Why is there only 19 categories in cityscapes_colors.txt and no background categories?

[Dumbledore-bit]

Thank for your contribution！And I have some questions I want to ask you。
Why is there only 19 categories in cityscapes_colors.txt and no background categories?
Why is it not necessary to calculate the loss function of the background class in calculating the loss function?
In the code, you set the value of the background to 255 and set ignore_label = 255. I think you ignore the calculation of the loss function of the background, and the output channel of the network is also 19 types, and there is no background prediction. Is this prediction chart accurate? It is also possible that I have overlooked the details of some code processing, and please provide guidance accordingly.