mastodon-ios/Mastodon/Vender/BlurHashEncode.swift

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4.9 KiB
Swift
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import UIKit
extension UIImage {
public func blurHash(numberOfComponents components: (Int, Int)) -> String? {
let pixelWidth = Int(round(size.width * scale))
let pixelHeight = Int(round(size.height * scale))
let context = CGContext(
data: nil,
width: pixelWidth,
height: pixelHeight,
bitsPerComponent: 8,
bytesPerRow: pixelWidth * 4,
space: CGColorSpace(name: CGColorSpace.sRGB)!,
bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue
)!
context.scaleBy(x: scale, y: -scale)
context.translateBy(x: 0, y: -size.height)
UIGraphicsPushContext(context)
draw(at: .zero)
UIGraphicsPopContext()
guard let cgImage = context.makeImage(),
let dataProvider = cgImage.dataProvider,
let data = dataProvider.data,
let pixels = CFDataGetBytePtr(data) else {
assertionFailure("Unexpected error!")
return nil
}
let width = cgImage.width
let height = cgImage.height
let bytesPerRow = cgImage.bytesPerRow
var factors: [(Float, Float, Float)] = []
for y in 0 ..< components.1 {
for x in 0 ..< components.0 {
let normalisation: Float = (x == 0 && y == 0) ? 1 : 2
let factor = multiplyBasisFunction(pixels: pixels, width: width, height: height, bytesPerRow: bytesPerRow, bytesPerPixel: cgImage.bitsPerPixel / 8, pixelOffset: 0) {
normalisation * cos(Float.pi * Float(x) * $0 / Float(width)) as Float * cos(Float.pi * Float(y) * $1 / Float(height)) as Float
}
factors.append(factor)
}
}
let dc = factors.first!
let ac = factors.dropFirst()
var hash = ""
let sizeFlag = (components.0 - 1) + (components.1 - 1) * 9
hash += sizeFlag.encode83(length: 1)
let maximumValue: Float
if ac.count > 0 {
let actualMaximumValue = ac.map({ max(abs($0.0), abs($0.1), abs($0.2)) }).max()!
let quantisedMaximumValue = Int(max(0, min(82, floor(actualMaximumValue * 166 - 0.5))))
maximumValue = Float(quantisedMaximumValue + 1) / 166
hash += quantisedMaximumValue.encode83(length: 1)
} else {
maximumValue = 1
hash += 0.encode83(length: 1)
}
hash += encodeDC(dc).encode83(length: 4)
for factor in ac {
hash += encodeAC(factor, maximumValue: maximumValue).encode83(length: 2)
}
return hash
}
private func multiplyBasisFunction(pixels: UnsafePointer<UInt8>, width: Int, height: Int, bytesPerRow: Int, bytesPerPixel: Int, pixelOffset: Int, basisFunction: (Float, Float) -> Float) -> (Float, Float, Float) {
var r: Float = 0
var g: Float = 0
var b: Float = 0
let buffer = UnsafeBufferPointer(start: pixels, count: height * bytesPerRow)
for x in 0 ..< width {
for y in 0 ..< height {
let basis = basisFunction(Float(x), Float(y))
r += basis * sRGBToLinear(buffer[bytesPerPixel * x + pixelOffset + 0 + y * bytesPerRow])
g += basis * sRGBToLinear(buffer[bytesPerPixel * x + pixelOffset + 1 + y * bytesPerRow])
b += basis * sRGBToLinear(buffer[bytesPerPixel * x + pixelOffset + 2 + y * bytesPerRow])
}
}
let scale = 1 / Float(width * height)
return (r * scale, g * scale, b * scale)
}
}
private func encodeDC(_ value: (Float, Float, Float)) -> Int {
let roundedR = linearTosRGB(value.0)
let roundedG = linearTosRGB(value.1)
let roundedB = linearTosRGB(value.2)
return (roundedR << 16) + (roundedG << 8) + roundedB
}
private func encodeAC(_ value: (Float, Float, Float), maximumValue: Float) -> Int {
let quantR = Int(max(0, min(18, floor(signPow(value.0 / maximumValue, 0.5) * 9 + 9.5))))
let quantG = Int(max(0, min(18, floor(signPow(value.1 / maximumValue, 0.5) * 9 + 9.5))))
let quantB = Int(max(0, min(18, floor(signPow(value.2 / maximumValue, 0.5) * 9 + 9.5))))
return quantR * 19 * 19 + quantG * 19 + quantB
}
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) }
}()
extension BinaryInteger {
func encode83(length: Int) -> String {
var result = ""
for i in 1 ... length {
let digit = (Int(self) / pow(83, length - i)) % 83
result += encodeCharacters[Int(digit)]
}
return result
}
}
private func pow(_ base: Int, _ exponent: Int) -> Int {
return (0 ..< exponent).reduce(1) { value, _ in value * base }
}