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，但是可以转换结果转换为 32 位浮点数 (Float)。本帖

32-bit to 16-bit Floating Point Conversion

包含大量关于 Half-precision floating-point format, and various conversion methods. The crucial hint however is in Ian Ollman's answer:

On OS X / iOS, you can use vImageConvert_PlanarFtoPlanar16F and vImageConvert_Planar16FtoPlanarF. See Accelerate.framework.

Ian 确实没有提供任何代码，所以这是一个可能的实现在 Swift:

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)
}

备注：

你需要import Accelerate.
请注意，您的代码分配了 totalBytes * bpp 个字节必要的 totalBytes.
模拟器不支持 kCIFormatRGBAh 像素格式（截至 Xcode 7），因此您必须在真实设备上测试代码。

更新： Swift 5.3（Xcode 12，目前处于测试阶段）引入了一种新的 Float16 类型，可用于 iOS14，见SE-0277 Float16关于Swift进化。

这简化了代码，因为不再需要转换为 Float。我还删除了从 iOS 12:

开始不推荐使用的 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: [=11=].baseAddress!, rowBytes: totalBytes,
                       bounds: filter.outputImage!.extent, format: CIFormat.RGBAh,
                       colorSpace: CGColorSpaceCreateDeviceRGB())
    }
    return rgbaFloat
}

如何在 Swift 中将字节转换为半浮点数？

How to convert bytes to half-floats in Swift?

ios

swift

core-image