We read the default bias by the provided software and obtain the default pos_thres=102%，neg_thres=73%. It's very curious. If we set the intensity of two images as I1 and I2. We can get a positive event only if I2 - I1 > 102% * I1. It seems not very reasonable. If the intensity range in 0~1, for pixels with intensity larger than 0.5, we can never get a positive event. 

Thus I'm curious about the default biase value. I do not know where I misunderstand.