[camera]remove CAS operation and use dispatch queue instead (#4973)

Author: hellohuanlin

packages/camera/camera/CHANGELOG.md

@@ -1,5 +1,6 @@
-## NEXT
+## 0.9.4+15
 
+* Uses dispatch queue for pixel buffer synchronization on iOS.
 * Minor iOS internal code cleanup related to queue helper functions.  
 
 ## 0.9.4+14

packages/camera/camera/example/ios/Runner.xcodeproj/project.pbxproj

@@ -28,6 +28,7 @@
 		E0C6E2002770F01A00EA6AA3 /* ThreadSafeMethodChannelTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0C6E1FD2770F01A00EA6AA3 /* ThreadSafeMethodChannelTests.m */; };
 		E0C6E2012770F01A00EA6AA3 /* ThreadSafeTextureRegistryTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0C6E1FE2770F01A00EA6AA3 /* ThreadSafeTextureRegistryTests.m */; };
 		E0C6E2022770F01A00EA6AA3 /* ThreadSafeEventChannelTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0C6E1FF2770F01A00EA6AA3 /* ThreadSafeEventChannelTests.m */; };
+		E0CDBAC227CD9729002561D9 /* CameraTestUtils.m in Sources */ = {isa = PBXBuildFile; fileRef = E0CDBAC127CD9729002561D9 /* CameraTestUtils.m */; };
 		E0F95E3D27A32AB900699390 /* CameraPropertiesTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E0F95E3C27A32AB900699390 /* CameraPropertiesTests.m */; };
 		E487C86026D686A10034AC92 /* CameraPreviewPauseTests.m in Sources */ = {isa = PBXBuildFile; fileRef = E487C85F26D686A10034AC92 /* CameraPreviewPauseTests.m */; };
 		F6EE622F2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m in Sources */ = {isa = PBXBuildFile; fileRef = F6EE622E2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m */; };
@@ -91,6 +92,8 @@
 		E0C6E1FD2770F01A00EA6AA3 /* ThreadSafeMethodChannelTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ThreadSafeMethodChannelTests.m; sourceTree = "<group>"; };
 		E0C6E1FE2770F01A00EA6AA3 /* ThreadSafeTextureRegistryTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ThreadSafeTextureRegistryTests.m; sourceTree = "<group>"; };
 		E0C6E1FF2770F01A00EA6AA3 /* ThreadSafeEventChannelTests.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; path = ThreadSafeEventChannelTests.m; sourceTree = "<group>"; };
+		E0CDBAC027CD9729002561D9 /* CameraTestUtils.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = CameraTestUtils.h; sourceTree = "<group>"; };
+		E0CDBAC127CD9729002561D9 /* CameraTestUtils.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = CameraTestUtils.m; sourceTree = "<group>"; };
 		E0F95E3C27A32AB900699390 /* CameraPropertiesTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = CameraPropertiesTests.m; sourceTree = "<group>"; };
 		E487C85F26D686A10034AC92 /* CameraPreviewPauseTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = CameraPreviewPauseTests.m; sourceTree = "<group>"; };
 		F63F9EED27143B19002479BF /* MockFLTThreadSafeFlutterResult.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = MockFLTThreadSafeFlutterResult.h; sourceTree = "<group>"; };
@@ -132,6 +135,8 @@
 				E071CF7127B3061B006EF3BA /* FLTCamPhotoCaptureTests.m */,
 				E071CF7327B31DE4006EF3BA /* FLTCamSampleBufferTests.m */,
 				E01EE4A72799F3A5008C1950 /* QueueUtilsTests.m */,
+				E0CDBAC027CD9729002561D9 /* CameraTestUtils.h */,
+				E0CDBAC127CD9729002561D9 /* CameraTestUtils.m */,
 				E487C85F26D686A10034AC92 /* CameraPreviewPauseTests.m */,
 				F6EE622E2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m */,
 				F63F9EED27143B19002479BF /* MockFLTThreadSafeFlutterResult.h */,
@@ -408,6 +413,7 @@
 				E071CF7427B31DE4006EF3BA /* FLTCamSampleBufferTests.m in Sources */,
 				E04F108627A87CA600573D0C /* FLTSavePhotoDelegateTests.m in Sources */,
 				F6EE622F2710A6FC00905E4A /* MockFLTThreadSafeFlutterResult.m in Sources */,
+				E0CDBAC227CD9729002561D9 /* CameraTestUtils.m in Sources */,
 				334733EA2668111C00DCC49E /* CameraOrientationTests.m in Sources */,
 				E032F250279F5E94009E9028 /* CameraCaptureSessionQueueRaceConditionTests.m in Sources */,
 				E0C6E2022770F01A00EA6AA3 /* ThreadSafeEventChannelTests.m in Sources */,

