Saturation processing specification for dynamic range mappings

A display apparatus comprising a display and an image decoder for decoding two images of at least a factor two different luminance dynamic range for a time instant, a first image of the two images being a high dynamic range image having a dynamic range higher than a second image of the two images and the second image having a peak brightness of 100 nit, the decoder comprising:

• a color mapper configured to derive the first image by applying functions received via an image input to the second image, the functions including a luminance transformation function and a saturation modification specification function,
• wherein the color mapper comprises a color saturation modification apparatus configured to:
• determine linear color differences defined as a linear additive primary color component of an input color minus a luminance of the input color,
• do a multiplication of the linear color differences by a gain determined as the result of applying the saturation modification specification function on a difference value being defined as the value of a highest one of the linear color differences,
• add the luminance to the result of the multiplication of the linear color differences by the gain, yielding an output color, and
• obtain the first image of the two images by applying the functions to the second image for display of one of the first image and the second image on the display.

A method of image encoding comprising acts of:

• receiving a first image of a first luminance dynamic range;
• applying a color transformation to the first image to obtain a second image of a different second luminance dynamic range which is different from the first dynamic range by at least a factor two, and a peak brightness of the second image being 100 nit;
• outputting an image signal comprising one of the first image and the second image of the second dynamic range, and metadata specifying functions used in the color transformation, wherein the color transformation comprises applying a method of color saturation including determining linear color differences defined as a linear additive primary color component of an input color minus a luminance of the input color, performing a multiplication of the linear color differences by a gain, wherein the gain is determined from a saturation modification specification function depending on a difference value being defined as a value of a highest one of the linear color differences, and adding the luminance to a result of the multiplication of the linear color differences by the gain, yielding an output color, and the metadata comprising a saturation modification specification function specifying a pixel gain for a highest linear color pixel of a pixel color;
• and
• displaying one of the first image and the second image on a display.

A method of image decoding comprising acts of:

• receiving a first image of a first luminance dynamic range;
• receiving a saturation modification specification function including gain values defined as output of a function of a highest difference value being defined as a value of a highest one of linear color differences;
• applying a color transformation to the first image, the color transformation comprising acts of:
• determining the linear color differences defined as a linear additive primary color component of an input color minus a luminance of the first color,
• determining a gain corresponding to the highest one of the linear color differences, and
• multiplying the linear color differences by the gain to obtain second image of a second luminance dynamic range which is at least a factor of two different, wherein the second has a peak brightness of 100 nit;
• outputting an image including one of the first image and the second image;
• and
• displaying one of the first image and the second image on a display.