Rotate frames correctly for back camera

media/capture/video/linux/video_capture_device_chromeos.cc

Index: media/capture/video/linux/video_capture_device_chromeos.cc
diff --git a/media/capture/video/linux/video_capture_device_chromeos.cc b/media/capture/video/linux/video_capture_device_chromeos.cc
index 1e6006c734ab71b66f6353794a6f244571b424ae..40ad58cebb9ff7af7dfc96683a1fe5bea6610bb5 100644
--- a/media/capture/video/linux/video_capture_device_chromeos.cc
+++ b/media/capture/video/linux/video_capture_device_chromeos.cc
@@ -7,6 +7,7 @@
 #include <stdint.h>
 
 #include "base/bind.h"
+#include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
@@ -17,6 +18,13 @@
 
 namespace media {
 
+namespace {
+
+base::LazyInstance<CameraFacingChromeOS>::Leaky g_camera_facing_ =
+    LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
 // This is a delegate class used to transfer Display change events from the UI
 // thread to the media thread.
 class VideoCaptureDeviceChromeOS::ScreenObserverDelegate
@@ -99,12 +107,55 @@ VideoCaptureDeviceChromeOS::VideoCaptureDeviceChromeOS(
     const VideoCaptureDeviceDescriptor& device_descriptor)
     : VideoCaptureDeviceLinux(device_descriptor),
       screen_observer_delegate_(
-          new ScreenObserverDelegate(this, ui_task_runner)) {}
+          new ScreenObserverDelegate(this, ui_task_runner)),
+      kLensFacing(
+          g_camera_facing_.Get().GetCameraFacing(device_descriptor.device_id,
+                                                 device_descriptor.model_id)) {}
 
 VideoCaptureDeviceChromeOS::~VideoCaptureDeviceChromeOS() {
   screen_observer_delegate_->RemoveObserver();
 }
 
+void VideoCaptureDeviceChromeOS::SetRotation(int rotation) {
+  // We assume external camera is facing the users. If not, the users can
+  // rotate the camera manually by themselves.
+  if (kLensFacing == CameraFacingChromeOS::LensFacing::BACK) {
+    // Original frame when |rotation| = 0
+    // -----------------------
+    // |          *          |
+    // |         * *         |
+    // |        *   *        |
+    // |       *******       |
+    // |      *       *      |
+    // |     *         *     |
+    // -----------------------
+    //
+    // |rotation| = 90, this is what back camera sees
+    // -----------------------
+    // |    ********         |
+    // |       *   ****      |
+    // |       *      ***    |
+    // |       *      ***    |
+    // |       *   ****      |
+    // |    ********         |
+    // -----------------------
+    //
+    // |rotation| = 90, this is what front camera sees
+    // -----------------------
+    // |         ********    |
+    // |      ****   *       |
+    // |    ***      *       |
+    // |    ***      *       |
+    // |      ****   *       |
+    // |         ********    |
+    // -----------------------
+    //
+    // Therefore, for back camera, we need to rotate (360 - |rotation|).

[mcasas, 2016/12/03 03:37:11]

All this is to say that, same as Android, rotation goes
backwards for the "back" camera, that is called 
"environment" there. 
Although I'm a fan of ASCII art myself, I wonder if you
could convey that meaning in a more succinct 
fashion here?

[shenghao, 2016/12/05 11:17:20]

My team members were confused by why back camera should rotate backward here and
in android code base several times. That's why I would like to draw this and
make it super clear.

+    rotation = (360 - rotation) % 360;
+  }
+  VideoCaptureDeviceLinux::SetRotation(rotation);
+}
+
 void VideoCaptureDeviceChromeOS::SetDisplayRotation(
     const display::Display& display) {
   if (display.IsInternal())