AP0101CS datasheet(19/42 Pages) ONSEMI

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AP0101CS/D Rev. 7, 1/16 EN

©Semiconductor Components Industries, LLC,2016.

AP0101CS HDR: Image Signal Processor (ISP)

Image Flow Processor

Adaptive Local Tone Mapping (ALTM)

Real world scenes often have very high dynamic range (HDR) that far exceeds the elec-

trical dynamic range of the imager. Dynamic range is defined as the luminance ratio

between the brightest and the darkest object in a scene. In recent years many technolo-

gies have been developed to capture the full dynamic range of real world scenes. For

example, the multiple exposure method is a widely adopted method for capturing high

dynamic range images, which combines a series of low dynamic range images of the

same scene taken under different exposure times into a single HDR image.

Even though the new digital imaging technology enables the capture of the full dynamic

range, low dynamic range display devices are the limiting factor. Today’s typical LCD

monitor has contrast ratio around 1,000:1; however, it is not atypical for an HDR image

having contrast ratio around 250,000:1. Therefore, in order to reproduce HDR images on

a low dynamic range display device, the captured high dynamic range must be

compressed to the available range of the display device. This is commonly called tone

mapping.

Tone mapping methods can be classified into global tone mapping and local tone

mapping. Global tone mapping methods apply the same mapping function to all pixels.

While global tone mapping methods provide computationally simple and easy to use

solutions, they often cause loss of contrast and detail. A local tone mapping is thus

necessary in addition to global tone mapping for the reproduction of visually more

appealing images that also reveal scene details that are important for automotive safety

and surveillance applications. Local tone mapping methods use a spatially varying

mapping function determined by the neighborhood of a pixel, which allows it to

increase the local contrast and the visibility of some details of the image. Local methods

usually yield more pleasing results because they exploit the fact that human vision is

more sensitive to local contrast.

ON Semiconductor’s ALTM solution significantly improves the performance over global

tone mapping. ALTM is directly applied to the Bayer domain to compress the dynamic

range from 20-bit to 12-bit. This allows the regular color pipeline to be used for HDR

image rendering.

Color Interpolation

In the raw data stream fed by the sensor core to the IFP, each pixel is represented by a 20-

or 12-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 ALTM, preserve

the one-color-per-pixel nature of the data stream, but after ALTM 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. The color correction matrix can be either programmed by

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