# SoftBank Robotics documentation What's new in NAOqi 2.8?

# ALBacklightingDetection

NAOqi Vision - Overview | API

## What it does

ALBacklightingDetection extractor checks if the images returned by the camera are backlit or not. Backlighting occurs when there is high contrast between a light source (e.g. a window) and the observed scene.

The following backlighting value is computed for each frame:

- 0: no backlight
- 1: possible backlight
- 2: backlight identified

This value is stored in the ALMemory. When a backlighting situation is detected for the first time (i.e the backlighting value just turned to 2) an event is raised.

## How it works

In order to compute the backlighting value, the extractor:

1. analyses in the current frame if more than 2% of pixels are clipped (a phenomenon that occurs when a pixel sensor receives more light that it can “store”, resulting in a ‘255’ value after digital conversion for that pixel).
2. checks if more than 60% of pixels present a luminance below ‘70’.

The final result is a weighted combination of both the percentage of pixels at ‘255’ and the percentage of pixels below ‘70’.

## Performances and Limitations

The camera AutoExposition parameter (and the AutoGain parameter, if it exists) of the camera must be activated for this module to work properly.

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