Rotate frames correctly for back camera

media/capture/video/linux/video_capture_device_linux.cc

Index: media/capture/video/linux/video_capture_device_linux.cc
diff --git a/media/capture/video/linux/video_capture_device_linux.cc b/media/capture/video/linux/video_capture_device_linux.cc
index a1b2855055e853b5e1d9210f5da139f8740ef7ec..f1987f704eb21e46d5cd3f2db1e62b2014b17550 100644
--- a/media/capture/video/linux/video_capture_device_linux.cc
+++ b/media/capture/video/linux/video_capture_device_linux.cc
@@ -11,6 +11,7 @@
 #include "base/bind.h"
 #include "base/single_thread_task_runner.h"
 #include "build/build_config.h"
+#include "media/capture/video/linux/camera_characteristics.h"
 #include "media/capture/video/linux/v4l2_capture_delegate.h"
 
 #if defined(OS_OPENBSD)
@@ -37,8 +38,12 @@ std::list<uint32_t> VideoCaptureDeviceLinux::GetListOfUsableFourCCs(
 
 VideoCaptureDeviceLinux::VideoCaptureDeviceLinux(
     const VideoCaptureDeviceDescriptor& device_descriptor)
-    : v4l2_thread_("V4L2CaptureThread"),
-      device_descriptor_(device_descriptor) {}
+    : v4l2_thread_("V4L2CaptureThread"), device_descriptor_(device_descriptor) {
+  CameraCharacteristics characteristics;

[wuchengli, 2016/11/22 07:59:34]

base::LazyInstance<CameraCharacteristics> g_camera_characteristics_ = 
    LAZY_INSTANCE_INITIALIZER;



is_back_camera_ =
g_camera_characteristics_.Get().GetCameraFacing(device_descriptor.model_id);

[shenghao, 2016/11/24 07:13:04]

Done.

+  is_back_camera_ =
+      characteristics.GetCameraFacing(device_descriptor.model_id) ==
+      DeviceInfo::LENS_FACING_BACK;
+}
 
 VideoCaptureDeviceLinux::~VideoCaptureDeviceLinux() {
   // Check if the thread is running.
@@ -113,6 +118,42 @@ void VideoCaptureDeviceLinux::SetPhotoOptions(
 
 void VideoCaptureDeviceLinux::SetRotation(int rotation) {
   if (v4l2_thread_.IsRunning()) {
+    // We assume external camera is facing the users. If not, the users can
+    // rotate the camera manually by theirselves.

[wuchengli, 2016/11/22 03:47:06]

s/theirselves/themselves/

[shenghao, 2016/11/24 07:13:04]

Done.

+    if (is_back_camera_) {
+      // Original frame when screen_rotation = 0
+      // -----------------------
+      // |                     |
+      // |          *          |
+      // |         * *         |
+      // |        *****        |

[wuchengli, 2016/11/22 03:47:06]

Great diagram! Can you make the A of the first one bigger to match the sizes of
the other two?

[shenghao, 2016/11/24 07:13:04]

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;
+    }
     v4l2_thread_.task_runner()->PostTask(
         FROM_HERE,
         base::Bind(&V4L2CaptureDelegate::SetRotation, capture_impl_, rotation));