriteGetDirection(Sprite sprite) {
 	     ? atan2(sprite->vy, sprite->vx)*(180/PI) : 0;
 }
 void spritePointTo(Sprite sprite, double x, double y)
-	{ spriteSetRotation( sprite, atan2(y - sprite->y, x - sprite->x)*(PI/180) ); }
+	{ spriteSetRotation( sprite, atan2(y - sprite->y, x - sprite->x)*(180/PI) ); }
 
 double spriteGetScaledWidth(Sprite sprite)
 	{ return sprite->width * sprite->scale; }
@@ -259,7 +259,9 @@ Sprite worldGetSprite(int i)
 
 static void drawSpriteDebug(cairo_t *cr, Sprite s) {
 	cairo_save(cr);
+	cairo_set_line_width(cr, 0.5);
 	cairo_translate(cr, s->x, s->y);
+	cairo_rotate(cr, s->rotation*(PI/180));
 	cairo_scale(cr, s->scale*s->mirrorX, s->scale*s->mirrorY);
 	
 	double hw = s->width*0.5;
@@ -282,10 +284,11 @@ static void drawSpriteDebug(cairo_t *cr, Sprite s) {
 	char buf[1024];
 	snprintf(buf, sizeof(buf)-1, "%f", s->depth);
 	
-	double s1 = hw*0.5;
+	double s1 = hw*0.25;
 	double s2 = hh*0.5;
 	cairo_set_font_size(cr, s1 < s2 ? s1 : s2);
-	cairo_move_to(cr, -hw*0.9, -hh*0.9);
+
+	cairo_move_to(cr, -hw*0.9, -hh*0.5);
 	cairo_show_text(cr, buf);
 
 	cairo_restore(cr);
@@ -294,6 +297,7 @@ static void drawSpriteDebug(cairo_t *cr, Sprite s) {
 static void drawSprite(cairo_t *cr, Sprite s) {
 	cairo_save(cr);
 	cairo_translate(cr, s->x, s->y);
+	cairo_rotate(cr, s->rotation*(PI/180));
 	cairo_scale(cr, s->scale*s->mirrorX, s->scale*s->mirrorY);
 	
 	double hw = s->width*0.5;
@@ -312,6 +316,7 @@ static void drawSprite(cairo_t *cr, Sprite s) {
 			cairo_save(cr);
 			cairo_scale(cr, hw/ihw, hh/ihh);
 			cairo_set_source_surface(cr, frame, -ihw, -ihh);
+			cairo_paint(cr);
 			cairo_restore(cr);
 		}
 	} else {
@@ -374,12 +379,13 @@ void drawSprites() {
 		}
 	}
 	
-	// draw
 	if (heliCairo) {
+		// draw
 		for(int i = 0; i < spritesSorted.count; ++i) {
 			Sprite s = (Sprite)(spritesSorted.items[i].value);
 			if (s->visible) drawSprite(heliCairo, s);
 		}
+		// draw debug marks
 		for(int i = 0; i < spritesSorted.count; ++i) {
 			Sprite s = (Sprite)(spritesSorted.items[i].value);
 			if (s->debug) drawSpriteDebug(heliCairo, s);
diff --git a/helianthus/world.c b/helianthus/world.c
index 2bbc96a..6a79902 100644
--- a/helianthus/world.c
+++ b/helianthus/world.c
@@ -20,8 +20,8 @@ static guint timerInterval;
 static Callback initCallback;
 static Callback drawCallback;
 
-static int width;
-static int height;
+static int width = 400;
+static int height = 400;
 static double frameRate = 30;
 
 static int cameraEnabled;
@@ -79,19 +79,24 @@ double mouseY()
 int mousePressedOver(Sprite sprite)
 	{ return buttonsPressed.count && mouseIsOver(sprite); }
 
+static void resize(int w, int h) {
+	width  = w > 200 ? w : 100;
+	height = h > 200 ? h : 200;
+	if (started) {
+		gtk_window_set_default_size(GTK_WINDOW(window), width, height);
+		gtk_widget_set_size_request(window, width, height);
+	}
+}
+	
 int worldGetWidth()
 	{ return width; }
-void worldSetWidth(int w) {
-	width = w;
-	if (started) gtk_widget_set_size_request(window, width, height);
-}
+void worldSetWidth(int w)
+	{ resize(w, height); }
 
 int worldGetHeight()
 	{ return height; }
-void worldSetHeight(int h) {
-	height = h;
-	if (started) gtk_widget_set_size_request(window, width, height);
-}
+void worldSetHeight(int h)
+	{ resize(width, h); }
 
 double worldGetFrameRate()
 	{ return frameRate; }
@@ -100,6 +105,9 @@ void worldSetFrameRate(double frameRate) {
 	if (!(frameRate < 100)) frameRate = 100;
 }
 
