Passing the output of a model as input to an other model , computer vision? i want to try my hands at a project where i have to detect the brands + the models of sportswears worn by persons participating to a marathon , the project is quite complex , so my first approach would be to use an already pre-trained model that will draw the bounding box of the persons , then use the outputed bonding boxes as input to an other model that will draw me the bounding box for the shirt , shoes , shorts ...ect , and follow this procedure until i get to the brand and then the model of the sportswear.
theorically , it's super simple , the problem is that , i'm having a hard time figuring out how to implement it by code , passing the output of one model as an input to an other model , is there any good exemples on the internet can illustrate that approach , by code i mean ?

Any suggestion would be much appreciated , thank you.
I find your question a little hard to understand. If I understand correctly you know how to obtain a bounding box for a person.
Using these coordinates, you can select this rectangle using open-cv or matplotlib or any image processing library. 
If your second model can handle multi-resolution or multiple sizes you can just use this selection as input.
If, on the other hand, your second model requires a fixed size you can resize the selection and use padding if necessary.