Still working my way thru the tutorial...

I have the camera and light_source inside a white box.
When I put the light_source close to one side of
the box, that side is rendered very dark.

This really surprises me.
Could someone please explain why this is, so that
I can anticipate this kind of behavior in the future?

The doc. says:
A pointlight ... illuminates  everything in the scene
equally no matter how far away from the light source
it may be.

Here is the .pov file:
-------------------------
#include "colors.inc"
camera {
    location <-10, 8, -19>  look_at <0, 5, 0>  angle 75
}
light_source { <-.3, 1, .3> color rgb 0.5 atmosphere off }

box { <-20,  0, -20>, <20, 20, 20>
    pigment { color White }
    finish { ambient 0.2 diffuse 0.5 }
}
// x-y-z axes
cylinder { -100*x, 100*x, .03  pigment {Gold} finish {ambient .8}}
cylinder { -100*z, 100*z, .03  pigment {Gold} finish {ambient .8}}
cylinder { -100*y, 100*y, .03  pigment {Gold} finish {ambient .8}}
--------------------------
Note how much darker the floor is.
If you move the light to <-.3, 19, .3>, the ceiling will be dark.
Change the value of "diffuse" and things don't change much.
Change the value of "ambient" and the floor is still
a lot darker than the walls.

Alan

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Hello Alan.

You have to remember that because the light it so near the floor (or
wall) that the rays will fall on it at a very shallow angle so it will
not be very bright except very close to the light itself. The rays will
almost be parallel to the floor. The light source does illuminate
everything equally in that the brightness of the light does not diminish
over distance unless you tell it to. But the angle the light falls on an
object affects how much the object is illuminated.

Hope that makes sense. (I hope I'm right! :-))

Mark.

yan### [at] pacbellnet wrote:
> 
> Still working my way thru the tutorial...
> 
> I have the camera and light_source inside a white box.
> When I put the light_source close to one side of
> the box, that side is rendered very dark.
> 
> This really surprises me.
> Could someone please explain why this is, so that
> I can anticipate this kind of behavior in the future?
> 
> The doc. says:
> A pointlight ... illuminates  everything in the scene
> equally no matter how far away from the light source
> it may be.
> 
[snip]

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The reason has already been explained, but to elaborate, this is a
realistic effect, since you have no falloff to the light.  In the real
world, the light would diminish in intensity the farther from the light
source.  If you place a light close to the ground in a room, you would
get that bright spot on the ground, but the walls around it would also
be very dark.

You actually explained the effect yourself, although you may not have
understood the implications of it:

The doc. says:
A pointlight ... illuminates  everything in the scene
equally no matter how far away from the light source
it may be.

This alone doesn't take into account the incident angle of the light,
only that the light doesn't dimish over distance.  The room will be
evenly illuminated if you add brilliance 1 to your finish, but then the
shadows dissapear.

Possible solutions would be to add some fill lighting to reduce the
darkness of the floor while adding a falloff distance to the light, or
to simply place the light higher up.  Typically, people opt for more
lights in a room to fill in dark corners but to retain a natural look.

-Mike

http://members.aol.com/amaltheaj5

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POVRAY (POV### [at] aolcom) wrote:
: This alone doesn't take into account the incident angle of the light,

  I wonder why the illumination of an object depends on the incident
angle of the light in real world. I just can't figure it. Has this
something to do with the amount of light per area unit of the surface?
How is light amount measured? I think there was a measure unit for this.

--
                                                              - Warp. -

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Thank you for your reply.

I'm glad you mentioned "brilliance", as that paragraph in the doc. explains it
very nicely.

--
Alan

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Nieminen Mika wrote in message <6h5a76$3em$2@oz.aussie.org>...
>POVRAY (POV### [at] aolcom) wrote:

>  I wonder why the illumination of an object depends on the incident
>angle of the light in real world. I just can't figure it. Has this
>something to do with the amount of light per area unit of the surface?


Well, looks like you already got it there!

This is the same reason, as for why the vineyards in the "relatively"
northern parts of Germany have to be sloped (to catch enough son), while the
ones in Italy (or California) don't have to care about that. Sun-collectors
on rooftops are also a good example: they are always perpendicualr to the
main sun direction.

Imagine that you could "count" the rays of the sun. Now imagine the sun
standing perpendicular to the earth's surface (like it can only happen
around the equator). You could then mark off a, lets say, 1 square meter
area and somehow "mark" the sunbeams that define the corners of that square.
Now angle the surface (in respect to the rays) and look at the shape that is
now defined by those four rays: It is now longer a square, but a
prespectively warped rectangle. And the area of this "shape" is larger than
the area of the original square and it will get even larger when you
increase the angle.
But the "number" of rays falling on this increasing area is still the same!
So (very simply put): The same amount of light falls on a larger area, thus,
while the area in total gets the same light, each square centimeter gets
less light (or less sun-energy, when going back to the vineyard /
sun-collector analogy).

Johannes.

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