iPhone image ratio captured from AVCaptureSession

Question

I am using somebody's source code for capturing image with AVCaptureSession. However,I found that CaptureSessionManager's previewLayer is shotter then the final captured image.

I found that the resulted image is always with ratio 720x1280=9:16. Now I want to crop the resulted image to an UIImage with ratio 320:480 so that it will only capture the portion visible in previewLayer. Any Idea? Thanks a lot.

Relevant Questions in stackoverflow(NO good answer yet): Q1, Q2

Source Code:

- (id)init {
if ((self = [super init])) {
    [self setCaptureSession:[[[AVCaptureSession alloc] init] autorelease]];
}
return self;
}

- (void)addVideoPreviewLayer {
[self setPreviewLayer:[[[AVCaptureVideoPreviewLayer alloc] initWithSession:[self captureSession]] autorelease]];
[[self previewLayer] setVideoGravity:AVLayerVideoGravityResizeAspectFill];

}

- (void)addVideoInput {
AVCaptureDevice *videoDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];   
if (videoDevice) {
    NSError *error;


    if ([videoDevice isFocusModeSupported:AVCaptureFocusModeContinuousAutoFocus] && [videoDevice lockForConfiguration:&error]) {
        [videoDevice setFocusMode:AVCaptureFocusModeContinuousAutoFocus];
        [videoDevice unlockForConfiguration];
    }        

    AVCaptureDeviceInput *videoIn = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:&error];
    if (!error) {
        if ([[self captureSession] canAddInput:videoIn])
            [[self captureSession] addInput:videoIn];
        else
            NSLog(@"Couldn't add video input");     
    }
    else
        NSLog(@"Couldn't create video input");
}
else
    NSLog(@"Couldn't create video capture device");
}

- (void)addStillImageOutput 
{
  [self setStillImageOutput:[[[AVCaptureStillImageOutput alloc] init] autorelease]];
  NSDictionary *outputSettings = [[NSDictionary alloc] initWithObjectsAndKeys:AVVideoCodecJPEG,AVVideoCodecKey,nil];
  [[self stillImageOutput] setOutputSettings:outputSettings];

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

  [[self captureSession] addOutput:[self stillImageOutput]];
}

- (void)captureStillImage
{  
AVCaptureConnection *videoConnection = nil;
for (AVCaptureConnection *connection in [[self 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: %@", [self stillImageOutput]);
[[self stillImageOutput] captureStillImageAsynchronouslyFromConnection:videoConnection 
                                                   completionHandler:^(CMSampleBufferRef imageSampleBuffer, NSError *error) { 
                                                     CFDictionaryRef exifAttachments = CMGetAttachment(imageSampleBuffer, kCGImagePropertyExifDictionary, NULL);
                                                     if (exifAttachments) {
                                                       NSLog(@"attachements: %@", exifAttachments);
                                                     } else { 
                                                       NSLog(@"no attachments");
                                                     }
                                                     NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageSampleBuffer];    
                                                     UIImage *image = [[UIImage alloc] initWithData:imageData];
                                                     [self setStillImage:image];
                                                     [image release];
                                                     [[NSNotificationCenter defaultCenter] postNotificationName:kImageCapturedSuccessfully object:nil];
                                                   }];
}

---

---

Edit after doing some more research and testing: AVCaptureSession's property "sessionPreset" has the following constants, I haven't checked each one of them, but noted that most of them ratio is either 9:16, or 3:4,

• NSString *const AVCaptureSessionPresetPhoto;
• NSString *const AVCaptureSessionPresetHigh;
• NSString *const AVCaptureSessionPresetMedium;
• NSString *const AVCaptureSessionPresetLow;
• NSString *const AVCaptureSessionPreset352x288;
• NSString *const AVCaptureSessionPreset640x480;
• NSString *const AVCaptureSessionPresetiFrame960x540;
• NSString *const AVCaptureSessionPreset1280x720;
• NSString *const AVCaptureSessionPresetiFrame1280x720;

In My project, I have the fullscreen preview(frame size is 320x480) also: [[self previewLayer] setVideoGravity:AVLayerVideoGravityResizeAspectFill];

I have done it in this way: take the photo in size 9:16 and crop it to 320:480, exactly the visible part of the previewlayer. It looks perfect.

The piece of code for resizing and croping to replace with old code is

NSData *imageData = [AVCaptureStillImageOutput  jpegStillImageNSDataRepresentation:imageSampleBuffer];
 UIImage *image = [UIImage imageWithData:imageData]; 
UIImage *scaledimage=[ImageHelper scaleAndRotateImage:image];
 //going to crop the image 9:16 to 2:3;with Width fixed 
float width=scaledimage.size.width; 
float height=scaledimage.size.height; 
float top_adjust=(height-width*3/2.0)/2.0;  
[self setStillImage:[scaledimage croppedImage:rectToCrop]];

Answer 1

iPhone's camera is natively 4:3. The 16:9 images you get are already cropped from 4:3. Cropping those 16:9 images again to 4:3 is not what you want. Instead get the native 4:3 images from iPhone's camera by setting self.captureSession.sessionPreset = AVCaptureSessionPresetPhoto (before adding any inputs/outputs to the session).