title: computer vision real world size pixel size, opencv
body: <p>My goal is to mount a web camera on a ground vehicle robot and have it recognize lanes and drive in between the lanes. In order to look for the appropriate size lines(lanes) in an image, I am trying to learn how to convert the size of a real world object to image pixel size.</p>
  
  <p>If I mount a usb camera so that its height above the ground is a known fixed vertical distance from the ground, and the angle between the center of the camera lens is fixed, how do I relate the size of a real world object to the number of pixels in the image.</p>
  
  <p>Example: My camera is two feet above the floor. If I define the y axis as pointing upward from the floor, my camera is on the y axis at real world coordinates (x = 0, y = 2 feet, z = 0). It has an angle of 15 degrees below the horizontal xz plane, meaning my camera is pointed downwards toward the floor.</p>
  
  <p>I take an image of some bright green tape on the floor. The tape is 3 feet long and 2 inches wide. How do I translate an object of 3 feet long and 2 inches wide to pixel dimensions? How do I calculate how many pixels represent 3 feet in my image? I am trying to recognize lines of the approximate size.</p>
  
  <p>Thank you in advance for your help.</p>