OETF (Opto-Electronic Transfer Function)

The OETF (Opto-Electronic Transfer Function) is the encoding side of a transfer function chain: the mathematical curve that converts scene luminance (optical input from a camera sensor or rendered linear light) into an encoded signal value (electrical signal stored in the file). It is the counterpart to the EOTF (the display side, converting signal back to luminance). For Rec.709 broadcast, the OETF is approximately a 1/2.0 power function — the camera encodes brightness raised to roughly the 0.5 power — paired with a 2.4-power EOTF on the display side. This asymmetry exists by historical accident and dim-viewing-room compensation, not as a pure mathematical inversion. For HDR formats, OETFs are more carefully defined: PQ does not technically have an OETF (it is defined as an EOTF only, since it is absolute-luminance based), while HLG defines both an OETF and an EOTF that are mutually consistent. Camera log curves (S-Log3, Log C, V-Log) are also OETFs — they encode wide-latitude scene luminance into limited code value space, with the corresponding decoding curve being the camera-specific reverse transform applied via LUT or IDT. The conceptual distinction between OETF and EOTF matters in modern color management because some operations (compositing, grading at scene-linear) need to undo the OETF, perform math, then re-encode. Reference: ITU-R BT.2100 distinguishes OETF and EOTF clearly for HLG and PQ.