Add pinch-to-zoom simulation

If Ctrl is hold when the left-click button is pressed, enable
pinch-to-zoom to scale and rotate relative to the center of the screen.

Fixes #24 <https://github.com/Genymobile/scrcpy/issues/24>

README.md

@@ -546,6 +546,19 @@ into the device clipboard. As a consequence, any Android application could read
 its content. You should avoid to paste sensitive content (like passwords) that
 way.
 
+
+#### Pinch-to-zoom
+
+To simulate "pinch-to-zoom": <kbd>Ctrl</kbd>+_click-and-move_.
+
+More precisely, hold <kbd>Ctrl</kbd> while pressing the left-click button. Until
+the left-click button is released, all mouse movements scale and rotate the
+content (if supported by the app) relative to the center of the screen.
+
+Concretely, scrcpy generates additional touch events from a "virtual finger" at
+a location inverted through the center of the screen.
+
+
 #### Text injection preference
 
 There are two kinds of [events][textevents] generated when typing text:
@@ -659,6 +672,7 @@ _<kbd>[Super]</kbd> is typically the <kbd>Windows</kbd> or <kbd>Cmd</kbd> key._
  | Synchronize clipboards and paste³           | <kbd>MOD</kbd>+<kbd>v</kbd>
  | Inject computer clipboard text              | <kbd>MOD</kbd>+<kbd>Shift</kbd>+<kbd>v</kbd>
  | Enable/disable FPS counter (on stdout)      | <kbd>MOD</kbd>+<kbd>i</kbd>
+ | Pinch-to-zoom                               | <kbd>Ctrl</kbd>+_click-and-move_
 
 _¹Double-click on black borders to remove them._  
 _²Right-click turns the screen on if it was off, presses BACK otherwise._  

app/scrcpy.1

@@ -316,6 +316,10 @@ Inject computer clipboard text as a sequence of key events
 .B MOD+i
 Enable/disable FPS counter (print frames/second in logs)
 
+.TP
+.B Ctrl+click-and-move
+Pinch-to-zoom from the center of the screen
+
 .TP
 .B Drag & drop APK file
 Install APK from computer

app/src/cli.c

@@ -279,6 +279,9 @@ scrcpy_print_usage(const char *arg0) {
         "    MOD+i\n"
         "        Enable/disable FPS counter (print frames/second in logs)\n"
         "\n"
+        "    Ctrl+click-and-move\n"
+        "        Pinch-to-zoom from the center of the screen\n"
+        "\n"
         "    Drag & drop APK file\n"
         "        Install APK from computer\n"
         "\n",

app/src/control_msg.h

@@ -17,6 +17,7 @@
 #define CONTROL_MSG_CLIPBOARD_TEXT_MAX_LENGTH (CONTROL_MSG_MAX_SIZE - 6)
 
 #define POINTER_ID_MOUSE UINT64_C(-1);
+#define POINTER_ID_VIRTUAL_FINGER UINT64_C(-2);
 
 enum control_msg_type {
     CONTROL_MSG_TYPE_INJECT_KEYCODE,

app/src/input_manager.c

@@ -70,6 +70,8 @@ input_manager_init(struct input_manager *im,
         im->sdl_shortcut_mods.data[i] = sdl_mod;
     }
     im->sdl_shortcut_mods.count = shortcut_mods->count;
+
+    im->vfinger_down = false;
 }
 
 static void
@@ -299,6 +301,36 @@ input_manager_process_text_input(struct input_manager *im,
     }
 }
 
