AN2676 Freescale Semiconductor / Motorola, AN2676 Datasheet - Page 6

• Manufacturer Part Number: AN2676
• Description: Image Capture with i.MX21
• Manufacturer: Freescale Semiconductor / Motorola

Bayer Statistics
6.1 White Balance
According to the Grey World model, to give a correct white color, the average (or sum) of every color in
an image should be equal. This is done by applying multipliers on 2 of the 3 primary color pixels. Usually
the green pixel is taken as the reference and used to generate the coefficient for blue and red.
6.2 Exposure Control
Exposure time is adjusted to a value that produces the greatest dynamic range with minimum noise level.
The adjustment is made at regular time intervals until the mean value of a specific color hits a target value.
For details, see the data sheet of the particular sensor.
6.3 Statistic Block Operation
The Bayer statistics are operated on image blocks. The image is divided into several square blocks; each
block has one set of statistics data output. One set of statistics includes Sum of Red, Sum of Green, Sum of
Blue, and Sum of Absolute Green Data (SOAD). Sum of Red, Green, and Blue are used in white balance
and exposure control, while SOAD is only used in focus control.
The image block size is pre-defined according to different live view resolutions (LVRM). The number of
blocks per row and per column is also pre-defined, which limits the aspect ratio (image width ÷ image
height) that is supported. To fit different image sizes, a line-skipping scheme is employed. Adjacent blocks
are separated by skipped lines. The number of skipped lines is programmable through register CR2.
Horizontal and vertical line skipping can be programmed independently. The rule is that sum of block size
plus the number of lines skipped must be equal to the image size.
The maximum supported resolution is equal to the maximum block size plus the skipped lines. For
LVRM = 0, HSC = 256, VSC = 256, the maximum resolution is 2560
Using the VGA case as an example, based on LVRM = 0, and HSC = VSC = 16, we have the results
shown in Figure 5.
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