ios經過AVCaptureSession獲取視頻流、照片

        在使用到AVCaptureSession獲取視頻流、照片時，因爲硬件採集到的數據跟設備方向有關，致使直接從緩衝區讀取到圖片或視頻，可能存在沒法正常顯示的問題。此時須要作相應的轉換。

        具體關於概念上的解釋，可參考http://www.cocoachina.com/ios/20150605/12021.html這篇博文。

調用硬件設備

- (AVCaptureVideoDataOutput *)captureOutput {
    if (!_captureOutput) {
        _captureOutput = [[AVCaptureVideoDataOutput alloc]
                                                   init];
        _captureOutput.alwaysDiscardsLateVideoFrames = YES;
        dispatch_queue_t queue;
        queue = dispatch_queue_create("cameraQueue", NULL);
        [_captureOutput setSampleBufferDelegate:self queue:queue];
        NSString* key = (NSString*)kCVPixelBufferPixelFormatTypeKey;
        NSNumber* value = [NSNumber
                           numberWithUnsignedInt:kCVPixelFormatType_32BGRA];
        NSDictionary* videoSettings = [NSDictionary
                                       dictionaryWithObject:value forKey:key];
        [_captureOutput setVideoSettings:videoSettings];
    }

    return _captureOutput;
}

- (AVCaptureSession *)captureSession {
    if (!_captureSession) {
        AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

        if ([device lockForConfiguration:nil]) {
            //設置幀率
            device.activeVideoMinFrameDuration = CMTimeMake(2, 3);
        }
        AVCaptureDeviceInput *captureInput = [AVCaptureDeviceInput
                                              deviceInputWithDevice:device  error:nil];

        _captureSession = [[AVCaptureSession alloc] init];
        [_captureSession addInput:captureInput];
        [_captureSession addOutput:self.captureOutput];
    }

    return _captureSession;
}

- (AVCaptureVideoPreviewLayer *)prevLayer {
    if (!_prevLayer) {
        _prevLayer = [AVCaptureVideoPreviewLayer
                          layerWithSession:self.captureSession];
        _prevLayer.frame = [self rectForPreviewLayer];
        _prevLayer.videoGravity = AVLayerVideoGravityResizeAspectFill;
    }

    return _prevLayer;
}

 如從緩衝區讀取照片時，

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

    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
    CVPixelBufferLockBaseAddress(imageBuffer,0);
    uint8_t *baseAddress = (uint8_t *)CVPixelBufferGetBaseAddress(imageBuffer);
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
    size_t width = CVPixelBufferGetWidth(imageBuffer);
    size_t height = CVPixelBufferGetHeight(imageBuffer);

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef newContext = CGBitmapContextCreate(baseAddress,
                                                    width, height, 8, bytesPerRow, colorSpace,
                                                    kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);
    CGImageRef newImage = CGBitmapContextCreateImage(newContext);

    CGContextRelease(newContext);
    CGColorSpaceRelease(colorSpace);

    //因爲這段代碼中，設備是home在下進行錄製，因此此處在生成image時，指定了方向
    UIImage *image = [UIImage imageWithCGImage:newImage scale:1.0
                                      orientation:UIImageOrientationRight];

    CGImageRelease(newImage);

    CVPixelBufferUnlockBaseAddress(imageBuffer,0);
}

上面只是正常從緩衝區獲取到了圖片，設置了正常顯示的方向，如作矩形檢測或人臉檢測的時候，因爲image的CGImage屬性沒有響應的調整方向，因此檢測的時候可能會出現point轉換的問題。爲避免point轉換這樣比較繁瑣的運算，因此能夠在生成image的時候，調整好image避免後續的轉換等問題

如在

UIImage *image = [UIImage imageWithCGImage:newImage scale:1.0
                                      orientation:UIImageOrientationRight];
//此行保證image的imageOrientation爲UIImageOrientationUp
        image = [image normalImage];

- (UIImage *)normalImage {

    if (self.imageOrientation == UIImageOrientationUp) return self;

    UIGraphicsBeginImageContextWithOptions(self.size, NO, self.scale);
    [self drawInRect:(CGRect){0, 0, self.size}];
    UIImage *normalizedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return normalizedImage;
}

經過以上方法，解決矩形檢測、人臉識別時，point轉換問題。

此爲圖片方向及檢測的問題解決方法，視頻暫未實踐