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..13e01af5c2ccfe9440f741e2226af9233d2b2a9c 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;
+  is_back_camera_ =
+      characteristics.GetCameraFacing(device_descriptor.model_id) ==
+      DeviceInfo::LENS_FACING_BACK;
+}
 
 VideoCaptureDeviceLinux::~VideoCaptureDeviceLinux() {
   // Check if the thread is running.
@@ -113,6 +118,12 @@ void VideoCaptureDeviceLinux::SetPhotoOptions(
 
 void VideoCaptureDeviceLinux::SetRotation(int rotation) {
   if (v4l2_thread_.IsRunning()) {
+    if (is_back_camera_) {

[wuchengli, 2016/11/18 05:08:50]

Add a comment to explain we assume an external camera is facing the users. If
it's not, users can manually rotate the camera.

[shenghao, 2016/11/21 06:24:53]

Done.

+      // For back camera, we need to rotate (360 - screen_rotation).
+      // Ex: when screen rotation is 90, we have to rotate the frame by 270
+      // clockwise.

[wuchengli, 2016/11/18 05:08:50]

Use ascii art to explain the image of front facing and back facing will be
opposite after rotation.

Example:
        //
        //     Original frame                          New frame
        // -----------------------             -----------------------
        // |                     |             |    ********         |
        // |          *          |             |       *   ****      |
        // |         * *         |     ==>     |       *      ***    |
        // |        *****        |             |       *      ***    |
        // |       *     *       |             |       *   ****      |
        // |                     |             |    ********         |
        // -----------------------             -----------------------
        //

[shenghao, 2016/11/21 06:24:53]

Done.

+      rotation = (360 - rotation) % 360;
+    }
     v4l2_thread_.task_runner()->PostTask(
         FROM_HERE,
         base::Bind(&V4L2CaptureDelegate::SetRotation, capture_impl_, rotation));