packages/camera/camera/example/ios/Runner/main.m

@@ -11,7 +11,7 @@ int main(int argc, char *argv[]) {
     // The setup logic in `AppDelegate::didFinishLaunchingWithOptions:` eventually sends camera
     // operations on the background queue, which would run concurrently with the test cases during
     // unit tests, making the debugging process confusing. This setup is actually not necessary for
-    // the unit tests, so here we want to skip the AppDelegate when running unit tests.
+    // the unit tests, so it is better to skip the AppDelegate when running unit tests.
     BOOL isTesting = NSClassFromString(@"XCTestCase") != nil;
     return UIApplicationMain(argc, argv, nil,
                              isTesting ? nil : NSStringFromClass([AppDelegate class]));

packages/camera/camera/example/ios/RunnerTests/CameraTestUtils.h

@@ -0,0 +1,18 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+@import camera;
+
+NS_ASSUME_NONNULL_BEGIN
+
+/// Creates an `FLTCam` that runs its capture session operations on a given queue.
+/// @param captureSessionQueue the capture session queue
+/// @return an FLTCam object.
+extern FLTCam *FLTCreateCamWithCaptureSessionQueue(dispatch_queue_t captureSessionQueue);
+
+/// Creates a test sample buffer.
+/// @return a test sample buffer.
+extern CMSampleBufferRef FLTCreateTestSampleBuffer(void);
+
+NS_ASSUME_NONNULL_END

packages/camera/camera/example/ios/RunnerTests/CameraTestUtils.m

@@ -0,0 +1,44 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import "CameraTestUtils.h"
+#import <OCMock/OCMock.h>
+@import AVFoundation;
+
+FLTCam *FLTCreateCamWithCaptureSessionQueue(dispatch_queue_t captureSessionQueue) {
+  id inputMock = OCMClassMock([AVCaptureDeviceInput class]);
+  OCMStub([inputMock deviceInputWithDevice:[OCMArg any] error:[OCMArg setTo:nil]])
+      .andReturn(inputMock);
+
+  id sessionMock = OCMClassMock([AVCaptureSession class]);
+  OCMStub([sessionMock addInputWithNoConnections:[OCMArg any]]);  // no-op
+  OCMStub([sessionMock canSetSessionPreset:[OCMArg any]]).andReturn(YES);
+
+  return [[FLTCam alloc] initWithCameraName:@"camera"
+                           resolutionPreset:@"medium"
+                                enableAudio:true
+                                orientation:UIDeviceOrientationPortrait
+                             captureSession:sessionMock
+                        captureSessionQueue:captureSessionQueue
+                                      error:nil];
+}
+
+CMSampleBufferRef FLTCreateTestSampleBuffer(void) {
+  CVPixelBufferRef pixelBuffer;
+  CVPixelBufferCreate(kCFAllocatorDefault, 100, 100, kCVPixelFormatType_32BGRA, NULL, &pixelBuffer);
+
+  CMFormatDescriptionRef formatDescription;
+  CMVideoFormatDescriptionCreateForImageBuffer(kCFAllocatorDefault, pixelBuffer,
+                                               &formatDescription);
+
+  CMSampleTimingInfo timingInfo = {CMTimeMake(1, 44100), kCMTimeZero, kCMTimeInvalid};
+
+  CMSampleBufferRef sampleBuffer;
+  CMSampleBufferCreateReadyWithImageBuffer(kCFAllocatorDefault, pixelBuffer, formatDescription,
+                                           &timingInfo, &sampleBuffer);
+
+  CFRelease(pixelBuffer);
+  CFRelease(formatDescription);
+  return sampleBuffer;
+}

