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:
- 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).
- 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.