79 lines
2.4 KiB
Swift
79 lines
2.4 KiB
Swift
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import Foundation
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/// Camera log transfer functions. Constants from vendor whitepapers:
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/// - ARRI LogC4 (ALEXA 35), ARRI LogC4 spec 2022
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/// - RED Log3G10 v3, RED IPP2 whitepaper
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/// - Sony S-Log3, Sony technical summary
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public enum TransferFunction: String, Sendable, CaseIterable, Codable {
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case logC4
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case log3G10
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case sLog3
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case linear
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/// Linear scene value -> encoded log signal (0..1 nominal).
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public func encode(_ x: Float) -> Float {
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switch self {
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case .linear:
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return x
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case .logC4:
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let c = LogC4.self
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if x < c.t { return (x - c.t) / c.s }
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return (log2(c.a * x + 64) - 6) / 14 * c.b + c.c
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case .log3G10:
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let c = Log3G10.self
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let y = x + c.c
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if y < 0 { return y * c.g }
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return c.a * log10(y * c.b + 1)
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case .sLog3:
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let c = SLog3.self
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if x >= c.linBreak {
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return (420 + log10((x + 0.01) / 0.19) * 261.5) / 1023
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}
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return (x * (171.2102946929 - 95) / c.linBreak + 95) / 1023
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}
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}
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/// Encoded log signal -> linear scene value.
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public func decode(_ y: Float) -> Float {
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switch self {
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case .linear:
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return y
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case .logC4:
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let c = LogC4.self
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if y < 0 { return y * c.s + c.t }
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return (exp2(14 * (y - c.c) / c.b + 6) - 64) / c.a
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case .log3G10:
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let c = Log3G10.self
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if y < 0 { return y / c.g - c.c }
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return (pow(10, y / c.a) - 1) / c.b - c.c
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case .sLog3:
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let c = SLog3.self
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if y >= 171.2102946929 / 1023 {
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return pow(10, (y * 1023 - 420) / 261.5) * 0.19 - 0.01
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}
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return (y * 1023 - 95) * c.linBreak / (171.2102946929 - 95)
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}
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}
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// MARK: Constants
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private enum LogC4 {
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static let a: Float = (exp2(18) - 16) / 117.45
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static let b: Float = (1023 - 95) / 1023
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static let c: Float = 95.0 / 1023.0
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static let s: Float = (7 * log(2.0 as Float) * exp2(7 - 14 * c / b)) / (a * b)
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static let t: Float = (exp2(14 * (-c / b) + 6) - 64) / a
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}
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private enum Log3G10 {
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static let a: Float = 0.224282
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static let b: Float = 155.975327
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static let c: Float = 0.01
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static let g: Float = 15.1927
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}
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private enum SLog3 {
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static let linBreak: Float = 0.01125000
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}
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}
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