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I am trying to get an average of photo luminance/brightness by using AVFoundation framework. What I am doing is taking pixel buffer (which comes with RAW capture only) from an AVCapturePhoto object, accessing its pixels by making UnsafeMutablePointer and retrieving luminance value at a specific point. The average value I expect to get should be between 528 and 4095 (black and white levels). I am not sure what kind of value do I get from this buffer and if I access its pixels in a right way. I am also not sure if this is the best approach, but the green value that I get from the image itself seems to be a compressed one as it never goes higher than 255.

Below is the code that I am using:

 // The pixel buffer stores an image in main memory.
 let pixelBuffer = photo.pixelBuffer!

 // Locks the BaseAddress of the PixelBuffer to ensure that the memory is accessible.
 CVPixelBufferLockBaseAddress(pixelBuffer, CVPixelBufferLockFlags(rawValue: 0))

 // Buffer width and height
 let bufferWidth = CVPixelBufferGetWidth(pixelBuffer) // 4032
 let bufferHeight = CVPixelBufferGetHeight(pixelBuffer) // 3024

 /*
 Returns the base address of the PixelBuffer.
 Retrieving the base address for a PixelBuffer requires that the buffer base address be locked via a successful call to CVPixelBufferLockBaseAddress.
 */

 let baseAddress = CVPixelBufferGetBaseAddress(pixelBuffer)
 let bytesPerRow = CVPixelBufferGetBytesPerRow(pixelBuffer) // 8064

 /*
 Returns the bits of the given instance, interpreted as having the specified type.
 Use this function only to convert the instance passed as x to a layout-compatible type when conversion through other means is not possible.
 */

 let pointer = unsafeBitCast(pixelBuffer, to: UnsafeMutablePointer<UInt16>.self) // what data type to use?
 //let pointer = baseAddress?.bindMemory(to: UInt16.self, capacity: CVPixelBufferGetDataSize(pixelBuffer))
 var lumaSum: Double = 0.0

 for y in 0..<bufferHeight 
 for x in 0..<bufferWidth 
 let index = y * bufferWidth + x // or y * bytesPerRow + x
 let luma = Double(pointer[index])
 lumaSum += luma
 
 

 let average = lumaSum / Double(bufferWidth * bufferHeight)
 print(average)

 CVPixelBufferUnlockBaseAddress(pixelBuffer, CVPixelBufferLockFlags(rawValue: 0))

Here is some information that might be useful:

Pixel format: kCVPixelFormatType_14Bayer_RGGB = 'rgg4', /* Bayer 14-bit Little-Endian, packed in 16-bits, ordered R G R G... alternating with G B G B... */

Buffer data size: 24385600

Contains RGB: true

Bits per component: 8

Bytes per row: 8064

Bits per block: 16