packages/camera/camera/example/ios/RunnerTests/FLTCamPhotoCaptureTests.m

@@ -7,7 +7,9 @@
 @import AVFoundation;
 @import XCTest;
 #import <OCMock/OCMock.h>
+#import "CameraTestUtils.h"
 
+/// Includes test cases related to photo capture operations for FLTCam class.
 @interface FLTCamPhotoCaptureTests : XCTestCase
 
 @end
@@ -22,7 +24,7 @@ - (void)testCaptureToFile_mustReportErrorToResultIfSavePhotoDelegateCompletionsW
   dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
   dispatch_queue_set_specific(captureSessionQueue, FLTCaptureSessionQueueSpecific,
                               (void *)FLTCaptureSessionQueueSpecific, NULL);
-  FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
+  FLTCam *cam = FLTCreateCamWithCaptureSessionQueue(captureSessionQueue);
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
   id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
   OCMStub([mockSettings photoSettings]).andReturn(settings);
@@ -61,7 +63,7 @@ - (void)testCaptureToFile_mustReportPathToResultIfSavePhotoDelegateCompletionsWi
   dispatch_queue_t captureSessionQueue = dispatch_queue_create("capture_session_queue", NULL);
   dispatch_queue_set_specific(captureSessionQueue, FLTCaptureSessionQueueSpecific,
                               (void *)FLTCaptureSessionQueueSpecific, NULL);
-  FLTCam *cam = [self createFLTCamWithCaptureSessionQueue:captureSessionQueue];
+  FLTCam *cam = FLTCreateCamWithCaptureSessionQueue(captureSessionQueue);
 
   AVCapturePhotoSettings *settings = [AVCapturePhotoSettings photoSettings];
   id mockSettings = OCMClassMock([AVCapturePhotoSettings class]);
@@ -92,23 +94,4 @@ - (void)testCaptureToFile_mustReportPathToResultIfSavePhotoDelegateCompletionsWi
   [self waitForExpectationsWithTimeout:1 handler:nil];
 }
 
-/// Creates an `FLTCam` that runs its operations on a given capture session queue.
-- (FLTCam *)createFLTCamWithCaptureSessionQueue:(dispatch_queue_t)captureSessionQueue {
-  id inputMock = OCMClassMock([AVCaptureDeviceInput class]);
-  OCMStub([inputMock deviceInputWithDevice:[OCMArg any] error:[OCMArg setTo:nil]])
-      .andReturn(inputMock);
-
-  id sessionMock = OCMClassMock([AVCaptureSession class]);
-  OCMStub([sessionMock alloc]).andReturn(sessionMock);
-  OCMStub([sessionMock addInputWithNoConnections:[OCMArg any]]);  // no-op
-  OCMStub([sessionMock canSetSessionPreset:[OCMArg any]]).andReturn(YES);
-
-  return [[FLTCam alloc] initWithCameraName:@"camera"
-                           resolutionPreset:@"medium"
-                                enableAudio:true
-                                orientation:UIDeviceOrientationPortrait
-                        captureSessionQueue:captureSessionQueue
-                                      error:nil];
-}
-
 @end

packages/camera/camera/example/ios/RunnerTests/FLTCamSampleBufferTests.m

@@ -7,31 +7,35 @@
 @import AVFoundation;
 @import XCTest;
 #import <OCMock/OCMock.h>
+#import "CameraTestUtils.h"
 
