Simple but versatile dynamic range coding

A method of image color processing to transform an input color having a linear red, green and blue color component representation of a pixel of an input image, which input image has a first luminance dynamic range, into a linear red, green and blue color component output color of a pixel of an output image, which output image has a second luminance dynamic range, whereby the peak luminance of the first dynamic range is at least 2 times higher than the peak luminance of the second dynamic range or vice versa, comprising:

• calculating a maximum of the linear red, green and blue color components of the input color;
• applying a uniformization function to the maximum as input, which function has a logarithmic shape and was predetermined to be of a fixed shape enabling to transform a linear input to a perceptually uniform output variable;
• receiving parameters defining a non-decreasing non-linear functional shape of a function, which was previously specified, and applying that function to the uniform output variable, yielding a transformed uniform value;
• transforming the transformed uniform value to a linear domain value by applying a linearization function;
• determining a multiplication factor being equal to the linear domain value divided by the maximum;
• and
• multiplying the linear red, green and blue color components by the multiplication factor, yielding the output color.

An image color processing apparatus arranged to transform an input color having a linear red, green and blue color component representation of a pixel of an input image, which input image has a first luminance dynamic range, into a linear red, green and blue color component output color of a pixel of an output image, which output image has a second luminance dynamic range, whereby the peak luminance of the first dynamic range is at least 2 times higher than the peak luminance of the second dynamic range or vice versa, comprising:

• a maximum determining unit arranged to calculate a maximum of the linear red, green and blue color components of the input color;
• a uniformization unit arranged to apply a function to the maximum as input, which function has a logarithmic shape and was predetermined to be of a fixed shape enabling to transform a linear input to a more perceptually uniform output variable;
• a coarse mapping unit arranged to arranged to apply a three-segment curve which consists of a linear segment for the darkest input values, which is controlled by a slope variable, a second linear segment for the lightest input values, which is controlled by a second slope variable, and a parabolic segment in between the two linear segments, and arranged to output a transformed uniform value;
• a linearization unit arranged to transform the transformed uniform value to a linear domain value;
• a multiplication factor determination unit arranged to determine a multiplication factor being equal to the linear domain value divided by the maximum;
• and
• a multiplier arranged to multiply the linear red, green and blue color components by the multiplication factor, yielding the output color.

A method of image color processing to transform an input color having a linear red, green and blue color component representation of a pixel of an input image, which input image has a first luminance dynamic range, into a linear red, green and blue color component output color a pixel of an output image, which output image has a second luminance dynamic range, whereby the peak luminance of the first dynamic range is at least 2 times higher than the peak luminance of the second dynamic range or vice versa, comprising:

• calculating a maximum of the linear red, green and blue color components of the input color;
• applying a uniformization function to the maximum as input, which function has a logarithmic shape and was predetermined to be of a fixed shape enabling to transform a linear input to a perceptually uniform output variable;
• applying a three-segment curve which consists of a linear segment for the darkest input values, which is controlled by a slope variable, a second linear segment for the lightest input values, which is controlled by a second slope variable, and a parabolic segment in between the two linear segments, yielding a transformed uniform value;
• transforming the transformed uniform value to a linear domain value by applying a linearization function;
• determining a multiplication factor being equal to the linear domain value divided by the maximum;
• and
• multiplying the linear red, green and blue color components by the multiplication factor, yielding the output color.