ASX340AT datasheet(15/54 Pages) ONSEMI

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ASX340AT

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TABLE 9. PAL (CONTINUED)

Line Numbers

Description

Field

311−312

Blanking

313−335

Blanking

336−623

Active video

624−625

Blanking

Black Level Subtraction and Digital Gain

Image stream processing starts with black level

subtraction and multiplication of all pixel values by a

programmable digital gain. Both operations can be

independently set to separate values for each color channel

(R, Gr., Gb, B). Independent color channel digital gain can

be adjusted with registers. Independent color channel black

level adjustments can also be made. If the black level

subtraction produces a negative result for a particular pixel,

the value of this pixel is set to 0.

Positional Gain Adjustments (PGA)

Lenses tend to produce images whose brightness is

significantly attenuated near the edges. There are also other

factors causing fixed pattern signal gradients in images

captured by image sensors. The cumulative result of all these

factors is known as image shading. The ASX340AT has an

embedded shading correction module that can be

programmed to counter the shading effects on each

individual R, Gb, Gr., and B color signal.

The Correction Function

The correction functions can then be applied to each pixel

value to equalize the response across the image as follows:

Pcorrected (row,col)=Psensor(row,col)*f(row,col)

(EQ 1)

where P is the pixel values and f is the color dependent

correction functions for each color channel.

Color Interpolation

In the raw data stream fed by the sensor core to the IFP,

each pixel is represented by a 10-bit integer number, which

can be considered proportional to the pixel’s response to a

one-color light stimulus, red, green, or blue, depending on

the pixel’s position under the color filter array. Initial data

processing steps, up to and including the defect correction,

preserve the one-color-per-pixel nature of the data stream,

but after the defect correction it must be converted to a

three-colors-per-pixel stream appropriate for standard color

processing. The conversion is done by an edge-sensitive

color interpolation module. The module pads the incomplete

color information available for each pixel with information

extracted from an appropriate set of neighboring pixels. The

algorithm used to select this set and extract the information

seeks the best compromise between preserving edges and

filtering out high frequency noise in flat field areas. The

edge threshold can be set through register settings.

Color Correction and Aperture Correction

To achieve good color fidelity of the IFP output,

interpolated RGB values of all pixels are subjected to color

correction. The IFP multiplies each vector of three pixel

colors by a 3 x 3 color correction matrix. The three

components of the resulting color vector are all sums of three

10-bit numbers. Since such sums can have up to 12

significant bits, the bit width of the image data stream is

widened to 12 bits per color (36 bits per pixel). The color

correction matrix can be either programmed by the user or

automatically selected by the auto white balance (AWB)

algorithm implemented in the IFP. Color correction should

ideally produce output colors that are corrected for the

spectral sensitivity and color crosstalk characteristics of the

image sensor. The optimal values of the color correction

matrix elements depend on those sensor characteristics and

on the spectrum of light incident on the sensor. The color

correction parameters can be adjusted through register

settings.

To increase image sharpness, a programmable 2D

aperture correction (sharpening filter) is applied to

color-corrected image data. The gain and threshold for 2D

correction can be defined through register settings.

Gamma Correction

The ASX340AT includes a block for gamma correction

that can adjust its shape based on brightness to enhance the

performance under certain lighting conditions. Two custom

gamma correction tables may be uploaded corresponding to

a brighter lighting condition and a darker lighting condition.

At power-up, the IFP loads the two tables with default

values. The final gamma correction table used depends on

the brightness of the scene and takes the form of an

interpolated version of the two tables.

The gamma correction curve (as shown in Figure 11) is

implemented as a piecewise linear function with 19 knee

points, taking 12-bit arguments and mapping them to 8-bit

output. The abscissas of the knee points are fixed at 0, 64,

128, 256, 512, 768, 1024, 1280, 1536, 1792, 2048, 2304,

2560, 2816, 3072, 3328, 3584, 3840, and 4096. The 8-bit

ordinates are programmable through registers.

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