Rotate frames correctly for back camera

media/capture/video/linux/video_capture_device_linux.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "media/capture/video/linux/video_capture_device_linux.h"
 
 #include <stddef.h>
 
 #include <list>
 
 #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)
 #include <sys/videoio.h>
 #else
 #include <linux/videodev2.h>
 #endif
 
 namespace media {
 
 // Translates Video4Linux pixel formats to Chromium pixel formats.
 // static
 VideoPixelFormat VideoCaptureDeviceLinux::V4l2FourCcToChromiumPixelFormat(
     uint32_t v4l2_fourcc) {
   return V4L2CaptureDelegate::V4l2FourCcToChromiumPixelFormat(v4l2_fourcc);
 }
 
 // Gets a list of usable Four CC formats prioritized.
 // static
 std::list<uint32_t> VideoCaptureDeviceLinux::GetListOfUsableFourCCs(
     bool favour_mjpeg) {
   return V4L2CaptureDelegate::GetListOfUsableFourCcs(favour_mjpeg);
 }
 
 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.
   // This means that the device has not been StopAndDeAllocate()d properly.
   DCHECK(!v4l2_thread_.IsRunning());
   v4l2_thread_.Stop();
 }
 
 void VideoCaptureDeviceLinux::AllocateAndStart(
     const VideoCaptureParams& params,
@@ skipping 54 matching lines @@
   if (!v4l2_thread_.IsRunning())
     return;  // Wrong state.
   v4l2_thread_.task_runner()->PostTask(
       FROM_HERE,
       base::Bind(&V4L2CaptureDelegate::SetPhotoOptions, capture_impl_,
                  base::Passed(&settings), base::Passed(&callback)));
 }
 
 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));
   }
 }
 
 // static
 int VideoCaptureDeviceLinux::TranslatePowerLineFrequencyToV4L2(
     PowerLineFrequency frequency) {
   switch (frequency) {
     case media::PowerLineFrequency::FREQUENCY_50HZ:
       return V4L2_CID_POWER_LINE_FREQUENCY_50HZ;
     case media::PowerLineFrequency::FREQUENCY_60HZ:
       return V4L2_CID_POWER_LINE_FREQUENCY_60HZ;
     default:
       // If we have no idea of the frequency, at least try and set it to AUTO.
       return V4L2_CID_POWER_LINE_FREQUENCY_AUTO;
   }
 }
 
+bool VideoCaptureDeviceLinux::GetCameraFacing(
+    const VideoCaptureDeviceDescriptor& descriptor) {
+  CameraCharacteristics characteristics;
+  return characteristics.GetCameraFacing(descriptor.model_id) ==
+         DeviceInfo::LENS_FACING_BACK;
+}
+
 }  // namespace media