logC4.py

from math import log
import vapoursynth as vs
core = vs.core


a = (pow(2.0, 18.0) - 16.0) / 117.45
b = (1023.0 - 95.0) / 1023.0
c = 95.0 / 1023.0
s = (7 * log(2) * pow(2.0, 7 - 14 * c / b)) / (a * b)
t = (pow(2.0, 14.0 * (-c / b) + 6.0) - 64.0) / a

# LogC4 Curve Encoding Function
def ex_relativeSceneLinearToNormalizedLogC4(src: vs.VideoNode) -> vs.VideoNode:
    return src.std.Expr(f'x {t} < x {t} - {s} / {a} x * 64 + log 2 log / 6 - 14 / {b} * {c} + ?')

# LogC4 Curve Decoding Function
def ex_normalizedLogC4ToRelativeSceneLinear(src: vs.VideoNode) -> vs.VideoNode:
    return src.std.Expr(f'x 0 < x {s} * {t} + 2 x {c} - 14 * {b} / 6 + pow 64 - {a} / ?')

def logC4ToACES(src: vs.VideoNode) -> vs.VideoNode:
    src_lin = ex_normalizedLogC4ToRelativeSceneLinear(src)
    r_lin, g_lin, b_lin = src_lin.std.SplitPlanes()

    # Matrix AWG4 + D65 --CAT02--> ACES AP0 + ACES RGB White Point
    rOut = core.std.Expr([r_lin, g_lin, b_lin], 'x 0.750957362824734131 * y 0.144422786709757084 * z 0.104619850465508965 * + +')
    gOut = core.std.Expr([r_lin, g_lin, b_lin], 'x 0.000821837079380207 * y 1.007397584885003194 * z -0.008219421964383583 * + +')
    bOut = core.std.Expr([r_lin, g_lin, b_lin], 'x -0.000499952143533471 * y -0.000854177231436971 * z 1.001354129374970370 * + +')
    return core.std.ShufflePlanes([rOut, gOut, bOut], [0, 0, 0], vs.RGB)