Gradual illumination changes: It is desirable that background model adapts to gradual changes of the appearance of the environment. For example in outdoor settings, the light intensity typically varies during day.
Sudden illumination changes: Sudden once-off changes are not covered by the background model. They occur for example with sudden switch of light, strongly affect the appearance of background, and cause false positive detections.
Dynamic background: Some parts of the scenery may contain movement, but should be regarded as background, according to their relevance. Such movement can be periodical or irregular (e.g., traffic lights, waving trees).
Camouflage: Intentionally or not, some objects may poorly differ from the appearance of background, making correct classification difficult. This is especially important in surveillance applications.
Shadows: Shadows cast by foreground objects often complicate further processing steps subsequent to background subtraction. Overlapping shadows of foreground regions for example hinder their separation and classification. Hence, it is preferable to ignore these irrelevant regions.
Bootstrapping: If initialization data which is free from foreground objects is not available, the background model has to be initialized using a bootstrapping strategy.
Video noise: Video signal is generally superimposed by noise. Background subtraction approaches for video surveillance have to cope with such degraded signals affected by different types of noise, such as sensor noise or compression artifacts.