CVPixelBufferLockBaseAddress why? Capture still image using AVFoundation - iphone

Question

I'm writing an iPhone app that creates still images from the camera using AVFoundation.
Reading the programming guide I've found a code that does almost I need to do, so I'm trying to "reverse engineering" and understand it.
I'm founding some difficulties to understand the part that converts a CMSampleBuffer into an image.
So here is what I understood and later the code.
The CMSampleBuffer represent a buffer in the memory where the image with additional data is stored. Later I call the function CMSampleBufferGetImageBuffer() to receive a CVImageBuffer back with just the image data.
Now there is a function that I didn't understand and I can only imagine its function: CVPixelBufferLockBaseAddress(imageBuffer, 0); I can't understand if it is a "thread lock" to avoid multiple operation on it or a lock to the address of the buffer to avoid changes during operation(and why should it change?..another frame, aren't data copied in another location?). The rest of the code it's clear to me.
Tried to search on google but still didn't find nothing helpful.
Can someone bring some light?
-(UIImage*) getUIImageFromBuffer:(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);
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 context = 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(context);
// Unlock the pixel buffer
CVPixelBufferUnlockBaseAddress(imageBuffer,0);
// Free up the context and color space
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
// Create an image object from the Quartz image
UIImage *image = [UIImage imageWithCGImage:quartzImage];
// Release the Quartz image
CGImageRelease(quartzImage);
return (image);
}
Thanks,
Andrea

Answer 1

The header file says that CVPixelBufferLockBaseAddress makes the memory "accessible". I'm not sure what that means exactly, but if you don't do it, CVPixelBufferGetBaseAddress fails so you'd better do it.
EDIT
Just do it is the short answer. For why consider that image may not live in main memory, it may live in a texture on some GPU somewhere (CoreVideo works on the mac too) or even be in a different format to what you expect, so the pixels you get are actually a copy. Without Lock/Unlock or some kind of Begin/End pair the implementation has no way to know when you've finished with the duplicate pixels so they would effectively be leaked. CVPixelBufferLockBaseAddress simply gives CoreVideo scope information, I wouldn't get too hung up on it.
Yes, they could have simply returned the pixels from CVPixelBufferGetBaseAddress and eliminate CVPixelBufferLockBaseAddress altogether. I don't know why they didn't do that.

Answer 2

I'd like to give more hints about this function, I made some tests so far and I can tell you that. When you get the base address you are probably getting the address of some shared memory resource. This becomes clear if you print the address of the base address, doing that you can see that base addresses are repeated while getting video frames. In my app I take frames at specific intervals and pass the CVImageBufferRef to an NSOperation subclass that converts the buffer in an image and saves it on the phone. I do not lock the pixel buffer until the operation starts to convert the CVImageBufferRef, even if pushing at higher framerates the base address of the pixel and the CVImageBufferRef buffer address are equal before the creation of the NSOperation and inside it. I just retain the CVImageBufferRef. I was expecting to se unmatching references and even if I didn't see it I guess that the best description is that CVPixelBufferLockBaseAddress locks the memory portion where the buffer is located, making it inaccessible from other resources so it will keep the same data, until you unlock it.