+/// Includes test cases related to sample buffer handling for FLTCam class.
 @interface FLTCamSampleBufferTests : XCTestCase
 
 @end
 
 @implementation FLTCamSampleBufferTests
 
 - (void)testSampleBufferCallbackQueueMustBeCaptureSessionQueue {
-  id inputMock = OCMClassMock([AVCaptureDeviceInput class]);
-  OCMStub([inputMock deviceInputWithDevice:[OCMArg any] error:[OCMArg setTo:nil]])
-      .andReturn(inputMock);
-
-  id sessionMock = OCMClassMock([AVCaptureSession class]);
-  OCMStub([sessionMock alloc]).andReturn(sessionMock);
-  OCMStub([sessionMock addInputWithNoConnections:[OCMArg any]]);  // no-op
-  OCMStub([sessionMock canSetSessionPreset:[OCMArg any]]).andReturn(YES);
-
   dispatch_queue_t captureSessionQueue = dispatch_queue_create("testing", NULL);
-  FLTCam *cam = [[FLTCam alloc] initWithCameraName:@"camera"
-                                  resolutionPreset:@"medium"
-                                       enableAudio:true
-                                       orientation:UIDeviceOrientationPortrait
-                               captureSessionQueue:captureSessionQueue
-                                             error:nil];
-  XCTAssertEqual(captureSessionQueue, cam.captureVideoOutput.sampleBufferCallbackQueue);
+  FLTCam *cam = FLTCreateCamWithCaptureSessionQueue(captureSessionQueue);
+  XCTAssertEqual(captureSessionQueue, cam.captureVideoOutput.sampleBufferCallbackQueue,
+                 @"Sample buffer callback queue must be the capture session queue.");
+}
+
+- (void)testCopyPixelBuffer {
+  FLTCam *cam = FLTCreateCamWithCaptureSessionQueue(dispatch_queue_create("test", NULL));
+  CMSampleBufferRef capturedSampleBuffer = FLTCreateTestSampleBuffer();
+  CVPixelBufferRef capturedPixelBuffer = CMSampleBufferGetImageBuffer(capturedSampleBuffer);
+  // Mimic sample buffer callback when captured a new video sample
+  [cam captureOutput:cam.captureVideoOutput
+      didOutputSampleBuffer:capturedSampleBuffer
+             fromConnection:OCMClassMock([AVCaptureConnection class])];
+  CVPixelBufferRef deliveriedPixelBuffer = [cam copyPixelBuffer];
+  XCTAssertEqual(deliveriedPixelBuffer, capturedPixelBuffer,
+                 @"FLTCam must deliver the latest captured pixel buffer to copyPixelBuffer API.");
+  CFRelease(capturedSampleBuffer);
+  CFRelease(deliveriedPixelBuffer);
 }
 
 @end

packages/camera/camera/example/ios/RunnerTests/FLTSavePhotoDelegateTests.m

@@ -53,7 +53,7 @@ - (void)testHandlePhotoCaptureResult_mustCompleteWithErrorIfFailedToWrite {
         [completionExpectation fulfill];
       }];
 
-  // We can't use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
+  // Do not use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
   // `XCTRunnerIDESession::logDebugMessage:`) on a private queue.
   id mockData = OCMPartialMock([NSData data]);
   OCMStub([mockData writeToFile:OCMOCK_ANY
@@ -82,7 +82,7 @@ - (void)testHandlePhotoCaptureResult_mustCompleteWithFilePathIfSuccessToWrite {
         [completionExpectation fulfill];
       }];
 
