Optimizing the displaying of hdr images

Abstract

1. To obtain in a pragmatic manner better watchable HDR images, we propose several variants of an image processing apparatus for processing an input image (Im_Comm) of an input video to obtain an output image of an output video, wherein the input image has pixels which have input luminances (Ln_in_pix) which fall within a first luminance dynamic range (DR_), which first luminance dynamic range has a first maximum luminance (PL_V_HDR), wherein the output image has pixels which have output luminances which can be calculated from the input luminances and which fall within a second luminance dynamic range, which second luminance dynamic range has a second maximum luminance (PL_V_MDR), wherein the apparatus comprises a video data input () arranged to receive the input image and a reference luminance mapping function (F_L) which is encoded as metadata associated with the input image, wherein the reference luminance mapping function specifies a relationship between luminances of a first reference image and luminances of a second reference image, wherein the first reference image is the input image, wherein the second reference image has a second reference maximum luminance (PL_V_SDR); wherein the apparatus comprises a display adaptation unit () arranged to determine an adapted luminance mapping function (FL_DA) which is based on the reference luminance mapping function (F_L), wherein the display adaptation unit uses a pre-fixed display adaptation algorithm, which specifies for each point on a diagonal, in a coordinate system of the input luminances normalized to a maximum being 1 and the output luminances normalized to a maximum being 1, a respective metric along a line segment oriented in a pre-fixed direction, the line segment starting on the diagonal, wherein the respective metric at each position is normalized by giving the point of intersection of the line segment and the locus of the luminance function (F_L) in the coordinate system the value 1, wherein the determining locates a position on each respective metric corresponding to the second maximum luminance (PL_V_MDR), wherein the collection of positions on the respective metrics is output as the adapted luminance mapping function (FL_DA); characterized in that the apparatus comprises a processing comprises a boost determination circuit () arranged to calculate a booster strength value (BO) comprising: —a histogram calculation unit () arranged to determine a histogram (hist) of intermediate luminances obtained by applying the adapted luminance mapping function (FL_DA) to the input luminances, —an average calculation circuit () arranged to calculate an average brightness measure (AB) on the basis of the histogram, —a first converter unit () arranged to calculate a first strength value (PosB) from the average brightness measure (AB), —a second measuring unit () arranged to calculate a weighted sum (BDE) of pixel counts in at least two configurable upper bins of the histogram, —a second conversion unit () arranged to calculate a second strength value (NegB) from the weighted sum (BDE), —wherein the boost determination circuit determines the booster strength value (BO) on the basis of a value (DV) which is equal to the first strength value (PosB) minus the second strength value (NegB); wherein the apparatus is further arranged to: use the display adaptation unit to calculate an adjusted adapted luminance mapping function (F_ALT_B), wherein the adjusted adapted luminance mapping function is calculated by using the display adaptation algorithm in a setting which outputs the locus of positions equal to the booster strength value on the metric as the adjusted adapted luminance mapping function (F_ALT_B); and the apparatus comprising a color transformer () arranged to apply the adjusted adapted luminance mapping function (F_ALT_B) to the input luminances to obtain the output luminances.