如何从灰度值重建灰度图像？

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。如何获取图像，将其转换为像素值数组，然后使用Swift中的像素数组使用CoreGraphics进行重建？

这个问题的答案的质量到处都是，所以我想要一个典型的答案。

Answer 1

获取像素值作为数组

这一功能可以很容易地扩展到彩色图像。为了简单起见，我使用的是灰度，但我已经对更改进行了注释，以获得RGB。

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

我重建一幅图像，在这种情况下，每像素8位灰度，返回到一个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
}

在游乐场中演示代码

您将需要将图像复制到Playground的Resources文件夹中，然后将下面的文件名和扩展名更改为匹配。最后一行的结果是从UIImage构造的CGImage。

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

我很难让Cameron上面的代码起作用，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将每个灰度值转换为ARGB值，只需对每个值进行映射：

/// 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)