如何在Swift中将字节转换为半浮点数？ | 如何在Swift中将字节转换为半浮点数

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问题描述

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

How can I convert two bytes (UInt8) to a half-precision (16-bit) Float in Swift, such as needed when reading the output of CIAreaHistogram with the kCIFormatRGBAh, as in the following example:

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)
}

推荐答案

Swift中没有16位浮点类型，但是您可以将结果转换
转换为32位浮点数（ Float ）。
此线程

There is no 16-bit floating point type in Swift, but you can convertthe results to 32-bit floating point numbers (Float).This thread

32-bit to 16-bit Floating Point Conversion

包含有关$ b的很多信息$ b ，以及各种转换方法。但是，关键提示在中：

contains a lot of information about theHalf-precision floating-point format, and various conversion methods. The crucial hint however is in Ian Ollman's answer:

Ian没有提供任何代码，因此这是在Swift中可能的实现
：

Ian did provide no code however, so here is a possible implementationin 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：

您需要导入加速。

请注意，您的代码分配了 totalBytes * bpp 字节，而不是必需的 totalBytes 中的
。

kCIFormatRGBAh Simulator（从Xcode 7开始）不支持像素格式，因此您必须在真实设备上测试代码。

You need to import Accelerate.
Note that your code allocates totalBytes * bpp bytes insteadof the necessary totalBytes.
The kCIFormatRGBAh pixel format is not supported on the Simulator (as of Xcode 7), so you have to test the code on a real device.

更新：：Swift 5.3（当前处于beta版的Xcode 12）引入了新的 Float16 类型，该类型可在iOS 14中使用参见 o n Swift快速开发。

Update: Swift 5.3 (Xcode 12, currently in beta) introduces a new Float16 type which is available in iOS 14, see SE-0277 Float16 on Swift Evolution.

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：

This simplifies the code because a conversion to Float is no longer necessary. I have also removed the use of OpenGL functions which are deprecated as of iOS 12:

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
}

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