RenderTexture.GetNativeTexturePtr()からネイティブ側でpngに変換+保存

NativeImageEncoder.mm

#import <Metal/Metal.h>
#import <UnityFramework/UnityFramework-Swift.h>

#ifdef __cplusplus
extern "C" {
#endif

// P/Invoke code.
// [DllImport("__Internal", EntryPoint = "encodeToPNG2")]
// static extern string EncodeToPNG(IntPtr nativeTexturePtr, string fileName);
// > encodeMethod(targetRenderTexture.GetNativeTexturePtr(), fileName);

const char* encodeToPNG1(unsigned char* nativeTexturePtr, const char* fineName) {
    NSString* str = [NSString stringWithCString:fineName encoding:NSUTF8StringEncoding];
    id <MTLTexture> texture = (__bridge id <MTLTexture>) (void*) nativeTexturePtr;
    NSString* path = [NativeImageEncoder convert1WithTexture:texture fileName:str];

    const char* strPtr = (char*) [path UTF8String];
    char* result = (char*) malloc(strlen(strPtr) + 1);
    strcpy(result, strPtr);
    result[strlen(strPtr)] = '\0';
    return result;
}

const char* encodeToPNG2(unsigned char* nativeTexturePtr, const char* fineName) {
    NSString* str = [NSString stringWithCString:fineName encoding:NSUTF8StringEncoding];
    id <MTLTexture> texture = (__bridge id <MTLTexture>) (void*) nativeTexturePtr;
    NSString* path = [NativeImageEncoder convert2WithTexture:texture fileName:str];

    const char* strPtr = (char*) [path UTF8String];
    char* result = (char*) malloc(strlen(strPtr) + 1);
    strcpy(result, strPtr);
    result[strlen(strPtr)] = '\0';
    return result;
}

#ifdef __cplusplus
}
#endif

NativeImageEncoder.swift

import UIKit
import CoreImage
import Accelerate
import MobileCoreServices.UTType

public class NativeImageEncoder: NSObject {

    // NOTE: Unityで言う`Application.temporaryCachePath`と同じ位置に保存
    static let cachePath = NSSearchPathForDirectoriesInDomains(
        .cachesDirectory, .userDomainMask, true)[0]

    static let mtlDevice = MTLCreateSystemDefaultDevice()!
    static let ciContext = CIContext(mtlDevice: mtlDevice)


    // MARK:- public methods

    @objc public static func convert1(texture: MTLTexture, fileName: String) -> String {
        return convertFromCIContext(texture: texture, fileName: fileName)
    }

    @objc public static func convert2(texture: MTLTexture, fileName: String) -> String {
        return convertFromCGImage(texture: texture, fileName: fileName)
    }


    // MARK:- private methods

    /// MTLTexture -> CIImage - UIImageベースの変換
    /// NOTE: UIImage().pngData()!を使うとiPhone 12 ProMaxで0.2sec近く掛かってゴリラ
    static func convertFromUIImage(texture: MTLTexture, fileName: String) -> String {

        // NOTE: Unityからは`.bgra8Unorm`で来る想定が有る
        precondition(texture.pixelFormat == .bgra8Unorm)

        guard let mtlTexture = texture.makeTextureView(pixelFormat: .bgra8Unorm_srgb),
              let ciImage = CIImage(mtlTexture: mtlTexture) else {
            fatalError("failed")
        }

        // MTLTextureをCIImage -> UIImageに変換しつつ、pngに変換するAPIを叩く
        // NOTE: こいつが0.2sec近く掛かってクッソ遅いのでゴリラ
        let uiImage = UIImage(ciImage: ciImage)
        let data = uiImage.pngData()!

        // pngの保存
        let fullPath = "\(cachePath)/\(fileName).png"
        let url = URL(fileURLWithPath: fullPath)
        do {
            try data.write(to: url)
        } catch {
            fatalError("\(error)")
        }

        return fullPath;
    }

    /// バックエンドにMetalを利用したCIContextベースの変換
    /// NOTE: 似た方式でJPEGへの変換も対応可能 (`ciContext.jpegRepresentation`がある)
    static func convertFromCIContext(texture: MTLTexture, fileName: String) -> String {

        // NOTE: Unityからは`.bgra8Unorm`で来る想定が有る
        precondition(texture.pixelFormat == .bgra8Unorm)

        // Unityから渡ってきたMTLTextureをsRGBに変換しつつ、
        // CoreImageベースでMTLTextureをpngに圧縮
        guard let mtlTexture = texture.makeTextureView(pixelFormat: .bgra8Unorm_srgb),
              let ciImage = CIImage(mtlTexture: mtlTexture),
              let data = ciContext.pngRepresentation(
                  of: ciImage,
                  format: .RGBA8,
                  colorSpace: CGColorSpaceCreateDeviceRGB()) else {
            fatalError("failed")
        }

        // pngの保存
        let fullPath = "\(cachePath)/\(fileName).png"
        let url = URL(fileURLWithPath: fullPath)
        do {
            try data.write(to: url)
        } catch {
            fatalError("\(error)")
        }
        return fullPath;
    }

    /// MetalでMTLTextureを変換しつつCGImageに変換して保存
    static func convertFromCGImage(texture: MTLTexture, fileName: String) -> String {

        let newTexture = texture.makeTextureView(pixelFormat: .bgra8Unorm_srgb)!
        let commandQueue = mtlDevice.makeCommandQueue()!
        let commandBuffer = commandQueue.makeCommandBuffer()!
        let blitEncoder = commandBuffer.makeBlitCommandEncoder()!

