rainbow-dragon/Tests/ForgeColorTests/TransferFunctionTests.swift

import XCTest
@testable import ForgeColor

final class TransferFunctionTests: XCTestCase {

    let samples: [Float] = [0.0, 0.001, 0.01, 0.045, 0.18, 0.5, 1.0, 4.0, 12.0]

    // MARK: Round trips lin -> log -> lin

    func testLogC4RoundTrip() {
        for x in samples {
            let y = TransferFunction.logC4.encode(x)
            let back = TransferFunction.logC4.decode(y)
            XCTAssertEqual(back, x, accuracy: max(1e-4, x * 1e-4), "LogC4 round trip at \(x)")
        }
    }

    func testLog3G10RoundTrip() {
        for x in samples {
            let y = TransferFunction.log3G10.encode(x)
            let back = TransferFunction.log3G10.decode(y)
            XCTAssertEqual(back, x, accuracy: max(1e-4, x * 1e-4), "Log3G10 round trip at \(x)")
        }
    }

    func testSLog3RoundTrip() {
        for x in samples {
            let y = TransferFunction.sLog3.encode(x)
            let back = TransferFunction.sLog3.decode(y)
            XCTAssertEqual(back, x, accuracy: max(1e-4, x * 1e-4), "S-Log3 round trip at \(x)")
        }
    }

    // MARK: Published anchor values

    // ARRI LogC4: 18% gray encodes to ~0.2783 (27.8% signal).
    func testLogC4MidGrayAnchor() {
        XCTAssertEqual(TransferFunction.logC4.encode(0.18), 0.2783, accuracy: 0.002)
    }

    // LogC4 encodes 0.0 to a small positive-ish value near 0.0929 region minus...
    // Zero linear must decode back to zero.
    func testLogC4ZeroStable() {
        let y = TransferFunction.logC4.encode(0)
        XCTAssertEqual(TransferFunction.logC4.decode(y), 0, accuracy: 1e-6)
    }

    // RED Log3G10: designed so 18% gray -> exactly 1/3.
    func testLog3G10MidGrayAnchor() {
        XCTAssertEqual(TransferFunction.log3G10.encode(0.18), 1.0 / 3.0, accuracy: 0.001)
    }

    // Sony S-Log3: 18% gray -> 420/1023.
    func testSLog3MidGrayAnchor() {
        XCTAssertEqual(TransferFunction.sLog3.encode(0.18), 420.0 / 1023.0, accuracy: 0.001)
    }

    // Monotonic increasing over sample range.
    func testMonotonic() {
        for tf in [TransferFunction.logC4, .log3G10, .sLog3] {
            var prev = -Float.infinity
            for x in samples {
                let y = tf.encode(x)
                XCTAssertGreaterThan(y, prev, "\(tf) not monotonic at \(x)")
                prev = y
            }
        }
    }

    // MARK: Gamut matrices

    // Camera gamut -> Rec709 matrices must preserve white: (1,1,1) -> (1,1,1).
    func testGamutMatricesPreserveWhite() {
        for m in [GamutMatrix.awg4ToRec709, .rwgToRec709, .sGamut3CineToRec709] {
            let w = m.apply(SIMD3<Float>(1, 1, 1))
            XCTAssertEqual(w.x, 1, accuracy: 1e-3)
            XCTAssertEqual(w.y, 1, accuracy: 1e-3)
            XCTAssertEqual(w.z, 1, accuracy: 1e-3)
        }
    }

    // Inverse matrix round-trips a color.
    func testGamutMatrixInverseRoundTrip() {
        let px = SIMD3<Float>(0.4, 0.25, 0.7)
        for m in [GamutMatrix.awg4ToRec709, .rwgToRec709, .sGamut3CineToRec709] {
            let back = m.inverted().apply(m.apply(px))
            XCTAssertEqual(back.x, px.x, accuracy: 1e-4)
            XCTAssertEqual(back.y, px.y, accuracy: 1e-4)
            XCTAssertEqual(back.z, px.z, accuracy: 1e-4)
        }
    }

    // ColorSpace bundles: full lin conversion pipeline camera-log -> linear Rec709.
    func testColorSpaceDecodeToLinearRec709() {
        // LogC4-encoded 18% gray decodes through AWG4->Rec709 to ~0.18 gray (neutral axis unchanged by matrix).
        let cs = ColorSpace.arriLogC4AWG4
        let gray = cs.toLinearRec709(SIMD3<Float>(repeating: cs.transfer.encode(0.18)))
        XCTAssertEqual(gray.x, 0.18, accuracy: 0.002)
        XCTAssertEqual(gray.y, 0.18, accuracy: 0.002)
        XCTAssertEqual(gray.z, 0.18, accuracy: 0.002)
    }
}