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..d1c69c8423845a07a55896f6f0c2675e3796578c 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)
@@ -38,7 +39,8 @@ std::list<uint32_t> VideoCaptureDeviceLinux::GetListOfUsableFourCCs(
 VideoCaptureDeviceLinux::VideoCaptureDeviceLinux(
     const VideoCaptureDeviceDescriptor& device_descriptor)
     : v4l2_thread_("V4L2CaptureThread"),
-      device_descriptor_(device_descriptor) {}
+      device_descriptor_(device_descriptor),
+      is_back_camera_(GetCameraFacing(device_descriptor)) {}
 
 VideoCaptureDeviceLinux::~VideoCaptureDeviceLinux() {
   // Check if the thread is running.
@@ -113,6 +115,12 @@ void VideoCaptureDeviceLinux::SetPhotoOptions(
 
 void VideoCaptureDeviceLinux::SetRotation(int rotation) {
   if (v4l2_thread_.IsRunning()) {
+    if (is_back_camera_) {
+      // For back camera, we need to rotate (360 - screen_rotation).
+      // Ex: when screen rotation is 90, we have to rotate the frame by 270

[wuchengli, 2016/11/17 09:26:04]

This example doesn't explain anything. This example is just a trivial rephrase
of the previous line. We want to know why from the comments.

+      // clockwise.
+      rotation = (360 - rotation) % 360;
+    }
     v4l2_thread_.task_runner()->PostTask(
         FROM_HERE,
         base::Bind(&V4L2CaptureDelegate::SetRotation, capture_impl_, rotation));
@@ -133,4 +141,11 @@ int VideoCaptureDeviceLinux::TranslatePowerLineFrequencyToV4L2(
   }
 }
 
+bool VideoCaptureDeviceLinux::GetCameraFacing(
+    const VideoCaptureDeviceDescriptor& descriptor) {
+  CameraCharacteristics characteristics;
+  return characteristics.GetCameraFacing(descriptor.model_id) ==
+         DeviceInfo::LENS_FACING_BACK;
+}
+
 }  // namespace media