ASX340AT_16 Datasheet, PDF (15/54 Pages) ON Semiconductor – 1/4‐inch Color CMOS NTSC/PAL Digital Image SOC

ASX340AT
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Description
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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