The source code for an object that helps you sample RGB values from ARFrames.

CapturedImageSampler.swift

//
//  CapturedImageSampler.swift
//  ARKitTest
//
//  Created by Joshua Sullivan on 9/22/17.
//  Copyright © 2017 Joshua Sullivan. All rights reserved.
//

import UIKit
import ARKit
import Accelerate

/// This object provides easy color sampling of the `capturedImage` property of an ARFrame.
///
/// - Warning: This class is NOT thread safe. The `rawRGBBuffer` property is shared between instances
///            and will cause a lot of headaches if 2 instances try to simultaneously access it.
///            If you need multi-threading, make the shared buffer an instance property instead.
///            Just remember to release it when you're done with it.
class CapturedImageSampler {
    
    /// This is the format of the pixel buffer included with the ARFrame.
    private static let expectedPixelFormat: OSType = kCVPixelFormatType_420YpCbCr8BiPlanarFullRange
    
    /// This is the YCbCr to RGB conversion opaque object used by the convert function.
    private static var conversionMatrix: vImage_YpCbCrToARGB = {
        var pixelRange = vImage_YpCbCrPixelRange(Yp_bias: 0, CbCr_bias: 128, YpRangeMax: 255, CbCrRangeMax: 255, YpMax: 255, YpMin: 1, CbCrMax: 255, CbCrMin: 0)
        var matrix = vImage_YpCbCrToARGB()
        vImageConvert_YpCbCrToARGB_GenerateConversion(kvImage_YpCbCrToARGBMatrix_ITU_R_709_2, &pixelRange, &matrix, kvImage420Yp8_CbCr8, kvImageARGB8888, UInt32(kvImageNoFlags))
        return matrix
    }()
    
    /// Since we'll generally be dealing with buffers of the same size, save processsing power
    /// by re-using a single, static one, rather than allocating a new buffer each time. This
    /// will be set by the initializer of the first instance of `CapturedImageSampler`.
    private static var rawRGBBuffer: UnsafeMutableRawPointer!
    
    /// Store the size information of the buffer.
    private static var rawBufferSize: CGSize = .zero
    
    /// The errors which can be produced.
    enum PixelError: Error {
        /// The `capturedImage` property's CVPixelBuffer was in an unexpected format.
        case incorrectPixelFormat
        
        /// Failed to allocate space for the conversion buffer.
        case systemFailure
        
        /// The conversion function returned an error.
        case conversionFailure(vImage_Error)
    }
    
    /// A private pointer to a cast version of CapturedImageSampler.rawRGBBuffer.
    private var rgb: UnsafePointer<UInt8>
    
    /// Stored buffer dimension information.
    private let rgbSize: BufferDimension
    
    /// Initialize the CapturedImageSampler with an `ARFrame` to sample RGB colors from the
    /// captured image that it contains.
    /// - Parameter frame: An `ARFrame` instance that you wish to sample for RGB colors.
    init(frame: ARFrame) throws {
        
        // Get the image pixel buffer.
        let pixelBuffer = frame.capturedImage
        
        // Double-check that the format hasn't changed unexpectedly.
        guard CVPixelBufferGetPixelFormatType(pixelBuffer) == CapturedImageSampler.expectedPixelFormat else {
            NSLog("ERROR: ARFrame.capturedImage had an unexpected pixel format.")
            throw PixelError.incorrectPixelFormat
        }
        
        // Lock the base address for work.
        CVPixelBufferLockBaseAddress(pixelBuffer, .readOnly)
        
        // Ensure that we can find both the Y and the CbCr planes.
        guard
            let rawyBuffer = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 0),
            let rawcbcrBuffer = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, 1)
            else {
                print("Unable to find correct planar data in source pixel buffer.")
                throw PixelError.incorrectPixelFormat
        }
        // Note: The Y plane has the same size as our output image. The CbCr plane is 1/2 resolution.
        //       However, the conversion method accounts for this, so we don't need to worry.
        let ySize = BufferDimension(pixelBuffer: pixelBuffer, plane: 0)
        let cSize = BufferDimension(pixelBuffer: pixelBuffer, plane: 1)
        
        // Convert the individual planes to the vImage_Buffer type via a convenience method on the size.
        var yBuffer = ySize.buffer(with: rawyBuffer)
        var cbcrBuffer = cSize.buffer(with: rawcbcrBuffer)
        
        // Check to see if the static RGB buffer is the correct size.
        if ySize.size != CapturedImageSampler.rawBufferSize && CapturedImageSampler.rawRGBBuffer != nil {
            // It's the wrong size. Free it and nil the reference so we can recreate it below.
            free(CapturedImageSampler.rawRGBBuffer)
            CapturedImageSampler.rawRGBBuffer = nil
        }
        
