games/galaxy: fix zoom

Zooming when far away from the center of gravity
of the galaxy would zoom the center of the screen
out of view. Now adjust the origin so that the
center of the screen stays centered

sys/src/games/galaxy/body.c

@@ -31,14 +31,23 @@ body(void)
 	return b;
 }
 
+Point
+topoint(Vector v)
+{
+	Point p;
+
+	p.x = v.x/scale + orig.x;
+	p.y = v.y/scale + orig.y;
+	return p;
+}
+
 void
 drawbody(Body *b)
 {
 	Point pos, v;
 	int s;
 
-	pos.x = b->x / scale + orig.x;
-	pos.y = b->y / scale + orig.y;
+	pos = topoint(b->Vector);
 	s = b->size/scale;
 	fillellipse(screen, pos, s, s, b->col, ZP);
 	v.x = b->v.x/scale*10;

sys/src/games/galaxy/galaxy.c

@@ -171,8 +171,7 @@ drawglxy(void)
 
 	draw(screen, screen->r, display->black, 0, ZP);
 	for(b = glxy.a; b < glxy.a + glxy.l; b++) {
-		pos.x = b->x / scale + orig.x;
-		pos.y = b->y / scale + orig.y;
+		pos = topoint(b->Vector);
 		s = b->size/scale;
 		fillellipse(screen, pos, s, s, b->col, ZP);
 		if(showv) {
@@ -198,8 +197,7 @@ setsize(Body *b)
 	Point pos, d;
 	double h;
 
-	pos.x = b->x / scale + orig.x;
-	pos.y = b->y / scale + orig.y;
+	pos = topoint(b->Vector);
 	d = subpt(mc->xy, pos);
 	h = hypot(d.x, d.y);
 	b->size = h == 0 ? scale : h*scale;
@@ -211,18 +209,10 @@ setvel(Body *b)
 {
 	Point pos, d;
 
-	pos.x = b->x / scale + orig.x;
-	pos.y = b->y / scale + orig.y;
+	pos = topoint(b->Vector);
 	d = subpt(mc->xy, pos);
-	b->v.x = (double)d.x*scale/10;
-	b->v.y = (double)d.y*scale/10;
-}
-
-void
-setpos(Body *b)
-{
-	b->x = (mc->xy.x - orig.x) * scale;
-	b->y = (mc->xy.y - orig.y) * scale;
+	b->v.x = d.x*scale/10;
+	b->v.y = d.y*scale/10;
 }
 
 void
@@ -275,7 +265,6 @@ dobody(void)
 	}
 
 	b = body();
-	setpos(b);
 	setvel(b);
 	setsize(b);
 	b->col = randcol();
@@ -290,7 +279,7 @@ dobody(void)
 		else if(mc->buttons == 5)
 			dovel(b);
 		else
-			setpos(b);
+			b->Vector = tovector(mc->xy);
 	}
 
 	CHECKLIM(b, f);
@@ -349,30 +338,41 @@ domove(void)
 	setcursor(mc, cursor);
 }
 
+Point
+screencenter(void)
+{
+	Point sc;
+
+	sc = divpt(subpt(screen->r.max, screen->r.min), 2);
+	return addpt(screen->r.min, sc);
+}
+
 void
 dozoom(void)
 {
-	Point z, d;
-	double f, olds;
+	Point oxy, d, sc, off;
+	Vector gsc;
+	double z, oscale;
 
 	setcursor(mc, &zoomcursor);
-
-	z = mc->xy;
-	olds = scale;
+	oxy = mc->xy;
+	oscale = scale;
 	for(;;) {
 		readmouse(mc);
 		if(mc->buttons != 2)
 			break;
-		d = subpt(mc->xy, z);
-		f = tanh((double)d.y/200) + 1;
+		d = subpt(mc->xy, oxy);
+		z = tanh((double)d.y/200) + 1;
+		sc = screencenter();
+		gsc = tovector(sc);
 		pause(0, 0);
-		scale = f*olds;
+		scale = z*oscale;
+		off = subpt(topoint(gsc), sc);
+		orig = subpt(orig, off);
 		drawglxy();
 		pause(1, 0);
 	}
-
 	setcursor(mc, cursor);
-	pause(1, 0);
 }
 
 void
@@ -579,6 +579,16 @@ Again:
 	}
 }
 
+Vector
+tovector(Point p)
+{
+	Vector v;
+
+	v.x = (p.x-orig.x) * scale;
+	v.y = (p.y-orig.y) * scale;
+	return v;
+}
+
 void
 usage(void)
 {
@@ -632,8 +642,7 @@ threadmain(int argc, char **argv)
 		sysfatal("initmouse failed: %r");
 
 	dt² = dt*dt;
-	orig = divpt(subpt(screen->r.max, screen->r.min), 2);
-	orig = addpt(orig, screen->r.min);
+	orig = screencenter();
 	glxyinit();
 	quadsinit();
 	if(doload)

sys/src/games/galaxy/galaxy.h

@@ -55,14 +55,16 @@ Body ZB;
 QB space;
 
 Image *randcol(void);
+Point topoint(Vector);
+Vector tovector(Point);
 
 Body *body(void);
 void drawbody(Body*);
 Vector center(void);
 void glxyinit(void);
+int Bfmt(Fmt*);
 void readglxy(int);
 void writeglxy(int);
-int Bfmt(Fmt*);
 
 void quadcalc(Body*, QB, double);
 int quadins(Body*, double);