+static bool
+simulate_virtual_finger(struct input_manager *im,
+                        enum android_motionevent_action action,
+                        struct point point) {
+    bool up = action == AMOTION_EVENT_ACTION_UP;
+
+    struct control_msg msg;
+    msg.type = CONTROL_MSG_TYPE_INJECT_TOUCH_EVENT;
+    msg.inject_touch_event.action = action;
+    msg.inject_touch_event.position.screen_size = im->screen->frame_size;
+    msg.inject_touch_event.position.point = point;
+    msg.inject_touch_event.pointer_id = POINTER_ID_VIRTUAL_FINGER;
+    msg.inject_touch_event.pressure = up ? 0.0f : 1.0f;
+    msg.inject_touch_event.buttons = 0;
+
+    if (!controller_push_msg(im->controller, &msg)) {
+        LOGW("Could not request 'inject virtual finger event'");
+        return false;
+    }
+
+    return true;
+}
+
+static struct point
+inverse_point(struct point point, struct size size) {
+    point.x = size.width - point.x;
+    point.y = size.height - point.y;
+    return point;
+}
+
 static bool
 convert_input_key(const SDL_KeyboardEvent *from, struct control_msg *to,
                   bool prefer_text, uint32_t repeat) {
@@ -512,10 +544,18 @@ input_manager_process_mouse_motion(struct input_manager *im,
         return;
     }
     struct control_msg msg;
-    if (convert_mouse_motion(event, im->screen, &msg)) {
+    if (!convert_mouse_motion(event, im->screen, &msg)) {
+        return;
+    }
+
     if (!controller_push_msg(im->controller, &msg)) {
         LOGW("Could not request 'inject mouse motion event'");
     }
+
+    if (im->vfinger_down) {
+        struct point mouse = msg.inject_touch_event.position.point;
+        struct point vfinger = inverse_point(mouse, im->screen->frame_size);
+        simulate_virtual_finger(im, AMOTION_EVENT_ACTION_MOVE, vfinger);
     }
 }
 
@@ -587,7 +627,9 @@ input_manager_process_mouse_button(struct input_manager *im,
         // simulated from touch events, so it's a duplicate
         return;
     }
-    if (event->type == SDL_MOUSEBUTTONDOWN) {
+
+    bool down = event->type == SDL_MOUSEBUTTONDOWN;
+    if (down) {
         if (control && event->button == SDL_BUTTON_RIGHT) {
             press_back_or_turn_screen_on(im->controller);
             return;
@@ -618,10 +660,36 @@ input_manager_process_mouse_button(struct input_manager *im,
     }
 
     struct control_msg msg;
-    if (convert_mouse_button(event, im->screen, &msg)) {
+    if (!convert_mouse_button(event, im->screen, &msg)) {
+        return;
+    }
+
     if (!controller_push_msg(im->controller, &msg)) {
         LOGW("Could not request 'inject mouse button event'");
+        return;
     }
+
+    // Pinch-to-zoom simulation.
+    //
+    // If Ctrl is hold when the left-click button is pressed, then
+    // pinch-to-zoom mode is enabled: on every mouse event until the left-click
+    // button is released, an additional "virtual finger" event is generated,
+    // having a position inverted through the center of the screen.
+    //
+    // In other words, the center of the rotation/scaling is the center of the
+    // screen.
+#define CTRL_PRESSED (SDL_GetModState() & (KMOD_LCTRL | KMOD_RCTRL))
+    if ((down && !im->vfinger_down && CTRL_PRESSED)
+            || (!down && im->vfinger_down)) {
+        struct point mouse = msg.inject_touch_event.position.point;
+        struct point vfinger = inverse_point(mouse, im->screen->frame_size);
+        enum android_motionevent_action action = down
+                                               ? AMOTION_EVENT_ACTION_DOWN
+                                               : AMOTION_EVENT_ACTION_UP;
+        if (!simulate_virtual_finger(im, action, vfinger)) {
+            return;
+        }
+        im->vfinger_down = down;
     }
 }
 

app/src/input_manager.h

@@ -30,6 +30,8 @@ struct input_manager {
         unsigned data[SC_MAX_SHORTCUT_MODS];
         unsigned count;
     } sdl_shortcut_mods;
+
+    bool vfinger_down;
 };
 
 void