如何将字节转换为Swift中的半浮点数？

Question

如何可以转换两个字节（UINT8）到半精度（16位）漂浮在夫特，如阅读CIAreaHistogram与kCIFormatRGBAh输出时，如在下面的例子中需要：

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

没有16位浮点型斯威夫特，但你可以转换 结果为32位浮点数（Float）。 这个线程

• 32-bit to 16-bit Floating Point Conversion

包含了很多关于 Half-precision floating-point format信息和各种转换方法。然而，关键的提示是在Ian Ollman's answer：

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

伊恩但是没有提供任何代码，所以这里是一个可能的实现 斯威夫特：

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totalBytes * bpp字节而不是必要的totalBytes 。
• 模拟器不支持kCIFormatRGBAh像素格式（从Xcode 7开始），因此您必须在真实设备上测试代码。