Color expression and color reproduction technology of color printing

I believe many people often feel that the color of the image captured by the same digital is not bright enough, or that the image after printing it is very different from the screen displayed on the monitor. So this time, let â€™s delve into the root causes of these problemsâ€”the characteristics of the human eye, and the essential differences in each related imaging product, and explain the â€œcolor reproductionâ€ technology.

The images taken by high-resolution digital cameras are printed by color printers. I believe most of them will encounter such situations-the clouds and forests are colored and slightly white. But these can be changed by appropriate settings, and the printing effect may be improved to some extent. This means that if the color standards between the digital camera and the printer can be properly corrected, the picture reproduction requirements can be met. of. Whether you use the image correction function built into most printer products or the image correction software provided by digital cameras, you can use a simple operation method to complete.

The adoption of such a correction method is basically based on the true and natural feeling of the user's eyes. In other words, if the "color characteristics" between the digital camera, printer and monitor can be unified, the required requirements can be achieved. Before explaining how to adjust, first of all, we must first understand the meaning of the terms such as color or brightness, and how the eyes feel "color".

The human eye recognizes light and color

For the eyes of the soul window, the light between purple and red (wavelength of about 380nm to 770nm) is actually divided into "value", "hue" and "chroma" Three qualities to sense. Structurally, it mainly uses the three visual cells "S cone", "M cone" and "L cone" arranged in a mosaic on the retina of the eye to sense the brightness and color of external light. Because of the different wavelengths (three primary colors) of light, the human eye can clearly distinguish the different colors. However, due to the difference in number, in terms of proportion, for each set of S cones, there are 16 sets of M cones and 32 sets of L cones, so the eye's perception of each light wavelength is not equal. . That is to say, if you want to show several specific colors with the same brightness, considering the actual perception ability of the eyes, you must correct the intensity of the individual colors among the three primary colors.

In addition, in the structure of the eye, there are about 130 million groups of rod visual cells with sensing functions in low-light situations, which is more than the total number of S, M, L cones with a total of 7 million groups. Out a lot. Due to the large number of rod cells, even in the case of insufficient external light, the human eye can generally capture more sufficient light information. However, because rod cells have only one species, and only have the characteristics of receiving black and white information, when staring at an object in the dark, the object is mostly presented in black and white. In addition, the eye's sensitivity to brightness is more sensitive than color, and the same is true.

Through the brightness information sensed by the cone and the rod, if the brightness can be emphasized between the cells adjacent to each other in the nerve in the retina, the eye's sensing ability will be more sensitive. And because the signal output of the cone is made of nerve cells in the retina, the difference between the brightness and "red-green" and "yellow-blue" is divided into signals to be processed. Therefore, the color processing generally needs to be replaced by Complementary colors can be called complementary color effects.

Since the cone has different brightness sensing capabilities (spectral sensitivity) for the primary colors of R (red), G (green), and B (blue), if R is set as the reference 1, then R is formed between the other two colors : G: B = 1: 4.5907: 0.0601 (referred to as the brightness count ratio of the eye). If this value is adjusted in a quantitative ratio, it will not be affected by any external factors, forming a CIE (International Lighting Society) color system represented by the hypothetical three primary colors (X, Y, Z), which is a formal Evaluation standard color expression form. In the CIE color system, any color can be displayed in 3D coordinates. Therefore, if the three primary color signals (X, Y, Z) are divided by the square root of (X2 + Y2 + Z2), then (x , Y, z) coordinates, generally called chromaticity coordinates. If the most vivid colors are used for drawing, the x, y coordinates are drawn using different monochromatic light wavelength values, and the color range seen by the human eye is depicted, and a horseshoe-shaped pattern (such as "chroma" The graph (consisting of JIS and CIE chromaticity diagrams) is the horseshoe shape of the xy chromaticity diagram. This figure goes from the four sides to the center, the color tends to be mixed, and the center gradually changes to white. In fact, most common image-related products such as CRT monitors, LCD monitors, or printers use this type of chart for color definitions and specifications. In other words, we must first regulate the range of colors visible to the human eye, and then determine the color space that the product can display or support.

