Question

我有一个输出纹理的内核，它是有效的MTLTexture对象。我想将其保存到项目工作目录中的png文件中。应该怎么做？

纹理格式为.bgra8Unorm，目标输出格式为PNG。纹理存储在MTLTexture对象中。

编辑：我在macOS XCode上。

Answer 1

如果您的应用程序在macOS上使用Metal，则首先需要确保CPU可以读取纹理数据。如果内核写入的纹理处于.private存储模式，则意味着您需要以.managed模式将纹理从blit（复制）到另一个纹理。如果纹理从.managed存储开始，则可能需要创建blit命令编码器并在纹理上调用synchronize(resource:)，以确保其在GPU上的内容反映在CPU上：

if let blitEncoder = commandBuffer.makeBlitCommandEncoder() {
    blitEncoder.synchronize(resource: outputTexture)
    blitEncoder.endEncoding()
}

一旦命令缓冲区完成（您可以通过调用waitUntilCompleted或在命令缓冲区中添加完成处理程序来等待），就可以复制数据并创建映像了：

func makeImage(for texture: MTLTexture) -> CGImage? {
    assert(texture.pixelFormat == .bgra8Unorm)

    let width = texture.width
    let height = texture.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()
    }

    texture.getBytes(imageBytes,
                     bytesPerRow: imageBytesPerRow,
                     from: MTLRegionMake2D(0, 0, width, height),
                     mipmapLevel: 0)

    swizzleBGRA8toRGBA8(imageBytes, width: width, height: height)

    guard let colorSpace = CGColorSpace(name: CGColorSpace.linearSRGB) else { return nil }
    let bitmapInfo = CGImageAlphaInfo.premultipliedLast.rawValue
    guard let bitmapContext = CGContext(data: nil,
                                        width: width,
                                        height: height,
                                        bitsPerComponent: 8,
                                        bytesPerRow: imageBytesPerRow,
                                        space: colorSpace,
                                        bitmapInfo: bitmapInfo) else { return nil }
    bitmapContext.data?.copyMemory(from: imageBytes, byteCount: imageByteCount)
    let image = bitmapContext.makeImage()
    return image
}

您会注意到在此函数中间对名为swizzleBGRA8toRGBA8的实用程序函数的调用。此函数交换图像缓冲区中的字节，以使它们处于CoreGraphics期望的RGBA顺序。它使用vImage（确保为import Accelerate），看起来像这样：

func swizzleBGRA8toRGBA8(_ bytes: UnsafeMutableRawPointer, width: Int, height: Int) {
    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))
}

现在，我们可以编写一个函数，使我们能够将纹理写入指定的URL：

func writeTexture(_ texture: MTLTexture, url: URL) {
    guard let image = makeImage(for: texture) else { return }

    if let imageDestination = CGImageDestinationCreateWithURL(url as CFURL, kUTTypePNG, 1, nil) {
        CGImageDestinationAddImage(imageDestination, image, nil)
        CGImageDestinationFinalize(imageDestination)
    }
}

Swift Metal将bgra8Unorm纹理保存到PNG文件

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