-  // We can't use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
+  // Do not use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
   // `XCTRunnerIDESession::logDebugMessage:`) on a private queue.
   id mockData = OCMPartialMock([NSData data]);
   OCMStub([mockData writeToFile:filePath options:NSDataWritingAtomic error:[OCMArg setTo:nil]])
@@ -107,7 +107,7 @@ - (void)testHandlePhotoCaptureResult_bothProvideDataAndSaveFileMustRunOnIOQueue
   const char *ioQueueSpecific = "io_queue_specific";
   dispatch_queue_set_specific(ioQueue, ioQueueSpecific, (void *)ioQueueSpecific, NULL);
 
-  // We can't use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
+  // Do not use OCMClassMock for NSData because some XCTest APIs uses NSData (e.g.
   // `XCTRunnerIDESession::logDebugMessage:`) on a private queue.
   id mockData = OCMPartialMock([NSData data]);
   OCMStub([mockData writeToFile:OCMOCK_ANY options:NSDataWritingAtomic error:[OCMArg setTo:nil]])

packages/camera/camera/ios/Classes/FLTCam.h

@@ -31,6 +31,13 @@ NS_ASSUME_NONNULL_BEGIN
 // Format used for video and image streaming.
 @property(assign, nonatomic) FourCharCode videoFormat;
 
+/// Initializes an `FLTCam` instance.
+/// @param cameraName a name used to uniquely identify the camera.
+/// @param resolutionPreset the resolution preset
+/// @param enableAudio YES if audio should be enabled for video capturing; NO otherwise.
+/// @param orientation the orientation of camera
+/// @param captureSessionQueue the queue on which camera's capture session operations happen.
+/// @param error report to the caller if any error happened creating the camera.
 - (instancetype)initWithCameraName:(NSString *)cameraName
                   resolutionPreset:(NSString *)resolutionPreset
                        enableAudio:(BOOL)enableAudio

packages/camera/camera/ios/Classes/FLTCam.m

@@ -52,7 +52,9 @@ @interface FLTCam () <AVCaptureVideoDataOutputSampleBufferDelegate,
 @property(readonly, nonatomic) AVCaptureSession *captureSession;
 
