Re: View port vs. view plane camera (was Re: Quesa - "Most Wanted" list)

[Edward K. C. <ek...@lg...>]

On Jun 3, 2004, at 17:52, James W. Walker wrote:

> That isn't how I'd do it.  I'd set up my camera with a wide field of 
> view, so that if the view port were at maximum, the camera could see 
> everything.  Then I'd set the view port to be something smaller.  To 
> pan around, I would then be sliding the view port, but not changing 
> its size nor moving the camera.

Well, what complicates things for me is that my projection model is 
rather rigidly defined (it's a geophysical modelling app which can 
incorporate survey data).  Whatever I decide to do with view ports, 
there are two properties which must always be satisfied: the scale of 
the rendered image must match real world units as much as possible 
(especially on a printout), and the amount of perspective should be a 
function of the observer's distance from the scale model.

So for example, the default scale factor is 1 cm = 100 m and the 
default viewing distance is 40 cm (in other words, your eye would be 
about 16" from the monitor).  That means a 1 km long survey line lying 
directly on the view plane (which corresponds to the plane of the 
monitor) should look about 10 cm long.  If it was closer to the camera 
by say 5 cm, it should look longer to the extent that an object 
hovering 5 cm in front of the screen would look larger to the human eye 
only 35 cm away.

Now, I am assuming the only way to adjust the perspective effect is to 
play with the field of view, so I think I need to stick to whatever 
value I have calculated for it.  On the other hand, I think there are 
several ways you can manipulate the scale of the image.  Reducing the 
distance of the camera from the view plane or shrinking the size of the 
view port should both have a linear dilation effect.  I suppose you 
could also do it at the geometry level with a scaling transform.

So if I understand these view ports correctly, I should be able to 
compensate for changes in view port dimensions by changing something 
else at the same time like the camera distance, and there should be no 
distortion of the image in either size or perspective.  Correct?

-Ted