Retaining CMSampleBufferRef cause random crashes - ios

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?

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.