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..aba144d0f7615c8de7d403812d6000ba5e2f0bd1 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<CameraCharacteristics> g_camera_characteristics_ =

[mcasas, 2016/11/29 19:12:56]

Use base::LazyInstane<>::Leaky to avoid slowing down shutdown [1].

[1]
https://www.chromium.org/developers/coding-style/important-abstractions-and-d...

[shenghao, 2016/12/01 07:34:21]

Done.

+    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)),
+      lens_facing_(g_camera_characteristics_.Get().GetCameraFacing(
+          device_descriptor.device_id,
+          device_descriptor.model_id)) {}

[mcasas, 2016/11/29 19:12:56]

Make  |lens_facing_| const and initialize it instead
of assigning it.

[shenghao, 2016/12/01 07:34:21]

I don't understand. Ain't I initializing it instead of assigning it right now?

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

You're assigning right now. As it is, the compiler 
will initialize |kLensFacing| in its constructor 
default initialization list, and then you  are assigning 
to it. You can make the member variable 
const and initialize it in the member initializer list:
  lens_facing_(g_camera_facing_.Get().GetCameraFacing(
    device_descriptor.device_id,
    device_descriptor.model_id), 
in l.109.

(See Scott Meyers, "Effective C++..." item 12:
"Prefer initialization to assignment in constructors.")

 
 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 (lens_facing_ == CameraDeviceInfo::LensFacing::kBack) {
+    // Original frame when screen_rotation = 0

[mcasas, 2016/11/29 19:12:56]

Here and elsewhere in these comments
s/screen_rotation/|rotation|/

[shenghao, 2016/12/01 07:34:21]

Done.

+    // -----------------------
+    // |          *          |
+    // |         * *         |
+    // |        *   *        |
+    // |       *******       |
+    // |      *       *      |
+    // |     *         *     |
+    // -----------------------
+    //
+    // screen_rotation = 90, this is what back camera sees
+    // -----------------------
+    // |    ********         |
+    // |       *   ****      |
+    // |       *      ***    |
+    // |       *      ***    |
+    // |       *   ****      |
+    // |    ********         |
+    // -----------------------
+    //
+    // screen_rotation = 90, this is what front camera sees
+    // -----------------------
+    // |         ********    |
+    // |      ****   *       |
+    // |    ***      *       |
+    // |    ***      *       |
+    // |      ****   *       |
+    // |         ********    |
+    // -----------------------
+    //
+    // Therefore, for back camera, we need to rotate (360 - screen_rotation).
+    rotation = (360 - rotation) % 360;
+  }
+  VideoCaptureDeviceLinux::SetRotation(rotation);
+}
+
 void VideoCaptureDeviceChromeOS::SetDisplayRotation(
     const display::Display& display) {
   if (display.IsInternal())