Paper 107: white balance of images

WhiteBalance: Refers to the process of image processing, the original material is a white object of the image color reduction, the effect of removing the color temperature of the external light source, so that it also shows white in the photo . This means that white objects can be restored to white, regardless of the light source.

White refers to the light reflected in the eyes of the human eye due to the same proportion of blue, green, red color and a certain degree of brightness formed by the visual reaction. We all know that white light by red, orange, yellow, green, green, blue, violet seven kinds of light, and these seven kinds of color by the red, green, blue three primary colors in different proportions of the formation, when a ray of the three primary colors in the same proportion of the same time, it is customary to call the color , black, white, gray, The light reflected by the gold and silver is color-dissipating. The popular understanding of white is the brightness that does not contain the color component. The white or other color seen by the human eye is related to the natural color of the object itself, the color temperature of the light source, the reflection or transmission characteristics of the object, the visual induction of the human eye and many other factors (see "Principles of Color"), for a simple example, when a light is illuminated to the Achromatic object, the object reflects the color When the white object is red under red light, and when two or more shades are illuminated at the same time, the object color is added color effect, such as red and green light at the same time the White object, the object appears yellow. The color of the object is a subtractive effect when light is illuminated on a colored object. If the yellow object is red under the magenta light, it is green under the cyan illumination, and appears gray or black under the blue light.

So, what is the color temperature?

The so-called color temperature, in short, is to quantify the Kelvin temperature (K) to indicate color. The well-known British physicist Kelvin believes that the assumption that a blackbody substance can absorb all the heat falling on it without loss, while simultaneously releasing the energy generated by the heat in the form of light, will become different colors depending on the heat. For example, when a blackbody is subjected to a heat equivalent to 500-550 ℃, it becomes dark red, when it reaches 1050-1150 ℃, it becomes yellow, and the temperature continues to rise in blue. The color component of the light source corresponds to the thermal temperature of the blackbody, and the temperature of any light is equivalent to the "temperature" at which the blackbody emits the same color, which is used to indicate the characteristic of a certain shade to distinguish the other, which is the color temperature. From the temperature of the 3300k~9300k, the color of the light it emits is red, white, and blue, respectively. In the process of iron, the black steel gradually turned red in the furnace temperature, this is the best example of the blackbody theory. Color temperature phenomenon in daily life is very common, I believe people are not unfamiliar with it. Light from tungsten filament (2900K) because the color temperature is lower than the yellow tone, different lamps will emit different colors of light, natural gas flame is blue, because the color temperature is higher. The temperature in the midday sunlight is about 5600 K, and cloudy is closer to the indoor color temperature 3200K. The color temperature at sunrise or sunset is about 2000K, and the temperature of the candle is about 1930K. Blue Sky (18000K), it is not difficult to find a rule: the higher the color temperature, the more blue color, the lower the color temperature is red. The color temperature of a certain shade is higher than other colors, indicating that the shade is more blue than other colors, while the other is red; Similarly, when a shade is blue than other colors, it indicates that the color temperature of the shade is high, and vice versa. The so-called warm and cold tones, is only a perceptual term.

< 3300K warm 3300~6000k white >6000k cool.

Because the human eye has a unique adaptability, so that we sometimes can not find the color temperature changes. For example, under the tungsten filament for a long time, and will not feel the white light under the red, if suddenly the fluorescent lamp changed to tungsten lighting, will be aware of the color of white paper is red, but this feeling can only last a while. The CCD camera does not have the same adaptability as the human eye, so if the color adjustment of the camera is inconsistent with the color temperature of the scene illumination, the bias color will occur.

The white balance is for different color temperature conditions, by adjusting the internal color circuit of the camera so that the image of the film offset color, closer to the visual habits of the human eye. Because the camera is no human eye so smart (the human eye when the object is seen, automatic color correction), so the camera set a range, if the photo of the color average falls within the set range, there is no need to fix, if deviate from the range, you need to adjust the parameters to let it fall within the range. This is the WB white balance correction process.

