为什么我可以在 AVCaptureSession 上处理灰度效果，而不能使用 AVAssetReader 处理？

Question

我正在开发 iPhone 应用程序。我想对库中的视频应用一些过滤器。经过研究，我从这篇文章开始 http ://www.sunsetlakesoftware.com/2010/10/22/gpu-accelerated-video-processing-mac-and-ios 和他的 ColorTracking 源代码。 从这段代码中，我可以使用 AVCaptureSession 实时应用灰度过滤器。正确的。但我想对图书馆中的视频做同样的事情。所以我使用 AVAssetReader 来读取源视频。与 AVCaptureSession 一样，我可以获得 CVImageBufferRef。它具有与 AVCaptureSession 相同的宽度、高度和数据大小，但我的 openGL 视图始终是黑色的。

我捕获帧的源代码：

    - (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {


    [self dismissModalViewControllerAnimated:NO];


    /// incoming video
    NSURL *videoURL = [info valueForKey:UIImagePickerControllerMediaURL];
    NSLog(@"Video : %@", videoURL);

    // AVURLAsset to read input movie (i.e. mov recorded to local storage)
    NSDictionary *inputOptions = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
    AVURLAsset *inputAsset = [[AVURLAsset alloc] initWithURL:videoURL options:inputOptions];

    // Load the input asset tracks information
    [inputAsset loadValuesAsynchronouslyForKeys:[NSArray arrayWithObject:@"tracks"] completionHandler: ^{


        NSError *error = nil;

        // Check status of "tracks", make sure they were loaded    
        AVKeyValueStatus tracksStatus = [inputAsset statusOfValueForKey:@"tracks" error:&error];
        if (!tracksStatus == AVKeyValueStatusLoaded)
            // failed to load
            return;




        /* Read video samples from input asset video track */
        AVAssetReader *reader = [AVAssetReader assetReaderWithAsset:inputAsset error:&error];

        NSMutableDictionary *outputSettings = [NSMutableDictionary dictionary];
        [outputSettings setObject: [NSNumber numberWithInt:kCVPixelFormatType_32BGRA]  forKey: (NSString*)kCVPixelBufferPixelFormatTypeKey];
        AVAssetReaderTrackOutput *readerVideoTrackOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:[[inputAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] outputSettings:outputSettings];


        // Assign the tracks to the reader and start to read
        [reader addOutput:readerVideoTrackOutput];
        if ([reader startReading] == NO) {
            // Handle error
            NSLog(@"Error reading");
        }

        NSAutoreleasePool *pool = [NSAutoreleasePool new];
        while (reader.status == AVAssetReaderStatusReading) {

            CMSampleBufferRef sampleBufferRef = [readerVideoTrackOutput copyNextSampleBuffer];
            if (sampleBufferRef) {
                CVImageBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBufferRef);
                [self processNewCameraFrame:pixelBuffer];

                CMSampleBufferInvalidate(sampleBufferRef);
                CFRelease(sampleBufferRef);
            }
        }
        [pool release];

        NSLog(@"Finished");
    }];
}

这是处理帧的代码：

- (void)processNewCameraFrame:(CVImageBufferRef)cameraFrame {


    CVPixelBufferLockBaseAddress(cameraFrame, 0);
    int bufferHeight = CVPixelBufferGetHeight(cameraFrame);
    int bufferWidth = CVPixelBufferGetWidth(cameraFrame);

    NSLog(@"Size : %i %i %zu", bufferWidth, bufferHeight, CVPixelBufferGetDataSize(cameraFrame));


    // Create a new texture from the camera frame data, display that using the shaders
    glGenTextures(1, &videoFrameTexture);
    glBindTexture(GL_TEXTURE_2D, videoFrameTexture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

    // This is necessary for non-power-of-two textures
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

    // Using BGRA extension to pull in video frame data directly
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, bufferWidth, bufferHeight, 0, GL_BGRA, GL_UNSIGNED_BYTE, CVPixelBufferGetBaseAddress(cameraFrame));

    GLenum err = glGetError();
    if (err != GL_NO_ERROR)
        NSLog(@"Error uploading texture. glError: 0x%04X", err);

    [self drawFrame];

    glDeleteTextures(1, &videoFrameTexture);

    CVPixelBufferUnlockBaseAddress(cameraFrame, 0);
}

对于在OpenGL视图中绘制帧：

- (void)drawFrame {    
    // Replace the implementation of this method to do your own custom drawing.
    static const GLfloat squareVertices[] = {
        -1.0f, -1.0f,
        1.0f, -1.0f,
        -1.0f,  1.0f,
        1.0f,  1.0f,
    };

    static const GLfloat textureVertices[] = {
        1.0f, 1.0f,
        1.0f, 0.0f,
        0.0f,  1.0f,
        0.0f,  0.0f,
    };

    [glView setDisplayFramebuffer];
    glUseProgram(grayScaleProgram);         

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, videoFrameTexture);

    // Update uniform values
    glUniform1i(uniforms[UNIFORM_VIDEOFRAME], 0);   

    // Update attribute values.
    glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, squareVertices);
    glEnableVertexAttribArray(ATTRIB_VERTEX);
    glVertexAttribPointer(ATTRIB_TEXTUREPOSITON, 2, GL_FLOAT, 0, 0, textureVertices);
    glEnableVertexAttribArray(ATTRIB_TEXTUREPOSITON);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);



    [glView presentFramebuffer];
}

我不添加代码，但如果需要帮助我，我可以......你有办法帮助我吗？

谢谢！

Answer 1

当您说灰度滤镜时，您的意思是您正在使用 kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange 视频帧吗？如果是这样，那么我假设您将纹理绑定为 GL_LUMINANCE 。但是，在您的 AVAssetReader 中，您使用 kCVPixelFormatType_32BGRA 像素格式，并在处理方法中将纹理绑定为 GL_RGBA。

我曾经偶然发现视频捕获像素格式和纹理绑定的错误组合，最终导致黑屏。检查 AVCaptureSession 设置代码中的 AVCaptureVideoDataOutput 设置。像素格式和纹理绑定应该相同。

编辑：
黑屏的另一个可能原因是 OpenGL 上下文不在线程之间共享。如果您对相机视频帧使用主队列 dispatch_get_main_queue() 以外的队列，则意味着该方法

- (void)captureOutput:(AVCaptureOutput *)captureOutput 
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer 
       fromConnection:(AVCaptureConnection *)connection  

是在后台线程上调用的，您无法从中更新 UI。

您可以尝试使用主调度队列设置捕获会话，看看会发生什么。

AVCaptureVideoDataOutput *videoOut = [[AVCaptureVideoDataOutput alloc] init];
// set video settings, frame rate .. etc
[videoOut setSampleBufferDelegate:self queue:dispatch_get_main_queue()];

Answer 2

看来您没有将您的cameraFrame 与videoFrameTexture 链接起来。
CVImageBufferRef 图像；
glBindTexture( CVOpenGLTextureGetTarget( 图像 ), CVOpenGLTextureGetName( 图像 ) );