# Revision history [back]

### How does the information from a urdf-file get to the code in cpp-file?

Hi,

I'm trying to understand how ros and gazebo work. I'm having a hard time with understanding how the information I specified in the urdf-file is used.

It seems that in urdf2gazebo.cpp the information is written to an object of type GazeboExtension. What I don't understand is what happens next. Like for example I set <laserRetro>0.9</laserRetro> in the urdf file and later the value appears in RayShape.cpp void SetRetro(float retro). But what are the steps in between? What is the file that actually calculates when a ray hits a surface?

I would be very thankful for any help I can get.

Cheers