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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

--- a/sys/src/games/galaxy/body.c

+++ b/sys/src/games/galaxy/body.c

@@ -31,6 +31,16 @@

 	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)

 {

@@ -37,8 +47,7 @@

 	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;

--- a/sys/src/games/galaxy/galaxy.c

+++ b/sys/src/games/galaxy/galaxy.c

@@ -171,8 +171,7 @@

 	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 @@

 	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;

@@ -211,21 +209,13 @@

 {

 	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;

+	b->v.x = d.x*scale/10;

+	b->v.y = d.y*scale/10;

 }

 void

-setpos(Body *b)

-{

-	b->x = (mc->xy.x - orig.x) * scale;

-	b->y = (mc->xy.y - orig.y) * scale;

-}

-

-void

 dosize(Body *b)

 {

 	Point p;

@@ -275,7 +265,6 @@

 	}

 	b = body();

-	setpos(b);

 	setvel(b);

 	setsize(b);

 	b->col = randcol();

@@ -290,7 +279,7 @@

 		else if(mc->buttons == 5)

 			dovel(b);

 		else

-			setpos(b);

+			b->Vector = tovector(mc->xy);

 	}

 	CHECKLIM(b, f);

@@ -349,30 +338,41 @@

 	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 @@

 	}

 }

+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 @@

 		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)

--- a/sys/src/games/galaxy/galaxy.h

+++ b/sys/src/games/galaxy/galaxy.h

@@ -55,14 +55,16 @@

 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);

-- 

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