[gameprogrammer] Re: culled objects and shadow volumes
- From: Sami Näätänen <sn.ml@xxxxxxxxxxxxxxx>
- To: gameprogrammer@xxxxxxxxxxxxx
- Date: Wed, 27 May 2009 16:04:28 +0300
On Friday 01 May 2009 03:37:21 Alan Wolfe wrote:
> interesting about the point lights, you are right.
>
> Someone suggested i make use of the vornoi regions of the frustum to find
> the closest point of the ray (ie get the closest point in each region then
> take the winner overall)
>
> Your projection of the corners onto the ray sounds intriguing (:
Did you find a solution to this?
And if you did, was it efective enough?
> On Thu, Apr 30, 2009 at 5:19 PM, Sami Näätänen <sn.ml@xxxxxxxxxxxxxxx>wrote:
> > On Wednesday 29 April 2009 19:09:50 Alan Wolfe wrote:
> > > Hey Sami,
> > >
> > > Here's what i was thinking for my final solution tell me what you
> >
> > think...
> >
> > > (heavily based on your idea, with a neat little twist for positional
> > > lights!)
> > >
> > > * when an object is first known to be frustum culled, iterate through
> > > all the lights that light the object (this is known inexpensively how i
> > > have
> >
> > it
> >
> > > set up)
> > > * then for each light:
> > > * Get the vector from the light source to the object (ie starting
> > > point and a vector) and make a ray from that vector, but using the
> > > objects location as the origin of the ray (or, for directional lights
> > > of course just use the direction vector as the vector)
> > > * Find the ray's closest point to the frustum
> > > * If the ray intersects the frustum you know it casts a shadow into
> > > the frustum so remember that you need to render the shadow for this
> > > object
> >
> > and
> >
> > > continue onto the next light (a flag is kept for each of the possible
> > > lights, up to 8 total)
> > > * If it doesn't intersect, check the distance of the closest point to
> >
> > the
> >
> > > frustum
> > > * If the light is directional, compare this distance to the bounding
> > > sphere radius. If it's closer than the bounding sphere radius, it
> > > could cast a shadow into the frustum so remember that you need to
> > > render the shadow for this object.
> > > * If the light is positional, calculate the "Radius Expansion
> >
> > Factor"...
> >
> > > you get this by figuring out the distance of the object to the light
> > > and dividing the sphere radius by that distance. What you have then is
> > > the knowledge of how big the shadow's radius will be along the ray at
> > > any distance from the light source. (ie it's the ratio that the radius
> > > of the shadow grows linearly over distance)
> > > * Get the distance along the ray where the closest point to frustum
> > > is and multiply that distance by the radius expansion factor to get the
> >
> > radius
> >
> > > of what the shadow is at that distance. Check if that point on the ray
> >
> > is
> >
> > > less than the expanded distance. If it is, then you know you have to
> > > render the shadow
> > >
> > > What do you think, does that sound decent?
> >
> > One thing to consider.
> >
> > To catch all possibilities you need two tests for positional lights.
> > 1. Nearest position to the frustum.
> > 2. Furthest possible point from the object so that a normal plain in this
> > point "touches" the frustum.
> >
> > Example of the case 2: (Frustum marked as a square to make this more
> > clear) '.' is light source
> > 'O' is the bounding sphere of an object.
> > 'f' is area inside the frustum.
> > 'F' marks the area inside the frustum where the shortest distance point
> > from
> > the light ray can be found.
> > 'S' is a special point.
> >
> > Using the nearest point test here tests at the corner marked with a F.
> > This is correct.
> > fffff
> > fffff
> > ffffF
> > . O
> >
> > But the next is not, even if the vector line would go a litle bit more
> > away from the frustum, than this apparently straight line down from the
> > light source through the center of the bounding sphere.
> > .
> > O
> > ffffF
> > fffff
> > ffffS
> >
> > This is because the expansion factor of the bounding sphere is higher
> > than the
> > factor of the change in distance. The correct testing point is in the
> > area marked with an S.
> >
> > > Hey also, i've been scouring my books (especially "real time collision
> > > detection") and the net and have been unable to find an algorithm for
> > > finding the closest point on a ray to a frustum. Would you happen to
> >
> > have
> >
> > > any hints there? (:
> >
> > I think you have to split it as separate planes and then test the
> > distance to
> > these planes. You of course need to restrict the planes, which I think is
> > the
> > hardest part.
> >
> > For finding the furthest possible test point touching the frustum, I
> > think you
> > could project the frustum corners to the line of the ray.
> >
> > This projection could be usefull for the first problem too, but I'm not
> > sure,
> > and I really don't have time now to put in to this. Although this is very
> > interesting problem.
> >
> >
> >
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