如何从强度值重建灰度图像？

Question

通常需要从图像中获取像素数据或从像素数据中重建该图像。如何拍摄图像，将其转换为像素值数组，然后使用Swift 中的像素数组使用CoreGraphics 重建它？

这个问题的答案质量到处都是，所以我想要一个规范的答案。

Answer 1

以数组形式获取像素值

这个功能可以很容易地扩展到彩色图像。为简单起见，我使用的是灰度，但我已经评论了获取 RGB 的更改。

func pixelValuesFromImage(imageRef: CGImage?) -> (pixelValues: [UInt8]?, width: Int, height: Int)
{
    var width = 0
    var height = 0
    var pixelValues: [UInt8]?
    if let imageRef = imageRef {
        let totalBytes = imageRef.width * imageRef.height
        let colorSpace = CGColorSpaceCreateDeviceGray()
        
        pixelValues = [UInt8](repeating: 0, count: totalBytes)
        pixelValues?.withUnsafeMutableBytes({
            width = imageRef.width
            height = imageRef.height
            let contextRef = CGContext(data: $0.baseAddress, width: width, height: height, bitsPerComponent: 8, bytesPerRow: width, space: colorSpace, bitmapInfo: 0)
            let drawRect = CGRect(x: 0.0, y:0.0, width: CGFloat(width), height: CGFloat(height))
            contextRef?.draw(imageRef, in: drawRect)
        })
    }

    return (pixelValues, width, height)
}

从像素值获取图像

我将图像（在本例中为每像素 8 位灰度）重建为 CGImage。

func imageFromPixelValues(pixelValues: [UInt8]?, width: Int, height: Int) ->  CGImage?
{
    var imageRef: CGImage?
    if let pixelValues = pixelValues {
        let bitsPerComponent = 8
        let bytesPerPixel = 1
        let bitsPerPixel = bytesPerPixel * bitsPerComponent
        let bytesPerRow = bytesPerPixel * width
        let totalBytes = width * height
        let unusedCallback: CGDataProviderReleaseDataCallback = { optionalPointer, pointer, valueInt in }
        let providerRef = CGDataProvider(dataInfo: nil, data: pixelValues, size: totalBytes, releaseData: unusedCallback)

        let bitmapInfo: CGBitmapInfo = [CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue), CGBitmapInfo(rawValue: CGImageByteOrderInfo.orderDefault.rawValue)]
        imageRef = CGImage(width: width,
                           height: height,
                           bitsPerComponent: bitsPerComponent,
                           bitsPerPixel: bitsPerPixel,
                           bytesPerRow: bytesPerRow,
                           space: CGColorSpaceCreateDeviceGray(),
                           bitmapInfo: bitmapInfo,
                           provider: providerRef!,
                           decode: nil,
                           shouldInterpolate: false,
                           intent: .defaultIntent)
    }

    return imageRef
}
    

在 Playground 中演示代码

您需要将图像复制到 Playground 的 Resources 文件夹中，然后更改下面的文件名和扩展名以匹配。最后一行的结果是从 CGImage 构造的 UIImage。

import Foundation
import CoreGraphics
import UIKit
import PlaygroundSupport

let URL = playgroundSharedDataDirectory.appendingPathComponent("zebra.jpg")
print("URL \(URL)")

var image: UIImage? = nil
if FileManager().fileExists(atPath: URL.path) {
    do {
        try NSData(contentsOf: URL, options: .mappedIfSafe)
    } catch let error as NSError {
        print ("Error: \(error.localizedDescription)")
    }
    image = UIImage(contentsOfFile: URL.path)
} else {
    print("File not found")
}

let (intensityValues, width, height) = pixelValuesFromImage(imageRef: image?.cgImage)
let roundTrippedImage = imageFromPixelValues(pixelValues: intensityValues, width: width, height: height)
let zebra = UIImage(cgImage: roundTrippedImage!)

Answer 2

我无法让上面的 Cameron 代码正常工作，所以我想测试另一种方法。我找到了Vacawama's code，它依赖于 ARGB 像素。您可以使用该解决方案并通过简单地映射每个值将每个灰度值转换为 ARGB 值：

/// Assuming grayscale pixels contains floats in the range 0...1
let grayscalePixels: [Float] = ...
let pixels = grayscalePixels.map { 
  let intensity = UInt8(round($0 / Float(UInt8.max)))
  return PixelData(a: UInt8.max, r: intensity, g: intensity, b: intensity)
}
let image = UIImage(pixels: pixels, width: width, height: height)