The CIE color difference calculation was performed in the previous Lab color space. Chromatic aberration generally refers to the color difference caused by different positions or different magnifications. In the field of image processing, especially for color processing, it is very important to eliminate chromatic aberration. Then we will discuss the improvement process of the CIE color difference formula, the meaning of each improvement, and the specific explanation of the formula.
CIE is the abbreviation of the International Commission on Illumination. In 1976, two color spaces, CIELUV and CIELAB, were proposed. Here we mainly introduce CIELAB, which mainly uses the theory of opposite colors. If it is represented by a graph as follows:
From this picture we can see that the different colors are not linearly separable. In our eyes, the color with a relatively large difference may actually have a small difference in RGB. We obviously feel that the difference on the left is smaller than that on the right.
Therefore, you cannot simply use formulas such as mean square error to judge the color difference of colors, so this is the meaning of the CIE color difference formula. The purpose of the CIE color difference formula is to find JND. People who understand quality evaluation should be familiar with this term, and can just identify the difference, that is, if the difference is within this threshold range, the human eye cannot distinguish it.
The previous blog about the LAB field has been introduced, and here we continue. CIE1976 is actually the mean square error. The specific formula is as follows: There is obviously an error in doing so,
because the impact of the three channels on the human eye must be different, and Different value ranges of the same channel have different effects on the human eye. The figure below shows the equichromatic aberration problem in different areas:
in this figure, the horizontal axis is a, and the vertical axis is b, so we can see that when the absolute values of a and b are larger, the area of the ellipse is larger. Large, which shows that the human eye is less sensitive to high chroma. After the above analysis, CIE improved the color difference formula again in 1994, and it became the form of the figure below: this formula is when the absolute value of a and b is large
, The sensitivity coefficient is also divided by a large number. The values of a and b are proportional to their respective sensitivity coefficients. The ratios are 0.045 and 0.015 respectively. In fact,
this improvement is far from enough, because all ellipses can be seen from the above figure Except for the blue area in the lower left corner, they all point to the origin of the coordinates. And for the blue area, the sensitivity of the human eye is higher, that is, the tolerance for changes is low. In fact, the short wavelength of blue light and the inability to produce blue light used to be a problem in the industry. Therefore, in 2000, CIE improved the chromatic aberration formula again for problems such as blue light, and changed it into the following form:
This is the chromatic aberration formula we still commonly use now. In daily use, we can decide whether to classify two colors into one category by adjusting the threshold. For the formula implementation of CIE2000, see the second reference article
Reference:
Introduction to Color Space-Huang Zong
The CIEDE2000 Color-Difference Formula: Implementation Notes, Supplementary Test Data, and Mathematical Observations