1. Field of the Invention
The present invention generally relates to image processing, and more particularly to a black level calibration method and system.
2. Description of the Related Art
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Image sensors such as CMOS or CCD sensors are made up of an array of individual pixels, each of which collects photons incident on the image sensor. The number of photons collected in each pixel is converted into an electrical charge by a photodiode and this charge is then converted into an analog voltage, which may be amplified, adjusted, and converted to a digital value by an analog-to-digital converter, so that the information obtained from the individual pixels can be processed, usually by a digital signal processor, into a final digital image.
Most image sensors require some form of calibration before use so that the data obtained from the image sensor can be used to produce digital images that faithfully reproduce the optical characteristics (e.g., intensity and color) of the scene or object whose image was captured. One type of calibration is referred to as black level calibration, which effectively sets a threshold below which digital data values obtained from the image sensor will be considered to represent a black level, or to represent the absence or substantial absence of light. Accurate black-level calibration helps to achieve a digital picture with full contrast and subtle details in dark shadow regions. If the black level is too low, information in dark areas may be lost. Conversely, if the black level is too high, signal range may be sacrificed.
In conventional systems, a border of an image-sensing array is surrounded with a number of rows and columns of light shielded, or black, pixels. These pixels provide black reference information or black pixel data to stabilize downstream image processing and establish the correct value for black in the output image.
Calibration purely in the digital domain reduces the range of the system and reduces image quality. On the other hand, to accomplish high resolution and a wide calibration range simultaneously in the analog domain, existing solutions often involve circuits with large size and high power consumption.