        // Check to see if the static buffer exists.
        if CapturedImageSampler.rawRGBBuffer == nil {
            // If it doesn't exist, create it. Size = width * height * 1 byte per channel (ARGB).
            guard let buffer = malloc(ySize.width * ySize.height * 4) else {
                print("ERROR: Unable to allocate space for RGB buffer.")
                throw PixelError.systemFailure
            }
            CapturedImageSampler.rawRGBBuffer = buffer
        }
        
        // At this point we know the static buffer exists. Use it to create the target RGB vImage_Buffer.
        var rgbBuffer: vImage_Buffer = vImage_Buffer(data: CapturedImageSampler.rawRGBBuffer, height: ySize.uHeight, width: ySize.uWidth, rowBytes: ySize.width * 4)
        
        // Put everything together to convert the Y and CbCr planes into a single, interleaved ARGB buffer.
        // Note: The declared constants for kvImageFlags are the wrong type: they're all Int, but should be UInt32.
        //       I've filed a radar about it.
        let error = vImageConvert_420Yp8_CbCr8ToARGB8888(&yBuffer, &cbcrBuffer, &rgbBuffer, &CapturedImageSampler.conversionMatrix, nil, 255, UInt32(kvImageNoFlags))
        
        // Check to see that the returned error type was No Error.
        if error != kvImageNoError {
            NSLog("Error converting buffer: \(error)")
            throw PixelError.conversionFailure(error)
        }
        
        // We're done with the original pixel buffer, unlock it.
        CVPixelBufferUnlockBaseAddress(pixelBuffer, .readOnly)
        
        // All we want is a buffer of bytes, we'll do the address math manually.
        rgb = unsafeBitCast(rgbBuffer.data, to: UnsafePointer<UInt8>.self)
        
        // Store the dimensions of the buffer so we can do offset math and check that our static buffer is
        // the correct size for the next instance of this class.
        rgbSize = BufferDimension(width: ySize.width, height: ySize.height, bytesPerRow: ySize.width * 4)
    }
    
    /// Get the RGB color of the pixel at the specified coordinates.
    ///
    /// - Parameter atX: A scalar float in the range 0.0..<1.0.
    /// - Parameter y: A scalar float in the range 0.0..<1.0.
    /// - Returns: An optional UIColor, based on whether or not valid coordinates were supplied.
    ///
    /// - Note: The scalar values for x and y are easily obtained by dividing the x and y of your
    ///         target point by the width and height of the image, respectively. This is done so
    ///         that there is agreement between the sample and the displayed image, even if the
    ///         latter is scaled up or down.
    func getColor(atX x: CGFloat, y: CGFloat) -> UIColor? {

        guard x >= 0 && x < 1 && y >= 0 && y < 1 else {
            // The coordinate is outside the valid range.
            return nil
        }
        
        // The buffer order is horizontally flipped from the displayed orientation, so we'll
        // flip the x coordinate. Make sure it never equals 1.0.
        let xFlipped = min(1 - x, 0.9999999)
        
        // The ARFrame's pixel buffer has the long axis horizontally, so we're going to swap the
        // x and y coordinates to ensure we get the color corresponding to the correct screen
        // location when the phone is in portrait orientation.
        let tx = Int(y * CGFloat(rgbSize.width))
        let ty = Int(xFlipped * CGFloat(rgbSize.height))
        
        // Calculate the starting index.
        let index = ty * rgbSize.bytesPerRow + tx * 4
        
        // Grab the ARGB bytes and convert them into scalar CGFloats.
        let a = CGFloat(rgb[index]) / 255.0
        let r = CGFloat(rgb[index + 1]) / 255.0
        let g = CGFloat(rgb[index + 2]) / 255.0
        let b = CGFloat(rgb[index + 3]) / 255.0
        
        // Return the resulting color.
        return UIColor(red: r, green: g, blue: b, alpha: a)
    }
    
    /// This helper method prints out a bunch of information about a `CVPixelBuffer`. Call this
    /// to see what is going on if you're having an incorrect pixel format error.
    static func inspect(pixelBuffer: CVPixelBuffer) {
        
