Anti-noise processing AG, the inevitable increase in noise, in addition, if the light is darker, the exposure time is too long, it will increase the number of noise (from the digital camera, mainly because of the long exposure, the sensor temperature rise, the current noise caused by the increase in the noise of the photosensitive element), The flaw in the photosensitive element itself is one of the sources of noise and even bad points. Therefore, it is common for the sensor integrated or backend ISP to have the relevant settings for noise reduction capabilities. 1.1.1 Start-up time according to the reason of the formation of noise, mainly AG or exptime after a certain value to start the noise reduction function, it is usually necessary to determine the threshold of the two parameters, too small and too much bad. Noise reduction from the following approach will be seen, the noise potential accompanying the image quality degradation, so early start noise reduction function, do not need to do noise reduction process not only increases the burden on the processor or ISP, but also may backfire. And the late start noise reduction function, when it was originally needed, does not play a corresponding role. 1.1.2 Determination principle and processing method so how to determine whether a point is a noise point. We start from the discussion of how people recognize noise, and for the human eye, to judge a point is a noise, no difference is that the brightness or color and the edge of most of the point differences too large. From the noise generation mechanism, the color anomaly should always be accompanied by the brightness of the anomaly, and the brightness of the abnormal processing work than the color anomalies, so usually the sensor ISP decision principle is a point of brightness and the brightness of the surrounding point is greater than a threshold value when the point is considered a noise point. The way to handle this is usually to replace the original value with the average of the surrounding points, which does not increase the amount of information, similar to a fuzzy algorithm. For high-end digital cameras, with a strong image processing chip, in the determination and processing of whether there are more complex algorithms, it is also possible to estimate. For example, brightness and color synthesis as a standard to determine the noise, using a larger computational interpolation algorithm to compensate, for sensor inherent bad point, noise, the use of shielding the way to abandon its data (Nikon is doing so, other manufacturers should be so) and so on. 1.1.3 effect for mobile phone sensor, the role of this noise reduction process, I personally think it should be very limited, after all, compared to digital cameras, cell phone sensor lens is too small, the light is small, so its benchmark AG is bound to be larger than the gain of the camera (such as the average home digital camera ISO800 level), so as to achieve the same brightness, so the impact of current noise will be much larger. In this way, even if the best situation, the noise will be a lot of fluctuations in the data itself is very large, which will cause us in the mobile phone photos are bound to see the dense flower point, if all do average, reduce the noise, the image will become blurred, so the cell phone noise judgment threshold will be set relatively high, so as to avoid too large, Blurred theOverall image. In this way, the data itself is poor, the second is the standard of noise reduction, resulting in poor overall results. 1.2 Digital zoom digital zoom can have two forms: one, is through the interpolation algorithm, the image interpolation, the size of the image to expand to the required specifications, this algorithm is not ideal for its effect, especially when used on the phone, the camera itself to get the data on the phone has a greater noise, interpolated, the resulting image is almost impossible to use. In fact, even digital camera digital zoom function does not have much practical value. If the interpolation algorithm does not have hardware support, it needs to be implemented at the application level. Our digital zoom for a platform is the way. Second, in fact, is a pseudo-digital zoom form, when the camera is not in the maximum resolution format, such as 1.3 million pixels of sensor using 640*480 specifications, still set sersor work in 1280*960 resolution, Then the image of the central part of the collection to obtain 640*480 photos, so that on the mobile phone looks like the size of the object is magnified one times. There are many mobile phones using this digital zoom, this approach requires little additional algorithm support, the image quality has no effect, the disadvantage is that only small size can be used. In addition, in the DV mode can also realize the so-called digital zoom magnification shooting function. (This should be a selling point, for DV, this digital zoom still has practical significance)
To use this zoom mode, the driver needs to support the windowing function to obtain the desired portion of the sensor image data.
1.3 Strobe suppression function
1.3.1 What is strobe
Daily use of ordinary light sources such as incandescent lamps, fluorescent lamps, quartz lights, etc. are directly used 220/50hz AC power, every second, positive and negative half-week change 50 times, resulting in light in 1 seconds in the blink of a 50x2, coupled with the instability of the mains voltage, flickering lights, This creates the so-called "strobe".
The following table shows the light intensity fluctuations of several light sources:
Because the human eye has certain hysteresis and adaptability to the light intensity change, it is usually not visible the brightness change of the source. But it still increases the fatigue level of the eye. So the market will have so-called non-strobe sales.
Suppression of 1.3.2 on strobe
For camera sensor, there is no hysteresis and adaptation of the human eye, so the change in brightness of the light source is more sensitive. If not suppressed, in preview and DV mode, there may be noticeable changes in the brightness of the image flashing.
How to solve it. Considering the periodicity of strobe, in a period, the cumulative value of light source brightness should be roughly the same, so, if the control exposure time is the whole multiple of the strobe period, then the brightness of each frame image is generally consistent, so that can effectively suppress the effect of strobe on the image brightness.
Therefore, in AE mode, the sensor adjusts the exposure time to the full multiples of its cycle, depending on the frequency of the strobe. Because of the different frequency of alternating current, so there are 50hz/60hz points.
When the relevant sensor registers are set, the number of clock cycles corresponding to the strobe period is calculated according to the current frequency and sensor clock frequency, resolution, etc.