        blitEncoder.copy(
            from: texture, sourceSlice: 0, sourceLevel: 0,
            sourceOrigin: MTLOrigin(x: 0, y: 0, z: 0),
            sourceSize: MTLSizeMake(texture.width, texture.height, texture.depth),
            to: newTexture, destinationSlice: 0, destinationLevel: 0,
            destinationOrigin: MTLOrigin(x: 0, y: 0, z: 0))

        blitEncoder.endEncoding()
        commandBuffer.commit()
        commandBuffer.waitUntilCompleted()

        let ciImage = CIImage(mtlTexture: newTexture)!
        let cgImage = ciContext.createCGImage(ciImage, from: ciImage.extent)!

        // pngの保存
        // NOTE: ここの処理が重め
        let fullPath = "\(cachePath)/\(fileName).png"
        let url = URL(fileURLWithPath: fullPath)
        if let imageDestination = CGImageDestinationCreateWithURL(
            url as CFURL, kUTTypePNG, 1, nil) {
            CGImageDestinationAddImage(imageDestination, cgImage, nil)
            CGImageDestinationFinalize(imageDestination)
        }

        return fullPath;
    }

    /// MetalでMTLTextureを変換しつつCGImageに変換して保存 (2)
    @objc public static func convertFromCGImage2(texture: MTLTexture, fileName: String) -> String {

        // MTLTexture間でsRGB変換
        let newTexture = texture.makeTextureView(pixelFormat: .bgra8Unorm_srgb)!
        let commandQueue = mtlDevice.makeCommandQueue()!
        let commandBuffer = commandQueue.makeCommandBuffer()!
        let blitEncoder = commandBuffer.makeBlitCommandEncoder()!

        blitEncoder.copy(
            from: texture, sourceSlice: 0, sourceLevel: 0,
            sourceOrigin: MTLOrigin(x: 0, y: 0, z: 0),
            sourceSize: MTLSizeMake(texture.width, texture.height, texture.depth),
            to: newTexture, destinationSlice: 0, destinationLevel: 0,
            destinationOrigin: MTLOrigin(x: 0, y: 0, z: 0))

        blitEncoder.endEncoding()
        commandBuffer.commit()
        commandBuffer.waitUntilCompleted()

        // MTLTextureの読み取り用ののバッファを確保
        // NOTE: 地味に2番目に時間掛かってる
        let width = newTexture.width
        let height = newTexture.height
        let pixelByteCount = 4 * MemoryLayout<UInt8>.size
        let imageBytesPerRow = width * pixelByteCount
        let imageByteCount = imageBytesPerRow * height
        let imageBytes = UnsafeMutableRawPointer.allocate(
            byteCount: imageByteCount, alignment: pixelByteCount)
        defer {
            imageBytes.deallocate()
        }
        newTexture.getBytes(
            imageBytes,
            bytesPerRow: imageBytesPerRow,
            from: MTLRegionMake2D(0, 0, width, height),
            mipmapLevel: 0)

        // フォーマットの変換など
        convertBuffers(imageBytes, width: width, height: height)

        // CGImageの作成
        guard let bitmapContext = CGContext(
            data: nil,
            width: width,
            height: height,
            bitsPerComponent: 8,
            bytesPerRow: imageBytesPerRow,
            space: CGColorSpaceCreateDeviceRGB(),
            bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue) else {
            fatalError("非対応")
        }
        bitmapContext.data?.copyMemory(from: imageBytes, byteCount: imageByteCount)
        let cgImage = bitmapContext.makeImage()!

        // pngの保存
        // NOTE: ここの処理が重め
        let fullPath = "\(cachePath)/\(fileName).png"
        let url = URL(fileURLWithPath: fullPath)
        if let imageDestination = CGImageDestinationCreateWithURL(
            url as CFURL, kUTTypePNG, 1, nil) {
            CGImageDestinationAddImage(imageDestination, cgImage, nil)
            CGImageDestinationFinalize(imageDestination)
        }

        return fullPath;
    }

    // ref:
    // - https://stackoverflow.com/questions/52920497/swift-metal-save-bgra8unorm-texture-to-png-file
    // - https://qiita.com/codelynx/items/e7d7da8621901467b64a
    static func convertBuffers(_ bytes: UnsafeMutableRawPointer, width: Int, height: Int) {

        // use Accelerate framework to convert from BGRA to RGBA
        var sourceBuffer = vImage_Buffer(data: bytes,
            height: vImagePixelCount(height),
            width: vImagePixelCount(width),
            rowBytes: width * 4)
        var destBuffer = vImage_Buffer(data: bytes,
            height: vImagePixelCount(height),
            width: vImagePixelCount(width),
            rowBytes: width * 4)
        var swizzleMask: [UInt8] = [2, 1, 0, 3] // BGRA -> RGBA
        vImagePermuteChannels_ARGB8888(&sourceBuffer, &destBuffer, &swizzleMask, vImage_Flags(kvImageNoFlags))

        // flipping image vertically
        var flippedBuffer = vImage_Buffer(data: bytes,
            height: vImagePixelCount(height),
            width: vImagePixelCount(width),
            rowBytes: width * 4)
        vImageVerticalReflect_ARGB8888(&destBuffer, &flippedBuffer, 0)
    }
}