Camera Coordinate Calibration Overview

###Let's say we have a photo with three plants in it whose locations and sizes we need to detect:
These example images were created with rotation to simulate a rotated camera. Background grid shown for reference.

Assumptions:

• We always know the coordinate location of the camera, since it is CNC-controlled.
• The image center is directly below the camera.
• The camera is the same distance away from the calibration objects and the objects to detect.
• The pixels in the image the camera takes are square.

Step 1: Take calibration image of known objects

With the (imaginary) camera at the same height from the subjects, we take a photo of two calibration dots of known spacing:

These example images were created with rotation to simulate a rotated camera. Background grid shown for reference.

We know that these objects are 1000 units apart and that they are square with our desired coordinate system.

Note: Camera coordinates are not needed for calibration. Calibration does require the orientation of the coordination objects to be known: that they are along the x-axis and that the origin is in the lower left.

Step 2: Run calibration

Using our known values, we determine that the camera is rotated 5 degrees from the coordinate system, and determine that the pixel to coordinate conversion scale is 1.72.

Step 3: Determine locations and sizes of "plants" in original image:

Using the calibration parameters determined in step 2, and the known location of the camera in this image (800, 500), we find the locations and sizes of the "plants":

X (calculated)	Y (calculated)	dia. (calculated)	X (actual)	Y (actual)	dia. (actual)
300	299	102	300	300	100
898	601	104	900	600	100
99	799	104	100	800	100