Retaining CMSampleBufferRef cause random crashes

Question

I'm using captureOutput:didOutputSampleBuffer:fromConnection: in order to keep track of the frames. For my use-case, I only need to store the last frame and use it in case the app goes to background.

That's a sample from my code:

@property (nonatomic, strong) AVCaptureVideoDataOutput *videoDataOutput;
@property (atomic) CMSampleBufferRef currentBuffer;

- (UIImage *) imageFromSampleBuffer:(CMSampleBufferRef) sampleBuffer
{
    // 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 con = 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(con);
    // Unlock the pixel buffer
    CVPixelBufferUnlockBaseAddress(imageBuffer,0);

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

    // Create an image object from the Quartz image
    //    UIImage *image = [UIImage imageWithCGImage:quartzImage];
    UIImage *image =  [UIImage imageWithCGImage:quartzImage scale:[[UIScreen mainScreen] scale] orientation:UIImageOrientationRight];

    // Release the Quartz image
    CGImageRelease(quartzImage);

    return (image);
}
//[self.videoDataOutput setSampleBufferDelegate:self queue:self.sessionQueue];

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

    @synchronized (self) {
        if (_currentBuffer) {
            CFRelease(_currentBuffer);
        }
        self.currentBuffer = sampleBuffer;
    }
}

- (void) goingToBackground:(NSNotification *) notification
{
    UIImage *snapshot = [self imageFromSampleBuffer:_currentBuffer];

    //Doing something with snapshot...
}

The problem is that in some cases I get this crash from within imageFromSampleBuffer:

<Error>: copy_read_only: vm_copy failed: status 1.

The crash happens on CGImageRef quartzImage = CGBitmapContextCreateImage(con);

What am I doing wrong?

Euryale

Could you show your code for the method imageFromSampleBuffer: – Thyroxine

@SheffieldKevin, I edited the question. – Euryale

Added answer how to take sampleBuffer ownership here: https://mcmap.net/q/1474402/-take-ownership-of-memory-from-cvimagebufferref – Yacht

Possible duplicate of AVCaptureSession returning blank image on iPhone 3G only – Leanora

Answer 1

I believe you need to copy the buffer, not just retain. From the description for captureOutput:didOutputSampleBuffer:fromConnection:

Note that to maintain optimal performance, some sample buffers directly reference pools of memory that may need to be reused by the device system and other capture inputs. [...] If multiple sample buffers reference such pools of memory for too long, inputs will no longer be able to copy new samples into memory and those samples will be dropped.

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