Inside the camera there are three CCD electronic coupling elements, they feel the blue, green, red light, in the pre-condition of these three photosensitive circuit The electron magnification ratio is the same, for the 1:1:1 relationship, the adjustment of the white balance is based on the adjusted scenery to change the proportional relationship. For example, the color of the blue, green, and red light is 2:1:1 (the ratio of Blu-ray, color temperature is high), then the proportion of white balance adjusted to 1:2:2, adjusted the proportion of the circuit magnification of the apparent blue decrease, increase the proportion of green and red, This is adjusted to the scene through the white balance adjustment circuit to the image taken, blue, green, red proportion will be the same. In other words, if the white offset is slightly blue, then the white balance adjustment will change the normal proportional relationship to weaken the blue circuit amplification, while increasing the ratio of green and red, so that the image remains white. That is to say, the color of the phenomenon when shooting under a certain light source, by strengthening the corresponding complementary colors to compensate. The colors produced by the photographs under various white balances show the complementary color of the compensation.。 When using a film camera, in order to compensate for these offsets, a variety of color filters are used when shooting. The basic principle of a digital camera is similar to that of a color filter, which is the white balance function. However, there is no similar automatic white balance filter in the color filter, there is a big difference between the two.      It is sufficient to choose automatic white balance (AWB) for general use, but if the hue is not ideal under certain conditions, you can choose to use a variety of other white balance options. In order to consider the problem of white balance adjustment, the camera will not "refuse" to be placed in front of the lens in the white balance adjustment tolerance, that is, the lens can adjust the white balance to any scene. Most of the time the White Board (card) to adjust the white balance, is because the white Board (card) can most effectively reflect the color temperature of the environment, in fact, many times the Whiteboard (card) is not white, more or less will be slightly blue or other colors, experienced camera will use the blue sky to adjust the white balance, To get the picture of the red and yellow tone. After figuring out how the white balance works, it will be bold to try different effects and enrich the video creation.

In life, the color temperature of the sun is constantly changing, it can be said that no two places the color temperature will be exactly the same, different regions, seasons, the ground environment, weather, morning and evening will affect the color temperature, there are a lot of artificial light, color temperature is not the same. The cameraman adjusts the white balance method coarse adjustment, fine adjustment and automatic tracking (ATW) Three kinds: coarse adjustment refers to the preset conditions to change the color temperature filter, so that the color temperature is close to the factory setting of 3200K; fine adjustment refers to the color temperature filter with the camera white balance adjustment function, A more precise adjustment result for a particular ambient color temperature; auto-tracking refers to the camera's automatic tracking function (ATW), which adjusts itself to the color temperature of the screen.

White balance is an indicator that describes the white accuracy of the red (R), Green (G), and Blue (B) tri-Primary colors in the display. White balance without defective display, in changing the color and brightness will not affect the white purity, that is, there will be no bias color, there is no other noise doping, because for a high-end large-screen dedicated display, even a very small "color" will affect the color quality of the screen.

So how do you fix the white balance? , first there are two important theories to know:
Gray World Theory: think of any image, when there is enough color change, its R, G, b component mean value will tend to balance (that is, RGB three values equal, that is, should be black and white gray type of color). This theory is obtained in the global white balance, which is characterized by the ability to use more image information to make judgments, but in the face of a single color of the image when it seems a little weak.
Total reflection Theory: The most luminance point in an image is the white point, which is assumed to be the most significant Y value in the YCbCr space, so that the whole image is corrected. Features are only considered the brightest part of the color, with the above the Gray world theory is the opposite, in the processing of color monotonous image, the effect is better, but the face of color-rich pictures, because the brightest point is not necessarily color, may appear biased color situation.
The two theories correspond to the theoretical basis of two color space RGB and YCbCr adjusting white balance: Judging whether a picture white balance is accurate, if inaccurate, how to quantify its deviation value. Several white balance algorithms: 1. Gray World algorithm: According to the Gray World theory, the RGB mean of the original graph is adjusted to r= G = B respectively. Imperfect place is this algorithm on the color of the image is not rich in general sensitivity, the processing effect will not be ideal, the limitations are great. 2. Standard deviation weighted Gray world algorithm: The algorithm is aimed at the improvement of the above algorithm, its principle is to divide the image into a few pieces, and then the use of statistical analysis of each block, look at the richness of color, color, color, less weight, the final sum to get a mean value, The RGB values are corrected based on this relatively accurate value. 3. Total reflection algorithm: The algorithm thinks that the brightest point is white, if not, the inverse correction for deviations from the white value. The disadvantage is that there is no high bright spot or image color complex, its correction effect will be relatively weak. Also: the brightness weighted gray world algorithm and the total reflection algorithm orthogonal combination algorithm, the algorithm is good, but the computational capacity is huge. However, it is convergent in itself and does not cause loss to the image. The above algorithm, see "White Balance correction algorithm".

Paper 107: white balance of images