I am modelling a fisheye lens for Blender using a mirror-ball and an orthogrpahic camera as shown below:
Black circle represents the mirror and red lines the orthographic camera and its output/incoming rays. The camera records the environment reflected on the mirror-ball, which gives an equisolid fisheye effect to the final image rendered.
The thing is that I have certain parameters of a lens and need to model the group camera+mirror accordingly as a fisheye equisolid. Using the equisolid projection equation I can calculate the radius of the ball, but I have doubts about the orthographic scale of the camera. I do not know what should it be in order to obtain the desired results.
For instance, I know that the sensor is 648x488px (4.032mm horizontal size), focal length = 1.134mm and FOV = 160deg (80+80). Then, radius of the ball should be r = 2.268mm according to the following equation:
$ Radius = 2*FocalLength*sin(AngleMax/2) $
I think that the orthographic scale is related to the sensor size and the radius probably, but I would appreciate if someone could help me work this out.
I have found the mirror-ball designed in the internet, but notice that I want to know how to design it myself so that I can create different ones for each lens.
I have the following setup:
Blue and red lines are separated 5 degrees and cover a range of 180 degrees. Yellow cylinder is located at 98 degrees from what I consider the center (negative y-axis, which goes into the half colored ball).
I place in the middle of the scene (where x-axis and y-axis cross) two systems:
1. A fisheye equisolid camera with certain parameters that has 196 degrees of horizontal field of view when rendering in cycles. The render of this camera:
2. A mirror ball on the center with an orthographic camera pointing at it. When rendering with Bledner Render (image below) the objective is to obtain the same image obtained in system 1 with Cycles. In other words, I need to design the mirror ball and orthographic camera pair in a way that the 196 degrees requirement is fulfilled and there is a very similar equisolid distortion (I know it will not be exact because of the nature of the procedure, but a good approximation is okey to me).
I am not sure which is the relation between the radius of the mirror ball and the orthographic camera scale (if there is any). I do not know how to calculate the radius of the ball in order to obtain the desired results or if it is an arbitrary value. I have found that radius_ball/(horizontal_sensor_size/2) = focal_length but I am not really sure about this. My question is how to design the mirror ball camera system so that the results are the desired ones.
I have found a pair of radius-orthographic scale values that do a pretty good job (see images above) but I would like to know the whole mathematics behind this and if my assumption is true.