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.