How Can i change image quality when using AVCaptureSessionPreset

Question

im looking for some help with AVFoundation. Currently i have following a step by step guide on how to capture a still image from this topic: How to save photos taken using AVFoundation to Photo Album?

My question is, how can i lower the quality of my saved image as well as using the AVCaptureSessionPreset640x480. I would like to half the image quality, or if there is another way to make the saved image as small a file as possible - possibly 320x280 - then that could be better than adjusting the actual quality.

I dont know if anyone else has asked this before or not but i have been searching the web for the past couple of days and cannot find an answer. Below is my code.

`

#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

You need to set kCVPixelBufferWidthKey and kCVPixelBufferHeightKey options on AVCaptureStillImageOutput object to set resolution of your choice. This width/height will override session preset width/height. Minimal sample as below (add error check).

    _session = [[AVCaptureSession alloc] init];
    _device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

    NSError * error;
    _sessionInput = [AVCaptureDeviceInput deviceInputWithDevice:_device error:&error];
    _stillImageOutput = [[AVCaptureStillImageOutput alloc] init];
    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                              [NSNumber numberWithDouble:320.0], (id)kCVPixelBufferWidthKey,
                              [NSNumber numberWithDouble:280.0], (id)kCVPixelBufferHeightKey,
                              [NSNumber numberWithUnsignedInt:kCVPixelFormatType_32BGRA], (id)kCVPixelBufferPixelFormatTypeKey,
                              nil];

    [_stillImageOutput setOutputSettings:options];

    [_session beginConfiguration ];
    [_session addInput:_sessionInput];
    [_session addOutput:_stillImageOutput];
    [_session setSessionPreset:AVCaptureSessionPresetPhoto];
     _avConnection = [_stillImageOutput connectionWithMediaType:AVMediaTypeVideo];
    [ _session commitConfiguration ];

.............

- (void) start
{
    [self.session startRunning];
}

.............

[self.stillImageOutput captureStillImageAsynchronouslyFromConnection:self.avConnection completionHandler: ^(CMSampleBufferRef imageSampleBuffer, NSError *error)
 {
     CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(imageSampleBuffer);
     CVPixelBufferLockBaseAddress(imageBuffer, 0);
     size_t width = CVPixelBufferGetWidth(imageBuffer);
     size_t height = CVPixelBufferGetHeight(imageBuffer);
     NSLog(@"%d : %d", height, width);

 }];

Note: I have only tried this on an mac. Ideally it should work for iOS too. Also try maintaining some aspect ratio.

Answer 2

Try using different Preset that will give you different resolution image.

As per the Apple Documentation, for iPhone4(Back) you will get the following resolution image for the following Preset for that session.

AVCaptureSessionPresetHigh : 1280x720

AVCaptureSessionPresetMedium : 480x360

AVCaptureSessionPresetLow : 192x144

AVCaptureSessionPreset640x480 : 640x480

AVCaptureSessionPreset1280x720 : 1280x720

AVCaptureSessionPresetPhoto : 2592x1936.This is not supported for video output

Hope this will help.