如何将字节转换为Swift中的半浮点数？
如何在Swift中将两个字节（UInt8）转换为半精度（16位）Float，例如在读取具有kCIFormatRGBAh的CIAreaHistogram的输出时所需的字节，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) } 在Swift中没有16位浮点types，但可以将结果转换为32位浮点数（ Float ）。 这个线程
- 32位到16位浮点转换
包含大量有关半精度浮点格式的信息以及各种转换方法。 然而，至关重要的是Ian Ollman的回答 ：
在OS X / iOS上，可以使用
vImageConvert_PlanarFtoPlanar16F和vImageConvert_Planar16FtoPlanarF。 请参阅Accelerate.framework。
伊恩没有提供代码，所以这是一个可能的实现在Swift中：
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
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testing代码。