Hue-preserving lightness and saturation modification in RGB color space for protanopia and deuteranopia

Colors are not seen in the same way among all people. For example, to dichromats such as protanope and deuteranope, red and green appear similar ochre-like colors. Therefore, it is difficult for dichromats to distinguish such color combinations. The purpose of this study is to propose a color conversion method that produces images that are easy for dichromats to discriminate colors, without significantly changing the impression of the original image for both dichromats and trichromats. The proposed method modifies the lightness of pixels to take into account colors that are difficult for dichromats to discriminate, thus enabling color discrimination. Furthermore, by also modifying the saturation of the pixels, the image quality degradation caused by overemphasis, which occurs when only modifying the lightness, is suppressed. To demonstrate the effectiveness of the proposed method, experiments using various images are conducted. From the resultant images, it can be confirmed that the proposed method achieves the color conversion that serves its purpose. The main results of quantitative evaluation are as follows. (1) In the index evaluating the degree of contrast improvement for colors which are difficult for dichromats to discriminate, the average value of the proposed method is about 0.84, and indicates that the proposed method is comparable to the conventional methods. (2) In the index represented by the ratio of the number of pixels clipped with white or black, the average of the proposed method is $4.38\times 10^{-4}$ and indicates that there are almost no clipped white and black pixels.