如何在 Swift 中将字节转换为半浮点数？

Question

如何在 Swift 中将两个字节 (UInt8) 转换为半精度（16 位）浮点数，例如在使用 kCIFormatRGBAh 读取 CIAreaHistogram 的输出时需要，如下例所示：

func areaHistogram(image : UIImage) {

    let inputImage = CIImage(image: image)

    let totalBytes : Int = bpp * BINS //8 * 64 for example
    let bitmap : UnsafeMutablePointer<Void> = calloc(totalBytes, bpp)

    let filter = CIFilter(name: "CIAreaHistogram")!
    filter.setValue(inputImage, forKey: kCIInputImageKey)
    filter.setValue(CIVector(x: 0, y: 0, z: image.size.width, w: image.size.height), forKey: kCIInputExtentKey)
    filter.setValue(BINS, forKey: "inputCount") 
    filter.setValue(1, forKey: "inputScale")

    let myEAGLContext = EAGLContext(API: .OpenGLES2)
    let options = [kCIContextWorkingColorSpace : kCFNull]
    let context : CIContext = CIContext(EAGLContext: myEAGLContext, options: options)
    context.render(filter.outputImage!, toBitmap: bitmap, rowBytes: totalBytes, bounds: filter.outputImage!.extent, format: kCIFormatRGBAh, colorSpace: CGColorSpaceCreateDeviceRGB())

    let bytes = UnsafeBufferPointer<UInt8>(start: UnsafePointer<UInt8>(bitmap), count: bpp * BINS)

    //HOW TO CONVERT TWO CONSECUTIVE BYTES AS 16-BIT FLOATS?
    //THIS CODE DOES NOT WORK (I guess because Float in Swift is 32-bit):

    for var i=0; i < self.bpp * self.BINS; i+=self.bpp {
        let bitsR = UnsafePointer<Float._BitsType>(self.queryHist!)[i+0].bigEndian
        let R = Float( Float._fromBitPattern(bitsR) )

        let bitsG = UnsafePointer<Float._BitsType>(self.queryHist!)[i+2].bigEndian
        let G = Float( Float._fromBitPattern(bitsG) )

        let bitsB = UnsafePointer<Float._BitsType>(self.queryHist!)[i+4].bigEndian
        let B = Float( Float._fromBitPattern(bitsB) )

        print("R/G/B = \(R) \(G) \(B)")
    }

    free(bitmap)
}

Answer 1

Swift 中没有 16 位浮点类型，但您可以将 结果转换为 32 位浮点数 ( Float)。这个线程

• 32 位到 16 位浮点转换

包含大量关于 半精度浮点格式的信息，以及各种转换方法。然而，关键的提示是在Ian Ollman 的回答中：

在 OS X / iOS 上，您可以使用vImageConvert_PlanarFtoPlanar16F和 vImageConvert_Planar16FtoPlanarF。请参阅 Accelerate.framework。

然而，Ian 没有提供任何代码，所以这里是一个可能的 Swift 实现：

func areaHistogram(image : UIImage) {
    
    let inputImage = CIImage(image: image)
    
    let totalBytes : Int = bpp * BINS //8 * 64 for example
    let bitmap = calloc(1, totalBytes)
    
    let filter = CIFilter(name: "CIAreaHistogram")!
    filter.setValue(inputImage, forKey: kCIInputImageKey)
    filter.setValue(CIVector(x: 0, y: 0, z: image.size.width, w: image.size.height), forKey: kCIInputExtentKey)
    filter.setValue(BINS, forKey: "inputCount") 
    filter.setValue(1, forKey: "inputScale")
    
    let myEAGLContext = EAGLContext(API: .OpenGLES2)
    let options = [kCIContextWorkingColorSpace : kCFNull]
    let context : CIContext = CIContext(EAGLContext: myEAGLContext, options: options)
    context.render(filter.outputImage!, toBitmap: bitmap, rowBytes: totalBytes, bounds: filter.outputImage!.extent, format: kCIFormatRGBAh, colorSpace: CGColorSpaceCreateDeviceRGB())

    // *** CONVERSION FROM 16-bit TO 32-bit FLOAT ARRAY STARTS HERE ***
    
    let comps = 4 // Number of components (RGBA)
    
    // Array for the RGBA values of the histogram: 
    var rgbaFloat = [Float](count: comps * BINS, repeatedValue: 0)
    
    // Source and image buffer structure for vImage conversion function:
    var srcBuffer = vImage_Buffer(data: bitmap, height: 1, width: UInt(comps * BINS), rowBytes: bpp * BINS)
    var dstBuffer = vImage_Buffer(data: &rgbaFloat, height: 1, width: UInt(comps * BINS), rowBytes: comps * sizeof(Float) * BINS)
    
    // Half-precision float to Float conversion of entire buffer:
    if vImageConvert_Planar16FtoPlanarF(&srcBuffer, &dstBuffer, 0) == kvImageNoError {
        for bin in 0 ..< BINS {
            let R = rgbaFloat[comps * bin + 0]
            let G = rgbaFloat[comps * bin + 1]
            let B = rgbaFloat[comps * bin + 2]
            print("R/G/B = \(R) \(G) \(B)")
        }
    }
    
    free(bitmap)
}

评论：

• 你需要import Accelerate。
• 请注意，您的代码分配totalBytes * bpp字节而不是必要的totalBytes.
• 的kCIFormatRGBAh像素格式不支持的仿真器（如Xcode的7），因此必须测试真实设备上的代码。

更新： Swift 5.3（Xcode 12，目前处于测试阶段）引入了一种Float16在 iOS 14 中可用的新类型，请参阅Swift Evolution 上的SE-0277 Float16。

这简化了代码，因为Float不再需要转换为。我还删除了自 iOS 12 起已弃用的 OpenGL 函数的使用：

func areaHistogram(image: UIImage, bins: Int) -> [Float16] {

    let comps = 4 // Number of components (RGBA)

    let inputImage = CIImage(image: image)
    var rgbaFloat = [Float16](repeating: 0, count: comps * bins)
    let totalBytes = MemoryLayout<Float16>.size * comps * bins

    let filter = CIFilter(name: "CIAreaHistogram")!
    filter.setValue(inputImage, forKey: kCIInputImageKey)
    filter.setValue(CIVector(x: 0, y: 0, z: image.size.width, w: image.size.height), forKey: kCIInputExtentKey)
    filter.setValue(bins, forKey: "inputCount")
    filter.setValue(1, forKey: "inputScale")

    let options: [CIContextOption : Any] = [.workingColorSpace : NSNull()]
    let context = CIContext(options: options)
    
    rgbaFloat.withUnsafeMutableBytes {
        context.render(filter.outputImage!, toBitmap: $0.baseAddress!, rowBytes: totalBytes,
                       bounds: filter.outputImage!.extent, format: CIFormat.RGBAh,
                       colorSpace: CGColorSpaceCreateDeviceRGB())
    }
    return rgbaFloat
}