swift metal save bgra8unorm纹理到png文件

如果你的应用程序在MacOS上使用金属,你需要做的第一件事就是确保你的纹理数据可以被CPU读取。如果内核正在写入的纹理 .private 存储模式,这意味着您需要在 .managed 模式。如果你的质地开始 管理的 存储,您可能需要创建一个blit命令编码器并调用 synchronize(resource:) 在纹理上确保其在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 或者通过向命令缓冲区添加一个完成处理程序),就可以复制数据并创建图像:

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
}

您会注意到在这个函数的中间有一个调用,调用了一个实用函数 swizzleBGRA8toRGBA8 . 此函数交换图像缓冲区中的字节,使它们按照coregraphics预期的rgba顺序排列。它使用Vimage(一定要 import Accelerate )看起来是这样的:

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)
    }
}