Explore gamma correction related properties on iOS

ExploreSRGB.swift

// test in playground targeting iOS
import UIKit
import QuartzCore

let sRGBSpace = CGColorSpace(name: CGColorSpace.sRGB)!
let linearSpace = CGColorSpace(name: CGColorSpace.linearSRGB)!

func dumpImageFirstPixel(_ image: UIImage) {
  guard let cgImage = image.cgImage,
    let dataBuffer = cgImage.dataProvider?.data as Data?,
    let colorspaceName = cgImage.colorSpace?.name else {
    return
  }
  print("\(Array(dataBuffer.prefix(through: 3))) in colorspace \(colorspaceName)")
}

// test 1
// blend red and green by default
let imageSize = CGSize(width: 20, height: 20)
let format = UIGraphicsImageRendererFormat()
format.scale = 1
let renderer = UIGraphicsImageRenderer(size: imageSize, format: format)
let image1 = renderer.image { context in
  let bounds = renderer.format.bounds
  UIColor.red.setFill()
  context.fill(bounds)
  
  UIColor.green.withAlphaComponent(0.5).setFill()
  context.fill(bounds, blendMode: .normal) // by default .copy may be used
}
dumpImageFirstPixel(image1)
// outputs [127, 128, 0, 255], but it's relatively dark

// test 2
// force input a non-linear color, the system will perform transformation for us
var buffer: [CGFloat] = [0.5, 0.5, 0, 1]
let linearBlendedColor = CGColor(colorSpace: linearSpace, components: &buffer)!
let linearUIColor = UIColor(cgColor: linearBlendedColor)
let image2 = renderer.image { context in
  let bounds = renderer.format.bounds
  linearUIColor.setFill()
  context.fill(bounds)
}
dumpImageFirstPixel(image2)
// outputs [188, 188, 0, 255]

// test 3
// use a linear color space directly
let alphaInfo: CGImageAlphaInfo = .premultipliedLast
let linearContext = CGContext(data: nil,
  width: Int(imageSize.width),
  height: Int(imageSize.height),
  bitsPerComponent: 8,
  bytesPerRow: 0,
  space: linearSpace,
  bitmapInfo: alphaInfo.rawValue)!
// we can use UIColor.red/green because we know the value 255 will not be transformed,
// other colors should be created using CGColor
linearContext.setFillColor(UIColor.red.cgColor)
linearContext.fill(CGRect(origin: .zero, size: imageSize))
linearContext.setFillColor(UIColor.green.withAlphaComponent(0.5).cgColor)
linearContext.setBlendMode(.normal)
linearContext.fill(CGRect(origin: .zero, size: imageSize))

let linearSpaceImage = linearContext.makeImage()!
let image3 = UIImage(cgImage: linearSpaceImage)
dumpImageFirstPixel(image3)
// outputs [127, 128, 0, 255]

// in 3D rendering, if we wan't to achieve accurate physically based effects, we should display colors in image2 & 3 on monitor, not image1