add an option to change the rate of zoom mode

While `--zoom-steps` is the number of button or key presses between
powers of 2, `--zoom-rate` represents the number of pixels between
powers of 2 that the user clicks and drags between, so if you added
the arg `--zoom-rate=1024`, it would result in a very slow zoom mode.
The default is 128 pixels.

Author: myQwil

man/feh.pre

@@ -1010,6 +1010,13 @@ Zoom images in and out by a factor of the nth root
 of 2 when using the zoom keys and buttons. The default is the 3rd root,
 which means that there will be 3 steps between powers of 2.
 .
+.It Cm --zoom-rate Ar index
+.
+Zoom images in and out by a factor of the nth root
+.Pq Ar index
+of 2 upon entering zoom mode. The default is the 128th root,
+which means that there will be 128 pixels between powers of 2.
+.
 .El
 .
 .Sh MONTAGE MODE OPTIONS

src/events.c

@@ -40,8 +40,6 @@ fehkey *feh_str_to_kb(char *action);
 
 feh_event_handler *ev_handler[LASTEvent];
 
-static const double zoom_rate = M_LN2 / 128.0;
-
 static void feh_event_handle_ButtonPress(XEvent * ev);
 static void feh_event_handle_ButtonRelease(XEvent * ev);
 static void feh_event_handle_LeaveNotify(XEvent * ev);
@@ -244,7 +242,7 @@ static void feh_event_handle_ButtonPress(XEvent * ev)
 		D(("click offset is %d,%d\n", ev->xbutton.x, ev->xbutton.y));
 		winwid->click_offset_x = ev->xbutton.x;
 		winwid->click_offset_y = ev->xbutton.y;
-		winwid->old_step = round(log(winwid->zoom) / zoom_rate);
+		winwid->old_step = round(log(winwid->zoom) / opt.zoom_rate);
 
 		/* required to adjust the image position in zoom mode */
 		winwid->im_click_offset_x = (winwid->click_offset_x
@@ -553,9 +551,9 @@ static void feh_event_handle_MotionNotify(XEvent * ev)
 		while (XCheckTypedWindowEvent(disp, ev->xmotion.window, MotionNotify, ev));
 
 		winwid = winwidget_get_from_window(ev->xmotion.window);
-		if (winwid) {
+		if (winwid && opt.zoom_rate > 0) {
 			int step = winwid->old_step + ev->xmotion.x - winwid->click_offset_x;
-			winwid->zoom = exp(step * zoom_rate);
+			winwid->zoom = exp(step * opt.zoom_rate);
 
 			if (winwid->zoom < ZOOM_MIN)
 				winwid->zoom = ZOOM_MIN;

src/options.c

@@ -63,6 +63,7 @@ void init_parse_options(int argc, char **argv)
 	opt.font = NULL;
 	opt.max_height = opt.max_width = UINT_MAX;
 
+	opt.zoom_rate = M_LN2 / 128.0;
 	opt.step_rate = M_LN2 / 3.0;
 
 	opt.start_list_at = NULL;
@@ -395,6 +396,7 @@ static void feh_parse_option_array(int argc, char **argv, int finalrun)
 		{"bg-center"     , 0, 0, OPTION_bg_center},
 		{"bg-scale"      , 0, 0, OPTION_bg_scale},
 		{"zoom"          , 1, 0, OPTION_zoom},
+		{"zoom-rate"     , 1, 0, OPTION_zoom_rate},
 		{"zoom-step"     , 1, 0, OPTION_zoom_step},
 		{"no-screen-clip", 0, 0, OPTION_no_screen_clip},
 		{"index-info"    , 1, 0, OPTION_index_info},
@@ -847,10 +849,18 @@ static void feh_parse_option_array(int argc, char **argv, int finalrun)
 		case OPTION_window_id:
 			opt.x11_windowid = strtol(optarg, NULL, 0);
 			break;
+		case OPTION_zoom_rate:
+			opt.zoom_rate = atof(optarg);
+			if ((opt.zoom_rate <= 0)) {
+				weprintf("Zoom mode disabled due to --zoom-rate=%f", opt.zoom_rate);
+			} else {
+				opt.zoom_rate = M_LN2 / opt.zoom_rate;
+			}
+			break;
 		case OPTION_zoom_step:
 			opt.step_rate = atof(optarg);
 			if ((opt.step_rate <= 0)) {
-				weprintf("Zooming disabled due to --zoom-step=%f", opt.step_rate);
+				weprintf("Zoom steps disabled due to --zoom-step=%f", opt.step_rate);
 			} else {
 				opt.step_rate = M_LN2 / opt.step_rate;
 			}

src/options.h

@@ -127,6 +127,7 @@ struct __fehoptions {
 	int offset_y;
 	int default_zoom;
 	int zoom_mode;
+	double zoom_rate;
 	double step_rate;
 	unsigned char adjust_reload;
 	int xinerama_index;
@@ -216,6 +217,7 @@ OPTION_bg_scale,
 OPTION_bg_fill,
 OPTION_bg_max,
 OPTION_zoom,
+OPTION_zoom_rate,
 OPTION_zoom_step,
 OPTION_zoom_in_rate,
 OPTION_zoom_out_rate,