Init a Metal texture with the type of 3D

Question

Recently, I am playing with SceneKit, I found the colorGrading property. The doc say that

The contents value for this material property must be a 3D color lookup table, or a 2D texture image that represents such a table arranged in a horizontal strip.

and a 3D color lookup table can read from a Metal texture.

You can provide data in this cubic format as a Metal texture with the type3D texture type.

so how can I set scnCamera.colorGrading.contents property.

Answer 1

Creating a 3D texture is very similar to creating a 2D texture, provided you have a buffer containing the image data in the appropriate layout. I assume you already have that. Here's how to create the texture itself, copy the data into it, and set it as the color grading texture:

var dim = 16

var values: UnsafeMutablePointer<Float> = ... // alloc and populate 3D array of pixels

let textureDescriptor = MTLTextureDescriptor()
textureDescriptor.textureType = .type3D
textureDescriptor.pixelFormat = .rgba32Float
textureDescriptor.width = dim
textureDescriptor.height = dim
textureDescriptor.depth = dim
textureDescriptor.usage = .shaderRead

let texture = device.makeTexture(descriptor: textureDescriptor)

texture.replace(region: MTLRegionMake3D(0, 0, 0, dim, dim, dim),
                mipmapLevel:0,
                slice:0,
                withBytes:values,
                bytesPerRow:dim * MemoryLayout<Float>.size * 4,
                bytesPerImage:dim * dim * MemoryLayout<Float>.size * 4)

camera.colorGrading.contents = texture

EDIT

Here's a complete parser that will turn a .cube file into a MTLTexture that is suitable for use with this property:

import Metal

class AdobeLUTParser {

    static func texture(withContentsOf url: URL, device: MTLDevice) -> MTLTexture? {

        let lutString = try! NSString(contentsOf: url, encoding: String.Encoding.utf8.rawValue)

        let lines = lutString.components(separatedBy: "\r\n") as [NSString]

        var dim = 2

        var values: UnsafeMutablePointer<Float>? = nil
        var index = 0

        for line in lines {

            if line.length == 0 { continue; } // skip blanks

            let firstChar = line.character(at: 0)

            if firstChar < 58 /*':'*/ {
                if values == nil {
                    print("Error: Got data before size in LUT")
                    break;
                }

                let numbers = line.components(separatedBy: " ") as [NSString]
                if numbers.count == 3 {
                    let r = numbers[0].floatValue
                    let g = numbers[1].floatValue
                    let b = numbers[2].floatValue
                    let a = Float(1)

                    values![index * 4 + 0] = r
                    values![index * 4 + 1] = g
                    values![index * 4 + 2] = b
                    values![index * 4 + 3] = a

                    index += 1
                }
            } else {
                if line.hasPrefix("LUT_3D_SIZE") {
                    let sizeString = line.components(separatedBy: " ")[1] as NSString
                    dim = Int(sizeString.intValue)
                    if dim < 2 || dim > 512 {
                        print("Error: insane LUT size: \(dim)")
                    }
                    let rawPointer = malloc(dim * dim * dim * 4 * MemoryLayout<Float>.size)
                    values = rawPointer!.bindMemory(to: Float.self, capacity: dim * dim * dim * 4)
                } else if line.hasPrefix("LUT_1D_SIZE") {
                    print("Error: 1D LUTs not supported")
                    break
                }
            }
        }

        if values == nil {
            print("Did not parse LUT successfully")
            return nil
        }

        let textureDescriptor = MTLTextureDescriptor()
        textureDescriptor.textureType = .type3D
        textureDescriptor.pixelFormat = .rgba32Float
        textureDescriptor.width = dim
        textureDescriptor.height = dim
        textureDescriptor.depth = dim
        textureDescriptor.usage = .shaderRead

        let texture = device.makeTexture(descriptor: textureDescriptor)

        texture.replace(region: MTLRegionMake3D(0, 0, 0, dim, dim, dim),
                        mipmapLevel:0,
                        slice:0,
                        withBytes:values!,
                        bytesPerRow:dim * MemoryLayout<Float>.size * 4,
                        bytesPerImage:dim * dim * MemoryLayout<Float>.size * 4)

        return texture
    }
}

Usage:

let mtlDevice = MTLCreateSystemDefaultDevice()

let lutURL = Bundle.main.url(forResource: "MyGradingTexture", withExtension: "cube")

let lutTexture = AdobeLUTParser.texture(withContentsOf: lutURL!, device: mtlDevice!)

camera.colorGrading.contents = lutTexture