        print("Beginning pixel buffer inspection.")
        let pixelFormat = CVPixelBufferGetPixelFormatType(pixelBuffer)
        print("Raw pixel format: \(pixelFormat)")
        let pixelFormatString = str(from: pixelFormat)
        print("Pixel format: \(pixelFormatString)")
        let isPlanar = CVPixelBufferIsPlanar(pixelBuffer)
        print("Is planar: \(isPlanar)")
        if (isPlanar) {
            let planeCount = CVPixelBufferGetPlaneCount(pixelBuffer)
            print("Plane count: \(planeCount)")
            for planeIndex in 0..<planeCount {
                print("[Plane \(planeIndex)]")
                if let baseAddress = CVPixelBufferGetBaseAddressOfPlane(pixelBuffer, planeIndex) {
                    print("baseAddress: \(baseAddress)")
                }
                let planeWidth = CVPixelBufferGetWidthOfPlane(pixelBuffer, planeIndex)
                let planeHeight = CVPixelBufferGetHeightOfPlane(pixelBuffer, planeIndex)
                let planeRowSize = CVPixelBufferGetBytesPerRowOfPlane(pixelBuffer, planeIndex)
                print("width: \(planeWidth) / height: \(planeHeight) / rowSize: \(planeRowSize)")
            }
        } else {
            let width = CVPixelBufferGetWidth(pixelBuffer)
            let height = CVPixelBufferGetHeight(pixelBuffer)
            let rowSize = CVPixelBufferGetBytesPerRow(pixelBuffer)
            print("width: \(width) / height: \(height) / rowSize: \(rowSize)")
        }
    }
    
    /// Get a human-readable string from an OSType object.
    static func str(from os: OSType) -> String {
        var str = ""
        str.append(String(UnicodeScalar((os >> 24) & 0xFF)!))
        str.append(String(UnicodeScalar((os >> 16) & 0xFF)!))
        str.append(String(UnicodeScalar((os >> 8) & 0xFF)!))
        str.append(String(UnicodeScalar(os & 0xFF)!))
        return str
    }
}

/// This struct stores dimensional information about a buffer, needed to correctly address its
/// consitiuent elements.
struct BufferDimension {
    let width: Int
    let height: Int
    let bytesPerRow: Int
    
    /// The total number of bytes in a buffer with this dimension. Useful for bounds-checking
    /// array access of the buffer's elements.
    var byteCount: Int {
        return width * bytesPerRow * height
    }
    
    /// Convience getter to cast the width as a UInt, which is required by vImage_Buffer.
    var uWidth: UInt {
        return UInt(width)
    }
    
    /// Convience getter to cast the height as a UInt, which is required by vImage_Buffer.
    var uHeight: UInt {
        return UInt(height)
    }
    
    /// Convenience getter to get the size (width, height) of the buffer.
    var size: CGSize {
        return CGSize(width: width, height: height)
    }
    
    /// Initialize the dimension with a CVPixelBuffer and optional plane index.
    /// This will misbehave if you have a multi-planar image and you don't specify a plane,
    /// or if you specify a plane index and it is not planar.
    init(pixelBuffer: CVPixelBuffer, plane: Int?) {
        if let plane = plane {
            self.width = CVPixelBufferGetWidthOfPlane(pixelBuffer, plane)
            self.height = CVPixelBufferGetHeightOfPlane(pixelBuffer, plane)
            self.bytesPerRow = CVPixelBufferGetBytesPerRowOfPlane(pixelBuffer, plane)
        } else {
            self.width = CVPixelBufferGetWidth(pixelBuffer)
            self.height = CVPixelBufferGetHeight(pixelBuffer)
            self.bytesPerRow = CVPixelBufferGetBytesPerRow(pixelBuffer)
        }
    }
    
    /// Alternative initializer for directly setting dimension values.
    init(width: Int, height: Int, bytesPerRow: Int) {
        self.width = width
        self.height = height
        self.bytesPerRow = bytesPerRow
    }
    
    /// Create and return a `vImage_Buffer` using the dimensions and a supplied buffer.
    func buffer(with data: UnsafeMutableRawPointer) -> vImage_Buffer {
        return vImage_Buffer(data: data, height: uHeight, width: uWidth, rowBytes: bytesPerRow)
    }
}

Nice helper for that cumbersome conversion, thanks for sharing!
It looks like the rawBufferSize never gets set and the rawRGBBuffer is always recreated.

Can you show me an example call to the getColor() function if I want to get the color of each pixel in the image

Thank you very much for providing such a clear and elegant solution, @JoshuaSullivan.

For everyone who's asking how to use the code, here's an elementary example (for clarity reasons, I've omitted to check for null/empty data):

public func session(_ session: ARSession, didUpdate frame: ARFrame) {

    let colorData = frame.capturedImage
    
    let sampler = try? CapturedImageSampler(pixelBuffer: buffer)
    let color = sampler!.getColor(atX: 0, y: 0)
}

Also, consider reading Joshua's blog post for tips and warnings.

Hi, is there any way to do, or explain how it would work, kCVPixelFormatType_420YpCbCr8PlanarFullRange to Bi-planar or RGB that AVAssetWriter can use?