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..6465e697285882502c1a44bf8a972017d4e1f911 100644
--- a/media/capture/video/linux/video_capture_device_chromeos.cc
+++ b/media/capture/video/linux/video_capture_device_chromeos.cc
@@ -11,6 +11,7 @@
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
+#include "media/capture/video/linux/v4l2_capture_delegate.h"
 #include "ui/display/display.h"
 #include "ui/display/display_observer.h"
 #include "ui/display/screen.h"
@@ -99,12 +100,63 @@ 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_(CameraDeviceInfo::UNINITIALIZED),
+      camera_characteristics_(LAZY_INSTANCE_INITIALIZER) {}
 
 VideoCaptureDeviceChromeOS::~VideoCaptureDeviceChromeOS() {
   screen_observer_delegate_->RemoveObserver();
 }
 
+void VideoCaptureDeviceChromeOS::SetRotation(int rotation) {
+  if (v4l2_thread_.IsRunning()) {
+    if (lens_facing_ == CameraDeviceInfo::UNINITIALIZED) {
+      camera_characteristics_.Get().InitializeDeviceInfo();

[kcwu, 2016/11/24 15:48:52]

Will VideoCaptureDeviceChromeOS be created more than once? If so, you will call
CameraCharacteistics's init more than once.

[shenghao, 2016/11/28 08:52:04]

Done.

+      lens_facing_ = camera_characteristics_.Get().GetCameraFacing(

[kcwu, 2016/11/24 15:48:52]

If there are two cameras starting capture at the same time, will two
VideoCaptureDeviceChromeOS() call
camera_characteristics_.Get().GetCameraFacing() at the same time?

[kcwu, 2016/11/28 05:48:43]

FYI, http://stackoverflow.com/questions/5912539/stl-map-find-thread-safe

[shenghao, 2016/11/28 08:52:04]

Methods like find() can be called safely according to c++ spec 23.2.2
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf

+          device_descriptor_.device_id, device_descriptor_.model_id);
+    }
+    // We assume external camera is facing the users. If not, the users can
+    // rotate the camera manually by themselves.
+    if (lens_facing_ == CameraDeviceInfo::LENS_FACING_BACK) {
+      // Original frame when screen_rotation = 0
+      // -----------------------
+      // |          *          |
+      // |         * *         |
+      // |        *   *        |
+      // |       *******       |
+      // |      *       *      |
+      // |     *         *     |
+      // -----------------------
+      //
+      // screen_rotation = 90, this is what back camera sees
+      // -----------------------
+      // |    ********         |
+      // |       *   ****      |
+      // |       *      ***    |
+      // |       *      ***    |
+      // |       *   ****      |
+      // |    ********         |
+      // -----------------------
+      //
+      // screen_rotation = 90, this is what front camera sees

[kcwu, 2016/11/24 15:48:52]

270 ?

[shenghao, 2016/11/28 08:52:04]

This is 90. We want to demonstrate the difference of what front and back camera
sees at the same rotation.

+      // -----------------------
+      // |         ********    |
+      // |      ****   *       |
+      // |    ***      *       |
+      // |    ***      *       |
+      // |      ****   *       |
+      // |         ********    |
+      // -----------------------
+      //
+      // Therefore, for back camera, we need to rotate (360 - screen_rotation).
+      rotation = (360 - rotation) % 360;
+    }
+    v4l2_thread_.task_runner()->PostTask(

[kcwu, 2016/11/24 15:48:52]

How about call
VideoCaptureDeviceLinux::SetRotation(rotation);
then VideoCaptureDeviceLinux don't need to expose many members to children and
don't have duplicated code.

[shenghao, 2016/11/28 08:52:04]

Done.

+        FROM_HERE,
+        base::Bind(&V4L2CaptureDelegate::SetRotation, capture_impl_, rotation));
+  }
+}
+
 void VideoCaptureDeviceChromeOS::SetDisplayRotation(
     const display::Display& display) {
   if (display.IsInternal())