A computer program having instructions which, when executed by a computing device or system, cause said computing device or system to: - receive image data with luminance values representing absolute luminance values L to be encoded; - encode the received image data into reference encoded image data; and - output the reference encoded image data, wherein said encoding comprises mapping absolute luminance values into digital code values, the luminance values of the received image data being represented by the digital code values in the reference encoded image data; wherein said mapping is based at least in part on an inverse of a functional model of: Y = max V 1 m − c 1 0 c 2 − c 3 V 1 m 1 n , wherein: Y is a normalized value of a corresponding one of the absolute luminance values L, wherein 0 ≤ Y ≤ 1, V is a normalized value of a corresponding one of the digital code values D that are output as reference encoded image data, wherein 0 ≤ V ≤ 1, and n, m, c 1 , c 2 , and c 3 are predetermined values, with: n = 2610 4096 × 1 4 ≃ 0.1593017578125 ; m = 2523 4096 × 128 ≃ 78.84375 ; c 1 = c 3 − c 2 + 1 = 3424 4096 ≃ 0.8359375 ; c 2 = 2413 4096 × 32 ≃ 18.8515625 ; and c 3 = 2392 4096 × 32 ≃ 18.6875 . - receive image data with luminance values representing absolute luminance values L to be encoded; - encode the received image data into reference encoded image data; and - output the reference encoded image data, wherein said encoding comprises mapping absolute luminance values into digital code values, the luminance values of the received image data being represented by the digital code values in the reference encoded image data; wherein said mapping is based at least in part on an inverse of a functional model of: Y = max V 1 m − c 1 0 c 2 − c 3 V 1 m 1 n , wherein: Y is a normalized value of a corresponding one of the absolute luminance values L, wherein 0 ≤ Y ≤ 1, V is a normalized value of a corresponding one of the digital code values D that are output as reference encoded image data, wherein 0 ≤ V ≤ 1, and n, m, c 1 , c 2 , and c 3 are predetermined values, with: n = 2610 4096 × 1 4 ≃ 0.1593017578125 ; m = 2523 4096 × 128 ≃ 78.84375 ; c 1 = c 3 − c 2 + 1 = 3424 4096 ≃ 0.8359375 ; c 2 = 2413 4096 × 32 ≃ 18.8515625 ; and c 3 = 2392 4096 × 32 ≃ 18.6875 . wherein said encoding comprises mapping absolute luminance values into digital code values, the luminance values of the received image data being represented by the digital code values in the reference encoded image data; wherein said mapping is based at least in part on an inverse of a functional model of: Y = max V 1 m − c 1 0 c 2 − c 3 V 1 m 1 n , wherein: Y is a normalized value of a corresponding one of the absolute luminance values L, wherein 0 ≤ Y ≤ 1, V is a normalized value of a corresponding one of the digital code values D that are output as reference encoded image data, wherein 0 ≤ V ≤ 1, and n, m, c 1 , c 2 , and c 3 are predetermined values, with: n = 2610 4096 × 1 4 ≃ 0.1593017578125 ; m = 2523 4096 × 128 ≃ 78.84375 ; c 1 = c 3 − c 2 + 1 = 3424 4096 ≃ 0.8359375 ; c 2 = 2413 4096 × 32 ≃ 18.8515625 ; and c 3 = 2392 4096 × 32 ≃ 18.6875 . Y is a normalized value of a corresponding one of the absolute luminance values L, wherein 0 ≤ Y ≤ 1, V is a normalized value of a corresponding one of the digital code values D that are output as reference encoded image data, wherein 0 ≤ V ≤ 1, and n, m, c 1 , c 2 , and c 3 are predetermined values, with: n = 2610 4096 × 1 4 ≃ 0.1593017578125 ; m = 2523 4096 × 128 ≃ 78.84375 ; c 1 = c 3 − c 2 + 1 = 3424 4096 ≃ 0.8359375 ; c 2 = 2413 4096 × 32 ≃ 18.8515625 ; and c 3 = 2392 4096 × 32 ≃ 18.6875 . <EOS>