title: Calibration for pixels to real world unit
body: <p>I am doing a project on parts inspection by measuring different geometric properties of an object(<a href="https://i.stack.imgur.com/Rz6BX.jpg" rel="nofollow noreferrer">drill bit</a>) right now I am using a circle to calibrate from pixel to mm but since the <a href="https://i.stack.imgur.com/jVazr.jpg" rel="nofollow noreferrer">platform</a> on which it is placed is at an angle of 20 degrees so now there is a lot of error. Is there another way of doing calibration for pixel to mm. I have done the camera calibration and I got the camera matrix, distance from the object to the camera is known, what else is required to get this calibration? </p>
  
  <p>By the algorithm, I already tried when the platform is flat I am able to measure the total length(actual=120mm) of the <a href="https://i.stack.imgur.com/xVmBp.jpg" rel="nofollow noreferrer">bit</a> to be 120mm +- 0.9mm but when the platform is angled it is difficult to get the accuracy of 0.01mm </p>
  
  <p>I expect the output to be accurate to 1/10 mm but now I am getting a range from 1-1.5mm. A proper explanation with equation/code would be helpful</p>