Use argument to select between two drag methods

Makefile

@@ -1,4 +1,4 @@
-VERSION = git-20180109
+VERSION = git-20180122
 
 srcdir = .
 VPATH = $(srcdir)

commands.c

@@ -321,9 +321,9 @@ bool ci_scroll_to_edge(arg_t dir)
 	return img_pan_edge(&img, dir);
 }
 
-bool ci_drag(arg_t _)
+bool ci_drag(arg_t mode)
 {
-	int x, y;
+	int x, y, ox, oy;
 	float px, py;
 	XEvent e;
 
@@ -333,12 +333,19 @@ bool ci_drag(arg_t _)
 	win_set_cursor(&win, CURSOR_DRAG);
 
 	win_cursor_pos(&win, &x, &y);
+	ox = x;
+	oy = y;
 
 	for (;;) {
-		px = MIN(MAX(0.0, x - win.w*0.1), win.w*0.8) / (win.w*0.8)
-		   * (win.w - img.w * img.zoom);
-		py = MIN(MAX(0.0, y - win.h*0.1), win.h*0.8) / (win.h*0.8)
-		   * (win.h - img.h * img.zoom);
+		if (mode == DRAG_ABSOLUTE) {
+			px = MIN(MAX(0.0, x - win.w*0.1), win.w*0.8) / (win.w*0.8)
+			   * (win.w - img.w * img.zoom);
+			py = MIN(MAX(0.0, y - win.h*0.1), win.h*0.8) / (win.h*0.8)
+			   * (win.h - img.h * img.zoom);
+		} else {
+			px = img.x + x - ox;
+			py = img.y + y - oy;
+		}
 
 		if (img_pos(&img, px, py)) {
 			img_render(&img);
@@ -349,6 +356,8 @@ bool ci_drag(arg_t _)
 		if (e.type == ButtonPress || e.type == ButtonRelease)
 			break;
 		while (XCheckTypedEvent(win.env.dpy, MotionNotify, &e));
+		ox = x;
+		oy = y;
 		x = e.xmotion.x;
 		y = e.xmotion.y;
 	}

config.def.h

@@ -151,7 +151,7 @@ static const keymap_t keys[] = {
 static const button_t buttons[] = {
 	/* modifiers    button            function              argument */
 	{ 0,            1,                i_cursor_navigate,    None },
-	{ 0,            2,                i_drag,               None },
+	{ 0,            2,                i_drag,               DRAG_ABSOLUTE },
 	{ 0,            3,                g_switch_mode,        None },
 	{ 0,            4,                g_zoom,               +1 },
 	{ 0,            5,                g_zoom,               -1 },

sxiv.h

@@ -93,6 +93,11 @@ typedef enum {
 	SCALE_ZOOM
 } scalemode_t;
 
+typedef enum {
+	DRAG_RELATIVE,
+	DRAG_ABSOLUTE
+} dragmode_t;
+
 typedef enum {
 	CURSOR_ARROW,
 	CURSOR_DRAG,