 @property(readonly, nonatomic) AVCaptureInput *captureVideoInput;
-@property(readonly) CVPixelBufferRef volatile latestPixelBuffer;
+/// Tracks the latest pixel buffer sent from AVFoundation's sample buffer delegate callback.
+/// Used to deliver the latest pixel buffer to the flutter engine via the `copyPixelBuffer` API.
+@property(readwrite, nonatomic) CVPixelBufferRef latestPixelBuffer;
 @property(readonly, nonatomic) CGSize captureSize;
 @property(strong, nonatomic) AVAssetWriter *videoWriter;
 @property(strong, nonatomic) AVAssetWriterInput *videoWriterInput;
@@ -76,6 +78,9 @@ @interface FLTCam () <AVCaptureVideoDataOutputSampleBufferDelegate,
 @property AVAssetWriterInputPixelBufferAdaptor *videoAdaptor;
 /// All FLTCam's state access and capture session related operations should be on run on this queue.
 @property(strong, nonatomic) dispatch_queue_t captureSessionQueue;
+/// The queue on which `latestPixelBuffer` property is accessed.
+/// To avoid unnecessary contention, do not access `latestPixelBuffer` on the `captureSessionQueue`.
+@property(strong, nonatomic) dispatch_queue_t pixelBufferSynchronizationQueue;
 /// The queue on which captured photos (not videos) are written to disk.
 /// Videos are written to disk by `videoAdaptor` on an internal queue managed by AVFoundation.
 @property(strong, nonatomic) dispatch_queue_t photoIOQueue;
@@ -92,6 +97,22 @@ - (instancetype)initWithCameraName:(NSString *)cameraName
                        orientation:(UIDeviceOrientation)orientation
                captureSessionQueue:(dispatch_queue_t)captureSessionQueue
                              error:(NSError **)error {
+  return [self initWithCameraName:cameraName
+                 resolutionPreset:resolutionPreset
+                      enableAudio:enableAudio
+                      orientation:orientation
+                   captureSession:[[AVCaptureSession alloc] init]
+              captureSessionQueue:captureSessionQueue
+                            error:error];
+}
+
+- (instancetype)initWithCameraName:(NSString *)cameraName
+                  resolutionPreset:(NSString *)resolutionPreset
+                       enableAudio:(BOOL)enableAudio
+                       orientation:(UIDeviceOrientation)orientation
+                    captureSession:(AVCaptureSession *)captureSession
+               captureSessionQueue:(dispatch_queue_t)captureSessionQueue
+                             error:(NSError **)error {
   self = [super init];
   NSAssert(self, @"super init cannot be nil");
   @try {
@@ -101,8 +122,10 @@ - (instancetype)initWithCameraName:(NSString *)cameraName
   }
   _enableAudio = enableAudio;
   _captureSessionQueue = captureSessionQueue;
+  _pixelBufferSynchronizationQueue =
+      dispatch_queue_create("io.flutter.camera.pixelBufferSynchronizationQueue", NULL);
   _photoIOQueue = dispatch_queue_create("io.flutter.camera.photoIOQueue", NULL);
-  _captureSession = [[AVCaptureSession alloc] init];
+  _captureSession = captureSession;
   _captureDevice = [AVCaptureDevice deviceWithUniqueID:cameraName];
   _flashMode = _captureDevice.hasFlash ? FLTFlashModeAuto : FLTFlashModeOff;
   _exposureMode = FLTExposureModeAuto;
@@ -355,12 +378,17 @@ - (void)captureOutput:(AVCaptureOutput *)output
   if (output == _captureVideoOutput) {
     CVPixelBufferRef newBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
     CFRetain(newBuffer);
-    CVPixelBufferRef old = _latestPixelBuffer;
-    while (!OSAtomicCompareAndSwapPtrBarrier(old, newBuffer, (void **)&_latestPixelBuffer)) {
-      old = _latestPixelBuffer;
-    }
-    if (old != nil) {
-      CFRelease(old);
+
+    __block CVPixelBufferRef previousPixelBuffer = nil;
+    // Use `dispatch_sync` to avoid unnecessary context switch under common non-contest scenarios;
+    // Under rare contest scenarios, it will not block for too long since the critical section is
+    // quite lightweight.
+    dispatch_sync(self.pixelBufferSynchronizationQueue, ^{
+      previousPixelBuffer = self.latestPixelBuffer;
+      self.latestPixelBuffer = newBuffer;
+    });
+    if (previousPixelBuffer) {
+      CFRelease(previousPixelBuffer);
     }
     if (_onFrameAvailable) {
       _onFrameAvailable();
@@ -420,7 +448,7 @@ - (void)captureOutput:(AVCaptureOutput *)output
 
         [planes addObject:planeBuffer];
       }
-      // Before accessing pixel data, we should lock the base address, and unlock it afterwards.
+      // Lock the base address before accessing pixel data, and unlock it afterwards.
       // Done accessing the `pixelBuffer` at this point.
       CVPixelBufferUnlockBaseAddress(pixelBuffer, kCVPixelBufferLock_ReadOnly);
 
@@ -575,11 +603,12 @@ - (void)dealloc {
 }
 
 - (CVPixelBufferRef)copyPixelBuffer {
-  CVPixelBufferRef pixelBuffer = _latestPixelBuffer;
-  while (!OSAtomicCompareAndSwapPtrBarrier(pixelBuffer, nil, (void **)&_latestPixelBuffer)) {
-    pixelBuffer = _latestPixelBuffer;
-  }
-
+  __block CVPixelBufferRef pixelBuffer = nil;
+  // Use `dispatch_sync` because `copyPixelBuffer` API requires synchronous return.
+  dispatch_sync(self.pixelBufferSynchronizationQueue, ^{
+    pixelBuffer = self.latestPixelBuffer;
+    self.latestPixelBuffer = nil;
+  });
   return pixelBuffer;
 }