HDR VIDEO RECONSTRUCTION BY CONVERTED TONE MAPPING

To enable better quality decoding of HDR images which are communicated as corresponding LDR images plus luma mapping functions, applicant teaches an encoder for encoding a high dynamic range image (Im HDR PSEU), which high dynamic range image is represented firstly by a matrix of pixel colors (Y_SDR, Cb_SDR, Cr SDR) of an image of lower dynamic range (Im_SDR) than the high dynamic range image, which image of lower dynamic range gets compressed for communication as a compressed lower dynamic range image (Im_C), and also represented secondly by metadata (SEI) of the image which comprises a luma mapping function for calculating high dynamic range pixel lumas (Y _HDR) of the high dynamic range image by applying the function to pixel lumas of the image of lower dynamic range, wherein the encoder comprises an input (997) for receiving the luma mapping function (LMF) from an inverse tone mapping system (200), which is arranged to derive said luma mapping function based on analyzed properties of a master low dynamic range image (Im_LDR_mastr) for constructing the corresponding high dynamic range image (Im _HDR_ PSEU), characterized in that the encoder comprises a mapping splitting unit (901) arranged to transform the luma mapping function into a stretched luma mapping function (LMF_HS) which has a shape which maps a maximum normalized input to a maximum normalized output, and arranged to determine a scaling value (SCAL), the encoder having a formatter (104) arranged to output this stretched luma mapping function (LMF _HS) and this scaling value (SCAL) as metadata of the image of lower dynamic range (Im_SDR) which is also output.