1.1 brightness sensing and exposure
1.1.1 photosensitive widthFrom the brightest to the darkest, assuming that the human eye can see a certain range, the film (or CCD and other electronic photosensitive devices) can be much smaller than the human eye can see, the limited range is the photosensitive width. The human eye is much more sensitive than the film, and the film is much more sensitive than the CCD of a digital camera! After learning about this concept, we can easily understand why the human eyes can see the buildings with backlights and the dazzling sky and clouds under the background of backlight. Once the photo is taken, either the cloud colors are gorgeous and the building turns into a dark silhouette, or the color details of the building are clear, while the original beautiful Cloud turns into a white one and looks at the structure of the human eye, there is a pupil that can control the amount of light, and there are rod-shaped photosensitive cells and vertebral photosensitive cells to adapt to different light intensity. It can be seen that even if the human eye has a high sensitivity and width, there is still a brightness adjustment system, to adapt to light intensity changes. For camera sensor, correct exposure is even more important!
1.1.2 Automatic Exposure and 18% grayHow does sensor determine whether the exposure is correct? The standard practice is to calculate the mean value of the Y value of the current image in the YUV space. Adjust various exposure parameter settings (automatically or manually) so that the mean value falls near a target value, and the correct exposure is obtained. So how can we determine the mean value of Y and adjust the parameter so that sensor can adjust the brightness of the current image to this range? This involves a concept of 18% gray. It is generally considered that the average reflective coefficient of indoor and outdoor scenes is about 18%, and the color mean, as described above, it can be considered as a gray color. In this way, you can adjust the exposure parameters by shooting the gray panel with a counter rate of 18% so that the color is close to the gray with a moderate brightness (the Y value is 128 ). Then, the normal scenes can be automatically correctly exposed. Of course, the AE function of this automatic determination of the exposure parameter is not omnipotent. For scenarios with the inverse rate deviation from the average value, such as snow scenes and night views, this method cannot be used to obtain the correct exposure. Therefore, in the software processing module of sensor, the exposure level setting function is usually provided to forcibly change the judgment standard of Automatic Exposure. For example, changing the expected brightness mean.
1.1.3 exposure level settingsThe exposure level setting function can be seen on most digital cameras and photo mobile phones. As mentioned above, this setting actually provides users with certain exposure control capabilities based on automatic exposure, forcibly change the exposure judgment standard of the camera sensor to obtain the desired effect. Generally, the pre-period value of the Y value is changed so that the sensor can automatically adjust the exptime and Ag Based on the New Y value during Automatic Exposure.
1.1.4 Gamma CorrectionThe mean of the impression is correct, which does not mean that the brightness distribution of the entire image is consistent with that seen by the human eye. In fact, the human eye's response to brightness is not a linear proportional relationship, and the input and output characteristic curves of various devices involved in photoelectric conversion are generally non-linear and present as a power function:
Y = xn,
Therefore, the transfer function of the entire image system is a power function. G = G1 × G2 ×... * Gn for sensor, the response is close to a linear relationship, so in order to correctly output images that conform to the human eye's response to brightness on various devices, correction is required. The reciprocal of the power function index is commonly referred to as the gamma value. Normalized gamma curve
The corrected function can be expressed as '> 1. Generally, for the output Display System of window, the gamma value is 2.2, while for the output Display System and printing system of Apple, the gamma value is 1.8.
In fact, when performing Gamma Correction, sensor usually converts 10 bit data in raw format to 8 bit data. Therefore, the formula at this time can be expressed
Contrast
To a certain extent, the contrast is actually adjusted to the gamma curve. Increasing the contrast is to increase the GAMMA value. For image processing, after hardware Gamma Correction, the software performs a similar power function transformation to adjust the contrast.
1.1.6 adjust the exposure parameterYou can adjust the exposure intensity by changing the exposure time or by changing the brightness gain AG. The exposure time is limited by the frequency limit. For example, if the camera requires 15 frames per second, the maximum exposure time cannot exceed 1/15 seconds. There may be other conditions, and the actual exposure time is shorter, in the case of weak light, adjusting the exposure time alone cannot meet the frame rate needs. At this time, you can adjust the gain AG to control the gain of the exposure and reduce the exposure time. However, the disadvantage is that Ag is enhanced at the cost of image quality, and the signal-to-noise ratio (SNR) is reduced and image noise is increased. Therefore, when the image quality is taken as the priority, the adjustment of the exposure parameter is usually to prioritize the adjustment of the exposure time, and then to consider the exposure gain. Of course, the exposure time cannot be too long to avoid blurred images due to jitter. When shooting a sports scene, the exposure time is more demanding.
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