AR0331 datasheet(20/52 Pages) ONSEMI

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AR0331

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HIGH DYNAMIC RANGE MODE

By default, the sensor powers up in HDR Mode. The HDR

scheme used is multi-exposure HDR. This allows the sensor

to handle up to 100 dB of dynamic range. In HDR mode, the

sensor sequentially captures two exposures by maintaining

two separate read and reset pointers that are interleaved

within the rolling shutter readout. The intermediate pixel

values are stored in line buffers while waiting for the two

exposure values to be present. As soon as a pixel’s two

exposure values are available, they are combined to create

a linearized 16-bit value for each pixel’s response.

Depending on whether HiSPi or Parallel mode is selected,

the full 16 bit value may be output, it can be compressed to

12 bits using Adaptive Local Tone Mapping (ALTM), or

companded to 12 or 14 bits.

Adaptive Local Tone Mapping

Real- world scenes often have a very high dynamic range

(HDR) that far exceeds the electrical dynamic range of the

imager. Dynamic range is defined as the luminance ratio

between the brightest and the darkest objects in a scene.

Even though the AR0331 can capture full dynamic range

images, the images are still limited by the low dynamic

range of display devices. Today’s typical LCD monitor has

a contrast ratio around 1000:1 while it is not atypical for an

HDR image having a contrast ratio of around 250000:1.

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. The

AR0331 has implemented an adaptive local tone mapping

(ALTM) feature to reproduce visually appealing images that

increase the local contrast and the visibility of the images.

When ALTM is enabled, the gamma in the backend ISP

should be set to 1 for proper display.

See the AR0331 Developer Guide for more information

on ALTM.

Companding

The 16-bit linearized HDR image may be compressed to

12 bits using on-chip companding. Figure 19 illustrates the

compression from 16- to 12-bits. Companding is enabled by

setting R0x31D0. Table 10 shows the knee points for the

different modes.

Figure 19. HDR Data Compression

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16-bit Code Input

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