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implement panning with mouse

--- a/view.c

+++ b/view.c

@@ -20,6 +20,7 @@

 Keyboardctl *kctl;

 Image *bg;

 Image *img;

+Point pos;

 Image*

 load9(char *filename)

@@ -104,12 +105,28 @@

 {

 	lockdisplay(display);

 	draw(screen, screen->r, bg, nil, ZP);

-	draw(screen, screen->r, img, nil, img->r.min);

+	draw(screen, rectaddpt(img->r, addpt(pos, screen->r.min)), img, nil, img->r.min);

 	flushimage(display, 1);

 	unlockdisplay(display);

 }

 void

+pan(Point Δ)

+{

+	Rectangle r, ir;

+

+	if(Δ.x == 0 && Δ.y == 0)

+		return;

+	r = rectaddpt(img->r, addpt(pos, screen->r.min));

+	pos = addpt(pos, Δ);

+	ir = rectaddpt(r, Δ);

+	lockdisplay(display);

+	draw(screen, screen->r, bg, nil, ZP);

+	draw(screen, ir, img, nil, img->r.min);

+	unlockdisplay(display);

+}

+

+void

 evtresize(int new)

 {

 	if(new && getwindow(display, Refnone)<0)

@@ -120,7 +137,18 @@

 void

 evtmouse(Mouse m)

 {

-	USED(m);

+	Point o;

+

+	if(m.buttons == 1){

+		for(;;){

+			o = mctl->xy;

+			if(readmouse(mctl) < 0)

+				break;

+			if((mctl->buttons & 1) == 0)

+				break;

+			pan(subpt(mctl->xy, o));

+		}

+	}

 }

 void

@@ -169,6 +197,7 @@

 	alts[Ekeyboard].c = kctl->c;

 	initbg();

 	img = load(*argv);

+	pos = subpt(ZP, img->r.min);

 	evtresize(0);

 	for(;;){

 		switch(alt(alts)){