I hope you can help me out with the following:

I use several area lights in my scene which highlight objects on a somewhat
reflective plane. However, the light doesn't fall off completely smoothly but
there seem to be darker concentric lines around the highlighted areas. The lines
are virtually absent (or overlapped) in well light areas but become noticable
(with decreasing interval) in parts where the light starts to fade.

I've tried to google my way to a solution and I've adjusted every light setting
I could think of, but no luck. Since I'm a noob at pov-ray I was hoping that one
of you could help me out with this.

Btw, my monitor is set to 32-bit colors and the effect is visible in different
viewing programs.

Here is the code stripped to the bare minimum. The concentric lines are best
visible from an upward angle:

#include "colors.inc"
#include "textures.inc"

camera { location <6, 10, -10>

look_at <2,0,-0.3> angle 60
translate x*-2
translate z*-2
}

light_source {
        <0,15,0>
        color rgb <.8, .7, .6>*1
fade_distance 7 fade_power 1.2
area_light x*3, y*3, 12, 12  adaptive 0 circular orient jitter

}

//Appearance surface of plane
#declare Surface =
texture {
pigment { rgb <.7,.95,1>*0.4 }
finish { specular .5 reflection .2 }}

plane
{ y, -1.4
  texture {Surface}
}

Many thanks in advance!

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I think what you are talking abaut are artifacts caused by the bit-depth of your
graphics display, actually noticable very well at grayscale - even with 32 bit
colours. So what you could do to solve the problem is to introduce a small
amount of noise by adding the following line to your plane:

plane
{ y, -1.4
  texture {Surface}
  finish {crand 1/16}  // a little bit noise
}

....or to the whole scene, if it is very dark:

#default {finish {crand 1/16}}

I hope this helps! :)

Regards, Florian

Post a reply to this message

Am 23.09.2010 11:34, schrieb Der_Minimalist:

> I use several area lights in my scene which highlight objects on a somewhat
> reflective plane. However, the light doesn't fall off completely smoothly but
> there seem to be darker concentric lines around the highlighted areas. The lines
> are virtually absent (or overlapped) in well light areas but become noticable
> (with decreasing interval) in parts where the light starts to fade.

The dark concentric lines are actually an optical illusion, occuring at 
the boundaries between areas of very slightly different brightness - 
just enough for the eye to notice the contrast at the boundaries, but 
not enough to perceive the adjacent areas as having different brightness 
as a whole.

The proper solution to this, as Florian already pointed out, would be to 
introduce a tiny bit of noise (ideally corresponding to +/- 0.5 bits).

Other than using crand, you can also use a higher-precision output 
format (OpenEXR, or 16-bit-per-channel variants of classic formats; 
Radiance HDR may also do, provided the gradient colors are not too 
saturated), then convert to a standard 8-bpc format using an external 
tool that supports dithering. (IC is my favorite tool for this kind of job.)

(Note that in this case, the render preview will still show the same 
banding artifacts, although the OpenEXR output will be perfectly smooth.)

Unfortunately, POV-Ray does not support automatic dithering for 
low-precision output formats yet.

Post a reply to this message

> I use several area lights in my scene which highlight objects on a somewhat
> reflective plane. However, the light doesn't fall off completely smoothly but
> there seem to be darker concentric lines around the highlighted areas. The lines
> are virtually absent (or overlapped) in well light areas but become noticable
> (with decreasing interval) in parts where the light starts to fade.
>

Why don't you post one of the output images (not just a screenshot) so 
we can view it on our computers and confirm that it's not just a display 
issue?

If it's an actual output problem, what version of POV-Ray are you using?

  - Slime

Post a reply to this message

First of all, thnx for all the help, really appreciate it!!

As to the last comment, I'm using pov-ray 3.6.2 x64 and my output file type is
bmp. Since I'm not allowed to post attachments in this newsgroup I reckoned that
including the stripped scene file would be the best way to show you the problem.
I would be interested to hear if any of you doesn't see the concentric lines I'm
talking about when you render the code.

The tip of Florian and clipka (thanks!) to add some noise to the plane indeed
removes (or masks)the concentric lines, but this makes the scene look slightly
less realistic. Therefore I will now try the other suggestion of clipka to
output to OpenEXR, which in my case will involve some more google work ;), and
convert to 8-bit in IC(?).

Will let you know if I make any progress on this.

Cheers

Post a reply to this message

Well, after some more surfing I was lucky enough to end up with an article which
shows precisely what I mean. It is actually an article of clipka!

link: http://wiki.povray.org/content/User:Clipka/Gamma

The two grey colored pictures of a ball show the banding on the plane (and also
the ball itself) that I also have in my scene.

Unfortunately the suggestion of clipka to output to OpenEXR in pov-ray 3.7, only
made the banding worse than ever :( (when opening the exr in photoshop cs4)

Since the picture is to appear in a magazine soon I am in desperate need of a
solution!

Post a reply to this message

Der_Minimalist wrote:

> Unfortunately the suggestion of clipka to output to OpenEXR in pov-ray 3.7, only
> made the banding worse than ever :( (when opening the exr in photoshop cs4)

the key word here was "dithering". Just using an output format
with a higher resolution does not give you any benefit when viewing
it on a normal 32-bit display.

It might help the printing process but I'm no expert on supported
file formats for professional printing. Actually, I'm not even sure
banding will be a problem when in print.

Anyway, just opening the image in photoshop is not enough,
you need to explicitly apply color reduction to 32-bit (don't know
the exact command in photoshop, sorry), and select options to
soften the banding by using "patterns" of pixels with different
gray values (this is similar in effect to the crand finish but
more controlled). Many printing processes also use dithering
to achieve gray levels with just one level of "ink" color.

Post a reply to this message

> Well, after some more surfing I was lucky enough to end up with an article which
> shows precisely what I mean. It is actually an article of clipka!
>
> link: http://wiki.povray.org/content/User:Clipka/Gamma
>
> The two grey colored pictures of a ball show the banding on the plane (and also
> the ball itself) that I also have in my scene.
>
> Unfortunately the suggestion of clipka to output to OpenEXR in pov-ray 3.7, only
> made the banding worse than ever :( (when opening the exr in photoshop cs4)
>
> Since the picture is to appear in a magazine soon I am in desperate need of a
> solution!
>
>

Another way to add some noise to your plane.
Use micro normals.
Change your plane to:

plane
{ y, -1.4
   texture {Surface}
   normal{granite 0.02 scale 0.0001}
}


This will make the reflection somewhat blurry.

If the image is to be printed, the printing process itself will 
introduce a fair amount of noise and dithering that can do a LOT in 
hiding the banding.
By the way, that banding can appears in low contrast parts of photographies.



Alain

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Well, thanks for all your replies! Now I at least know this isn't something that
is solved easily by a simple pov-ray command (OK, besides adding noise to the
plane ;) It isn't that bad on screen and according to your replies it will not
get worse in print (which was my major concern).

Thanks again for your time, I've learned a lot! (had never heard of dither,.)

Cheers

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