mastodon-ios/Mastodon/Vender/BlurHashDecode.swift

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import UIKit
extension UIImage {
public convenience init?(blurHash: String, size: CGSize, punch: Float = 1) {
guard blurHash.count >= 6 else { return nil }
let sizeFlag = String(blurHash[0]).decode83()
let numY = (sizeFlag / 9) + 1
let numX = (sizeFlag % 9) + 1
let quantisedMaximumValue = String(blurHash[1]).decode83()
let maximumValue = Float(quantisedMaximumValue + 1) / 166
guard blurHash.count == 4 + 2 * numX * numY else { return nil }
let colours: [(Float, Float, Float)] = (0 ..< numX * numY).map { i in
if i == 0 {
let value = String(blurHash[2 ..< 6]).decode83()
return decodeDC(value)
} else {
let value = String(blurHash[4 + i * 2 ..< 4 + i * 2 + 2]).decode83()
return decodeAC(value, maximumValue: maximumValue * punch)
}
}
let width = Int(size.width)
let height = Int(size.height)
let bytesPerRow = width * 3
guard let data = CFDataCreateMutable(kCFAllocatorDefault, bytesPerRow * height) else { return nil }
CFDataSetLength(data, bytesPerRow * height)
guard let pixels = CFDataGetMutableBytePtr(data) else { return nil }
for y in 0 ..< height {
for x in 0 ..< width {
var r: Float = 0
var g: Float = 0
var b: Float = 0
for j in 0 ..< numY {
for i in 0 ..< numX {
let basis = cos(Float.pi * Float(x) * Float(i) / Float(width)) * cos(Float.pi * Float(y) * Float(j) / Float(height))
let colour = colours[i + j * numX]
r += colour.0 * basis
g += colour.1 * basis
b += colour.2 * basis
}
}
let intR = UInt8(linearTosRGB(r))
let intG = UInt8(linearTosRGB(g))
let intB = UInt8(linearTosRGB(b))
pixels[3 * x + 0 + y * bytesPerRow] = intR
pixels[3 * x + 1 + y * bytesPerRow] = intG
pixels[3 * x + 2 + y * bytesPerRow] = intB
}
}
let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.none.rawValue)
guard let provider = CGDataProvider(data: data) else { return nil }
guard let cgImage = CGImage(width: width, height: height, bitsPerComponent: 8, bitsPerPixel: 24, bytesPerRow: bytesPerRow,
space: CGColorSpaceCreateDeviceRGB(), bitmapInfo: bitmapInfo, provider: provider, decode: nil, shouldInterpolate: true, intent: .defaultIntent) else { return nil }
self.init(cgImage: cgImage)
}
}
private func decodeDC(_ value: Int) -> (Float, Float, Float) {
let intR = value >> 16
let intG = (value >> 8) & 255
let intB = value & 255
return (sRGBToLinear(intR), sRGBToLinear(intG), sRGBToLinear(intB))
}
private func decodeAC(_ value: Int, maximumValue: Float) -> (Float, Float, Float) {
let quantR = value / (19 * 19)
let quantG = (value / 19) % 19
let quantB = value % 19
let rgb = (
signPow((Float(quantR) - 9) / 9, 2) * maximumValue,
signPow((Float(quantG) - 9) / 9, 2) * maximumValue,
signPow((Float(quantB) - 9) / 9, 2) * maximumValue
)
return rgb
}
private func signPow(_ value: Float, _ exp: Float) -> Float {
return copysign(pow(abs(value), exp), value)
}
private func linearTosRGB(_ value: Float) -> Int {
let v = max(0, min(1, value))
if v <= 0.0031308 { return Int(v * 12.92 * 255 + 0.5) }
else { return Int((1.055 * pow(v, 1 / 2.4) - 0.055) * 255 + 0.5) }
}
private func sRGBToLinear<Type: BinaryInteger>(_ value: Type) -> Float {
let v = Float(Int64(value)) / 255
if v <= 0.04045 { return v / 12.92 }
else { return pow((v + 0.055) / 1.055, 2.4) }
}
private let encodeCharacters: [String] = {
return "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz#$%*+,-.:;=?@[]^_{|}~".map { String($0) }
}()
private let decodeCharacters: [String: Int] = {
var dict: [String: Int] = [:]
for (index, character) in encodeCharacters.enumerated() {
dict[character] = index
}
return dict
}()
extension String {
func decode83() -> Int {
var value: Int = 0
for character in self {
if let digit = decodeCharacters[String(character)] {
value = value * 83 + digit
}
}
return value
}
}
private extension String {
subscript (offset: Int) -> Character {
return self[index(startIndex, offsetBy: offset)]
}
subscript (bounds: CountableClosedRange<Int>) -> Substring {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start...end]
}
subscript (bounds: CountableRange<Int>) -> Substring {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start..<end]
}
}