AVCaptureSession specify resolution and quality of captured images obj-c iphone app

问题

Hi I want to setup AV capture session to capture images with specific resolution (and, if possible, with specific quality) using iphone camera. here's setupping AV session code

// Create and configure a capture session and start it running
- (void)setupCaptureSession 
{
    NSError *error = nil;

    // Create the session
    self.captureSession = [[AVCaptureSession alloc] init];

    // Configure the session to produce lower resolution video frames, if your 
    // processing algorithm can cope. We'll specify medium quality for the
    // chosen device.
    captureSession.sessionPreset = AVCaptureSessionPresetMedium;

    // Find a suitable AVCaptureDevice
    NSArray *cameras=[AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
    AVCaptureDevice *device;
    if ([UserDefaults camera]==UIImagePickerControllerCameraDeviceFront)
    {
        device =[cameras objectAtIndex:1];
    }
    else
    {
        device = [cameras objectAtIndex:0];
    };

    // Create a device input with the device and add it to the session.
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:device error:&error];
    if (!input)
    {
        NSLog(@"PANIC: no media input");
    }
    [captureSession addInput:input];

    // Create a VideoDataOutput and add it to the session
    AVCaptureVideoDataOutput *output = [[AVCaptureVideoDataOutput alloc] init];
    [captureSession addOutput:output];
    NSLog(@"connections: %@", output.connections);

    // Configure your output.
    dispatch_queue_t queue = dispatch_queue_create("myQueue", NULL);
    [output setSampleBufferDelegate:self queue:queue];
    dispatch_release(queue);

    // Specify the pixel format
    output.videoSettings = [NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA] forKey:(id)kCVPixelBufferPixelFormatTypeKey];


    // If you wish to cap the frame rate to a known value, such as 15 fps, set 
    // minFrameDuration.


    // Assign session to an ivar.
    [self setSession:captureSession];
    [self.captureSession startRunning];
}

and setSession:

-(void)setSession:(AVCaptureSession *)session
{
    NSLog(@"setting session...");
    self.captureSession=session;
    NSLog(@"setting camera view");
    self.previewLayer = [AVCaptureVideoPreviewLayer layerWithSession:session];
    //UIView *aView = self.view;
    CGRect videoRect = CGRectMake(20.0, 20.0, 280.0, 255.0);
    previewLayer.frame = videoRect; // Assume you want the preview layer to fill the view.
    [previewLayer setBackgroundColor:[[UIColor grayColor] CGColor]];
    [self.view.layer addSublayer:previewLayer];
    //[aView.layer addSublayer:previewLayer];
}

and output methods:

// Delegate routine that is called when a sample buffer was written
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer 
   fromConnection:(AVCaptureConnection *)connection
{ 
    //NSLog(@"captureOutput: didOutputSampleBufferFromConnection");

    // Create a UIImage from the sample buffer data
    self.currentImage = [self imageFromSampleBuffer:sampleBuffer];

    //< Add your code here that uses the image >
}

// Create a UIImage from sample buffer data
- (UIImage *) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer 
{
    //NSLog(@"imageFromSampleBuffer: called");
    // Get a CMSampleBuffer's Core Video image buffer for the media data
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer); 
    // Lock the base address of the pixel buffer
    CVPixelBufferLockBaseAddress(imageBuffer, 0); 

    // Get the number of bytes per row for the pixel buffer
    void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer); 

    // Get the number of bytes per row for the pixel buffer
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer); 
    // Get the pixel buffer width and height
    size_t width = CVPixelBufferGetWidth(imageBuffer); 
    size_t height = CVPixelBufferGetHeight(imageBuffer); 

    // Create a device-dependent RGB color space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); 

    // Create a bitmap graphics context with the sample buffer data
    CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8, bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst); 
    // Create a Quartz image from the pixel data in the bitmap graphics context
    CGImageRef quartzImage = CGBitmapContextCreateImage(context); 
    // Unlock the pixel buffer
    CVPixelBufferUnlockBaseAddress(imageBuffer,0);


    // Free up the context and color space
    CGContextRelease(context); 
    CGColorSpaceRelease(colorSpace);

    // Create an image object from the Quartz image
    UIImage *image = [UIImage imageWithCGImage:quartzImage];

    // Release the Quartz image
    CGImageRelease(quartzImage);

    return (image);
}

Everything is quite standard. But where and what should I change to specify the resolution of captured image and it's quality. Help me please

回答1:

Refer to Apple's guide Capturing Still Images section regarding which sizes you'll get if you set one or another preset.

The parameter you should change is captureSession.sessionPreset

回答2:

Try to go with something like this where cx and cy are your custom resolutions:

NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                              AVVideoScalingModeResizeAspectFill,AVVideoScalingModeKey,
                           AVVideoCodecH264, AVVideoCodecKey,
                           [NSNumber numberWithInt:cx], AVVideoWidthKey,
                           [NSNumber numberWithInt:cx], AVVideoHeightKey,
                           nil];
_videoInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];

来源：https://stackoverflow.com/questions/9312832/avcapturesession-specify-resolution-and-quality-of-captured-images-obj-c-iphone