Content-optimized ambient light HDR video adaptation

A device arranged to calculate an output image comprising:

• an input circuit,
• wherein the input circuit is arranged to receive a reference luminance mapping function as metadata associated with an input image;
• wherein the reference luminance mapping function specifies a relationship between luminances of the input image and luminances of a reference image,
• wherein the input image has an input maximum luminance and the reference image has a reference maximum luminance,
• wherein the input image has pixels,
• wherein the pixels have input luminances,
• wherein the input luminances fall within an input luminance dynamic range,
• wherein input luminance dynamic range is bounded by the input maximum luminance;
• a color calculation circuit,
• wherein the color calculation circuit comprises a luminance mapping function calculation circuit;
• and
• an output circuit,
• wherein the output circuit is arranged to output the output image,
• wherein the output image is calculated by the color calculation circuit on the basis of the input image,
• wherein the luminance mapping function calculation circuit is arranged to calculate a locus of points so as to form an adapted luminance mapping function,
• wherein each point on the locus is a point on a metric,
• wherein the metric has a first endpoint and a second endpoint,
• wherein the first endpoint is on a reference luminance mapping function,
• wherein the second endpoint is on a diagonal,
• wherein the locus of points depends on a control parameter,
• wherein the adapted luminance mapping function is applied to the input pixel luminances so as to obtain output luminances of the output image,
• wherein luminance mapping function calculation circuit is arranged to receive a maximum luminance of a display and a black level of the display,
• wherein the color calculation circuit is arranged to calculate the control parameter,
• wherein the control parameter specifies how much the adapted luminance mapping function deviates from the reference luminance function,
• wherein the control parameter is defined based on a maximum luminance of a display and a black level of the display,
• wherein the calculating of the control parameter comprises:
• applying an inverse of an electro-optical transfer function to the maximum luminance of the display to produce a first result;
• subtracting the black level from the first result to produce a second result;
• and
• applying the electro-optical transfer function to the second result.