+double worldGetTimeStep()
+	{ return 1/frameRate; }
+
 int worldGetFrameCount()
 	{ return (int)frameCount; }
 double worldGetSeconds()
@@ -150,7 +158,7 @@ static gboolean timeout(gpointer data G_GNUC_UNUSED) {
 	guint interval = (guint)round(1000/frameRate);
 	if (interval != timerInterval) {
 		timerInterval = interval;
-		g_timeout_add(timerInterval, timeout, NULL);
+		g_timeout_add_full(G_PRIORITY_LOW, timerInterval, timeout, NULL, NULL);
 		return FALSE;
 	}
 	return TRUE;
@@ -160,30 +168,22 @@ static gboolean draw(GtkWidget *widget G_GNUC_UNUSED, cairo_t *cr, gpointer data
 	guint64 newTime = g_get_monotonic_time();
 	heliCairo = cr;
 
-	if (!frameCount) {
-		startTime = currentTime = newTime;
-		if (initCallback) initCallback();
+	double dt = 1/frameRate;
+	guint64 timeStep = (guint64)round(dt*1e6);
+	while(currentTime < newTime) {
+		cairo_save(cr);
 		heliDrawingPrepareFrame();
 		if (drawCallback) drawCallback();
+		cairo_restore(cr);
+
+		currentTime += timeStep;
+		++frameCount;
+		heliSpriteUpdate(dt);
+
 		heliArrayClear(&keysPressedInFrame);
 		heliArrayClear(&keysReleasedInFrame);
 		heliArrayClear(&buttonsPressedInFrame);
 		heliArrayClear(&buttonsReleasedInFrame);
-		++frameCount;
-	} else {
-		double dt = 1/frameRate;
-		guint64 timeStep = (guint64)round(dt*1e6);
-		while(currentTime < newTime) {
-			currentTime += timeStep;
-			++frameCount;
-			heliSpriteUpdate(dt);
-			heliDrawingPrepareFrame();
-			if (drawCallback) drawCallback();
-			heliArrayClear(&keysPressedInFrame);
-			heliArrayClear(&keysReleasedInFrame);
-			heliArrayClear(&buttonsPressedInFrame);
-			heliArrayClear(&buttonsReleasedInFrame);
-		}
 	}
 
 	heliCairo = NULL;
@@ -200,6 +200,7 @@ static gboolean handleEvent(GtkWidget *widget G_GNUC_UNUSED, GdkEvent *event, gp
 		heliLowercase(keyname);
 		heliStringmapAdd(&keysPressed, keyname, NULL, NULL);
 		heliStringmapAdd(&keysPressedInFrame, keyname, NULL, NULL);
+		//printf("key pressed: %s\n", keyname);
 		free(keyname);
 		return TRUE;
 	case GDK_KEY_RELEASE:
@@ -241,6 +242,8 @@ static void activate(GtkApplication* app, gpointer data G_GNUC_UNUSED) {
 	window = gtk_application_window_new(app);
 	g_signal_connect(window, "draw", G_CALLBACK(draw), NULL);
 	g_signal_connect(window, "event", G_CALLBACK(handleEvent), NULL);
+	gtk_window_set_resizable(GTK_WINDOW(window), FALSE);
+	gtk_window_set_default_size(GTK_WINDOW(window), width, height);
 	gtk_widget_add_events( window,
 						   GDK_POINTER_MOTION_MASK
 						 | GDK_BUTTON_PRESS_MASK
@@ -249,9 +252,12 @@ static void activate(GtkApplication* app, gpointer data G_GNUC_UNUSED) {
 						 | GDK_KEY_RELEASE_MASK );
 	gtk_widget_set_size_request(window, width, height);
 	gtk_widget_show(window);
-	timerInterval = (guint)round(1000/frameRate);
-	g_timeout_add(timerInterval, timeout, NULL);
+
 	if (initCallback) initCallback();
+	
+	currentTime = g_get_monotonic_time();
+	timerInterval = (guint)round(1000/frameRate);
+	g_timeout_add_full(G_PRIORITY_LOW, timerInterval, timeout, NULL, NULL);
 }
 
 void worldRun() {
diff --git a/helianthus/world.h b/helianthus/world.h
index 8faff8b..2b8b377 100644
--- a/helianthus/world.h
+++ b/helianthus/world.h
@@ -41,6 +41,8 @@ void worldSetHeight(int height);
 double worldGetFrameRate();
 void worldSetFrameRate(double frameRate);
 
+double worldGetTimeStep();
+
 int worldGetFrameCount();
 double worldGetSeconds();