The Munsell color system that determines the color space through the three primary colors that make up light and color

Before talking about the three primary colors, you need to understand that the object itself belongs to the luminous source, and the generated three primary colors are (RGB: red, green, and blue), and the objects such as pigments are reflective light sources, and the formed three primary colors are (YMC: yellow, indigo, and magenta ), But also to understand the difference between these two systems. RGB uses the mixing of three colors to make the colors brighter, that is, the primary color mixing rule formed by the additive color method. And YMC is the darker color after the color is mixed, which belongs to the primary color mixing rule formed by the subtractive color method.

Generally speaking, most of the scenery or objects felt by human eyes are formed by reflecting light sources. However, if you are using a digital camera to shoot a landscape or an object, you use a CCD to convert the color of the object to RGB, which is converted into the three primary colors that the monitor can support, and the printer converts the image to the object color and prints it out. It can be seen from this that the computer processes the image by converting between the object color and the light source color. In other words, between the RGB system of the light source color and the YMC system of the object color, it can be transformed according to a simple function calculation. In the common common print output or image processing, black will also be added in the YMC system to improve the color reproduction effect after color mixing.

However, it is assumed that the color space in the CIE color system formed by the X, Y, and Z coordinates is not related to existing related products, but in practical applications, what the eye needs to recognize color is "lightness", " "Color" and "hue" three elements (such as "lightness, saturation, hue color space map"). Brightness is the degree of brightness, and hue represents the hue arranged on the hue circle. Chromaticity means that starting from the colorless central axis, the color is more vivid toward the edge.

Painter A. H. Munsell discovered this phenomenon and combined lightness (chroma) and hue (hue) with the feeling of human eyes to make it a swatch (index). This is the famous Munsell color system. Since then, the American Lighting Association has developed a revised version of the Munsell color system, and calculated the values â€‹â€‹corresponding to the X, Y, and Z color system coordinates. However, in terms of the physical color material (color material) of the object, there are still certain display restrictions, which will form a twisted three-dimensional cone like the "Munsell color stereogram", which is generally called "color three-dimensional" .

Because the Munsell color system (Munsell color system) is calculated based on empirical system coordinates, not based on physical substance. Therefore, if the color is to be quantified, it must be obtained through actual calculation. According to the color space formed by the Munsell value, the "L * a * b * color space" can be extended and defined. This is represented by L (white to black axis), and ab axes showing chroma and hue, where a represents the axis from red to cyan, and b represents the axis from yellow to cyan.

In the "L * a * b * color space", two different color axes can be used for calculation, so it has also been adopted by the color management of Windows, Macintosh and other systems. Since the introduction of Windows 98, the color management definition file used by the monitor has been listed as standard equipment, and the definition file required by the printer or scanner is provided, which is the "ICC definition file". The ICC definition file is a standardized data format developed by ICC (International Color Consotium). As long as the definition file is provided in the product, the colors can be correctly converted and combined with each other. Between different peripheral devices, the relevant color definitions are handled through the "L * a * b * color space". The conversion of most products in the color space is an example, and the "L * a * b *" between different products The conversion function of "color space" is actually the true face of the ICC definition file. As long as ICC definition values â€‹â€‹are used, the performance of the image output person can be easily regulated. At present, although the number of products that provide ICC definition files is not very large, I believe that the provision of ICC definition files in the future will become a choice for users. An important basis for products.

By analyzing the energy distribution state, the color perception of the light source is quantitatively expressed as the color temperature. As mentioned in the previous article, usually the eye uses the reflected light of the object for color recognition. Due to the different color of the light source, the color of the observed object will also change. The color mentioned here actually has a clear definition in physics, usually expressed by the term color temperature.

Simply put, according to the definition of Plankian radiator, assuming an object that can completely absorb incident light, express the temperature of this all-black object as K (Kelvin), which is the definition of color temperature. Has nothing to do with the material. When the temperature of this object is changed, the distribution of light energy released by it will also change accordingly, so the color of the light source can be defined in a quantitative way. For example, in the CIE color system, multiple sets of standard light source values â€‹â€‹are defined, of which the most commonly used standard sunlight is 6504K.

