Data processing device and computer-implemented method combining two images and an overlay color using a uniform color space

A data processing device for computing an output color of an output pixel in an output color image by mixing a background color of a first pixel in a background color image with an overlay color depending on a light intensity value of a second pixel in a second image, wherein the data processing device is configured to perform operations comprising:

• retrieve background color coordinates of the background color in a uniform color space, the background color having a predetermined maximum luminance value and a predetermined minimum luminance value in the uniform color space;
• retrieve overlay color coordinates of the overlay color in the uniform color space;
• retrieve the light intensity value, the light intensity value being in a range extending from and including a predetermined minimum light intensity value to and including a predetermined maximum light intensity value;
• compute a luminance line in the uniform color space, the luminance line extending from the overlay color coordinates to an end point, a luminance at the end point being lower than a luminance at the overlay color coordinates, the luminance line extending along points having a chroma of the overlay color coordinates;
• determine a first point on the luminance line, wherein
• a ratio of
• a difference between the predetermined maximum luminance value and a luminance of the background color coordinates and
• a difference between the luminance of the background color coordinates and the predetermined minimum luminance value
• corresponds to a ratio of
• a length of the luminance line between the first point and the overlay color coordinates and
• a length of the luminance line between the end point and the first point;
• compute an overlay color line that extends straight from the background color coordinates to the first point;
• determine a second point on the overlay color line, wherein
• a ratio of
• a difference between the predetermined maximum light intensity value and the light intensity value and
• a difference between the light intensity value and the predetermined minimum light intensity value
• corresponds to a ratio of
• a length of the overlay color line between the second point and the first point and
• a length of the overlay color line between the background color coordinates and the second point;
• and
• output color coordinates of the second point in the uniform color space as the output color of the output pixel.