"clipka" <nomail@nomail> wrote:
> "RC" <nomail@nomail> wrote:
> > My understanding is
> > that my clock values are equal divisions of my final frame value,
>
> Yes, that is right.
>
> > so with that
> > in mind if I have a 720 or 1920 pixel wide image rendering and I want to move
> > my object one pixel/frame, then all I would need to do is make my final frame
> > equal to the pixel width and use my clock value as my one pixel/frame motion
> > value.
> >                  translate <clock,0,0>
>
> Actually this is the wrong math already. It only holds true if your camera
> happens to be 1.0 POV units away from the object's trajectory with an opening
> angle of 90 degrees - or a suitable other distance if the opening angle is
> different.
>
> > BUT, what if I am not exactly <0,0,0> in my camera position, and view? How can I
> > calculate how much my clock is moving an object? Are there any other ways to do
> > this?
>
> Simple advice: Try to get away from pixel-based thinking. It's almost never the
> natural units to work with in POV, and it's incompatible with changes of the
> image resolution.
>
> As for the math: With the most common camera settings
>
>   camera {
>     perspective
>     location  MyCameraLocation
>     right     x*image_width/image_height
>     up        y
>     direction z
>     look_at   MyObjectLocation
>     angle     MyCameraAngle
>   }
>
> you can convert from POV units and vice versa using the following formulae (see
> diagram in section 3.3.1.1 "Placing the Camera" of POV's online help):
>
>   (1) tan(MyCameraAngle/2) * unitDistance = 0.5
>   or  unitDistance = 0.5 / tan(MyCameraAngle/2)
>
>   (2) pixels / image_width = size / (distance/unitDistance)
>   or  pixels = image_width * size / (distance/unitDistance)
>   or  size = (pixels/image_width) * (distance/unitDistance)
>
> Formula (1) describes the distance at which 1 POV unit will appear to be equal
> to the image width, while (2) describes the relation between size in pixels and
> size in POV units.

This is helpful thanks. I hate to admit it but so far to get this to work, I've
made marks with a dry erase marker on my LCDs to determine how far my objects
are moving between frames in front of a grid pattern. Ya, not so accurate...

Post a reply to this message