When the user performs image processing, due to the color temperature of the display used by the personal computer and the color provided can vary, so be careful when using it. In general, in order to adapt to users who prefer cool colors and higher definition, including TV sets, many display products on the market are based on 9300K. In other words, this benchmark is significantly higher than the standard sunlight value in the CIE color system. Therefore, for mid-to-upper display products, additional setting options to adjust the color temperature to 6500K are provided, Standard sunlight in natural tones is displayed. This can be said to be an indispensable important setting mode for workers who are required to faithfully present the original photos or images.

If the user is used to the screen displayed above 9000K, for the display mode around 6500K, the overall screen will feel reddish. Therefore, the faithful presentation of the picture will not meet everyone's needs because of the user's personal preferences, especially in different environments, the preferred color presentation may also be different. This gives rise to the concept of "color rendering", which is the direction of thinking of "tend to like color".

Reproduce the color of the favorite color and the related correction method

R. W. G. In 1970, Hunt distinguished the objectives of color reproduction into: "reproducible optical color reproduction", "optical optical color reproduction", "correct color reproduction", "equal color reproduction", "correspondence" Six types of classifications: "Reproduction of Sexual Color" and "Favorite Sexual Color Reproduction". In "Reproducible Color Reproduction", even if the external illumination (color) changes, the color reproduction of the object will not change, so in reality, it is still impossible to achieve this goal today. However, "optical color reproduction" and "correct color reproduction" belong to the category of color reproduction of measuring instruments, and have little practical significance for the perception of the human eye. "Equal color reproduction" is like watching a bright stage from a dark auditorium. The phenomenon of white tones will appear. Through this feature, the original chroma and contrast are emphasized to show the original effect.

As for "corresponding color reproduction", the most representative example is to make the place different from the original brightness and light source, or to view the picture under different external conditions and restore it to the original color tone. TV set. "Preference color reproduction" is aimed at users who like to display pictures with high color temperature or the warm-color lighting environment in literary and romantic films, for the purpose of matching or supporting color processing. In practice, in fact, it is commonly seen in the development of photos, through adjustments to increase or decrease skin to achieve a healthier effect, which can be regarded as the effect of "favor sexual reproduction". In addition, when taking photographs, color filters or polarizers with vivid colors are often used to increase color rendering.

Theoretically, the reproducibility of color is obtained through the linear RGB or YMCK color system. However, due to different purposes, the color reproducibility of digital cameras' CCDs, monitors, color printers, and scanners cannot be expressed in a straight line. Therefore, if you want to adjust to the target characteristics of the product, you need to first understand the color space characteristics of the product itself, convert it into a virtual XYZ color table system, convert it to L * a * b * color space, and then find the required correlation The color difference value of the function can really adjust the color difference caused by the image input and output.

One of the common techniques is to use Y correction to complete. For example, when viewing the pictures taken by a digital camera directly from the display, abnormal levels of dark parts often occur. At this time, as long as the image file is Y-corrected, the subtle parts can be clearly recognized in the dark screen. The so-called Y correction is actually named after correcting the non-linear characteristics (y characteristics) of the device with inverse characteristics. For example, if you want to output the image through the printer, you must correct all devices with y, so that the narrow color reproduction characteristics on the printer can be displayed with the best effect in the relevant dynamic range.

Afterword

In fact, the excellent image quality and color reproducibility cannot be achieved in the use environment mainly based on the color management mode of the ICC definition file, and it is particularly obvious that the performance in color reproducibility is very insufficient. In contrast, the common output methods of digital cameras and printers today are quite different from the photography methods using silver salt photos, mainly due to the advantages of photo paper pigments, so digital photography is still unable to reproduce 100% of the original color. The picture is also caused by the so-called high flexibility in the simulated world. If you want to reproduce the original picture, you must use at least a 32-bit color system with RGB three primary colors each having 10-bit data. However, the increase or decrease of the number of digits does not fully reflect the advantages of the analog display. It is necessary to add non-linear curve compression and other techniques before using the full full digit. Of course, for the 32-bit color system, we will not talk about it in detail here, but it seems that we must pay attention to the development trend of future updates.

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