ios - 使用 AVCaptureSession 预设时如何更改图像质量

Question

我正在寻找有关 AVFoundation 的帮助。目前，我已经按照有关如何从此主题捕获静止图像的分步指南进行操作:How to save photos taken using AVFoundation to Photo Album?

我的问题是，如何降低我保存的图像的质量以及使用 AVCaptureSessionPreset640x480。我想将图像质量减半，或者如果有另一种方法可以使保存的图像尽可能小(可能是 320x280)，那么这可能比调整实际质量更好。

我不知道以前是否有人问过这个问题，但过去几天我一直在网上搜索但找不到答案。下面是我的代码。

`

#import "ViewController.h"
#import <ImageIO/ImageIO.h>

@interface ViewController ()

@end

@implementation ViewController

@synthesize imagePreview;
@synthesize iImage;
@synthesize stillImageOutput;

-(IBAction)captureNow {
    AVCaptureConnection *videoConnection = nil;
    for (AVCaptureConnection *connection in stillImageOutput.connections)
    {
        for (AVCaptureInputPort *port in [connection inputPorts])
        {
            if ([[port mediaType] isEqual:AVMediaTypeVideo] )
            {
                videoConnection = connection;
                break;
            }
        }
        if (videoConnection)
        {
            break;
        }
    }

    NSLog(@"about to request a capture from: %@", stillImageOutput);
    [stillImageOutput captureStillImageAsynchronouslyFromConnection:videoConnection completionHandler: ^(CMSampleBufferRef imageSampleBuffer, NSError *error)
     {
         CFDictionaryRef exifAttachments = CMGetAttachment( imageSampleBuffer, kCGImagePropertyExifDictionary, NULL);
         if (exifAttachments)
         {
             // Do something with the attachments.
             NSLog(@"attachements: %@", exifAttachments);
         } else {
             NSLog(@"no attachments");
         }

         NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageSampleBuffer];
         //NSData *data2 = [NSData dataWithData:UIImageJPEGRepresentation(image, 0.5f)]];
         UIImage *image = [[UIImage alloc] initWithData:imageData];

         self.iImage.image = image;

         UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil);
     }];
}

-(void)viewDidAppear:(BOOL)animated
{
    AVCaptureSession *session = [[AVCaptureSession alloc] init];
    session.sessionPreset = AVCaptureSessionPreset640x480;

    CALayer *viewLayer = self.imagePreview.layer;
    NSLog(@"viewLayer = %@", viewLayer);

    AVCaptureVideoPreviewLayer *captureVideoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:session];

    captureVideoPreviewLayer.frame = self.imagePreview.bounds;
    [self.imagePreview.layer addSublayer:captureVideoPreviewLayer];

    AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

    NSError *error = nil;
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:device error:&error];
    if (!input) {
        // Handle the error appropriately.
        NSLog(@"ERROR: trying to open camera: %@", error);
    }
    [session addInput:input];

    stillImageOutput = [[AVCaptureStillImageOutput alloc] init];
    NSDictionary *outputSettings = [[NSDictionary alloc] initWithObjectsAndKeys: AVVideoCodecJPEG, AVVideoCodecKey, nil];
    [stillImageOutput setOutputSettings:outputSettings];
    [session addOutput:stillImageOutput];

    [session startRunning];
}


- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

`

Answer 1

尝试使用不同的预设，这将为您提供不同分辨率的图像。

根据 Apple 文档，对于 iPhone4(背面)，您将获得该 session 的以下预设的以下分辨率图像。

AVCaptureSessionPresetHigh : 1280x720

AVCaptureSessionPresetMedium : 480x360

AVCaptureSessionPresetLow : 192x144

AVCaptureSessionPreset640x480 : 640x480

AVCaptureSessionPreset1280x720 : 1280x720

AVCaptureSessionPresetPhoto : 2592x1936.这不支持视频输出

希望这会有所帮助。