The scope of this article is to take you through the 3A functions which are critical part of the ISP pipeline.
The 3A functions stand for:-
1. Auto focus
2. Auto exposure
3. Auto white balance
Auto-focus ISP calculates the correct focus by performing analysis of the image entering the optical system.
The 2 main popular methods used by ISPs to achieve auto-focus are
Contrast Measurement method
Contrast detection based auto-focus is performed by measuring the contrast within the sensor. The intensity difference between adjacent pixels of the sensor is measured. For an image that is perfectly in focus, the intensity difference is the highest. The optical system is adjusted until the maximum contrast is detected. This method does not provide us any direction or distance measurement. Thus, this method is tricky when we are looking at moving objects as this method wont be able to detect if the object is moving towards or away from the camera.
Phase Detection Method
Phase detection method is performed by dividing the incoming light into pairs of images and comparing them. In CMOS sensors, this is implemented with the assistance of microlenses. Two microlenses capture the light entering the camera from opposite side of the lens and divert it to the sensor. The 2 images are calculated for similar light intensity patterns and the separation error is calculated in order to identify whether the object is in front focus or back focus position. This method helps is determining the direction and estimation of movement of lens assembly required to reach focus position.
The most common AF implementation on embedded camera modules is the contrast based Auto-Focus.
Auto-exposure is a feature that is implemented on the digital cameras that enables the camera to automatically determine the correct exposure for capturing images. The ISP works with the sensor to detect the current luminance level and then it controls the gain level and exposure time to keep the brightness of the image constant.
Auto White Balance
As we discussed before, the sensor, ISP and rest of camera components aim to capture the image and present it to the user in a format that enables the use to perceive the scene as how a human eye perceives. The human eye is, however, very color flexible and it enables us to sense the original color of the objects even when the illuminating light source changes. In order to acquire natural images, the camera also needs to have this function that corrects the color depending on the illuminating source. This is called White Balance function. The ISP taking care of this function without user intervention is called Auto-White Balance.
These are the 3A Functions that are critical part of the ISP pipeline. The quality of the 3A functions depends on the ISP performance, Sensor-ISP compatibility, Lens selection and so on. Thus, when selecting the camera configuration, we need to have a clear understanding on the performance expected from the camera to satisfy that use case and then analyze if the sensor-ISP-Lens combination satisfies the requirement.
Our expert team at Regami Solutions shall be able to assist you with the same.
Please feel free to get in touch with us in case of any queries.