Alain Martel <kua### [at] videotronca> wrote:

> > Alain Martel <kua### [at] videotronca> wrote:

> >>> Hello Forum,
> >>>
> >>> I am relatively new to POV-RAY, but have a functional script .
> >>>
> >>> I am simulating air bubbles in a glass tube full of gel.  I am running into a
> >>> funny problem where, if I add too many bubbles to the script, they go from being
> >>> transparent and rendered correctly, to a completely opaque, black object through
> >>> which no light shines.
> >>>
> >>> Things that don't seem to work:
> >>>
> >>> 1) Increasing the  max_trace_level
> >>>
> >>> 2) I have tried to be sure that the inner surface of the gel is not in contact
> >>> with the air bubble by reducing the bubble radius by epsilon (.0001).  Oddly
> >>> enough, the script with up to 4 bubbles works only when the two surfaces are
> >>> coincident, and subtracting an epsilon value (.0001,.001,.01) causes the bubbles
> >>> to turn black, no matter the number.
> >>>
> >>> I would love your input on this issue as I am stumped.  I cannot figure out why
> >>> this renders without issue when I only have a certain number of objects defined.
> >>>
> >>>
> >>> The order of my file:
> >>>
> >>> In my scene, centered at the origin, I have bubbles, inside of gel, inside of a
> >>> glass tube, inside of water, inside of a plexiglass cylinder.
> >>>
> >>> My code
> >>>
> >>> 1) I define variables, set trace level, and place the camera
> >>>
> >>> 2) I create a distributed area light source on a rectangular box
> >>>
> >>> 3) I define the bubbles I want in the gel
> >>>
> >>> 4) I create the glass tube into which the gel will sit
> >>>
> >>> 5) I create the gel region with voids into which the bubbles sit.
> >>>
> >>> 6) I then add water around the glass tube, and then a plexiglass perimeter.
> >>>
> >>>
> >>>
> >>>
> >>
> >> The first thing that I would to would be to increase max_trace_level
> >> further still. It can go up to 255.
> >>
> >> Note : Your area_light is oblique. If you want it to be flat, then, it's
> >> size should be something like this :
> >> area_light 160*x 200*z 65 65
> >> adaptive 0 // start with zero, then, increase if needed
> >> Size of area_light adjusted to match the light_box dimensions.
> >>
> >> During testing, you should disable the area_light. It makes testing and
> >> debugging faster.
> >>
> >> The 100 by 100 get silently increased to 129 by 129 when adaptive is
> >> used. That increase DO NOT affect the rendering time.
> >>
> >> Using dispersion 1 is the same as not using dispersion. It must be
> >> slightly different than 1. Realistic values are usually in the 1.01 to
> >> 1.08. Values less than 1 result in inverted dispersion.
> >>
> >> You are using fade_power 1000 !? You should use 1 or 1001.
> >> Using 1000 may cause some issues.
> >
> > Hello Alain,
> >
> > Thank you so much for your reply and suggestions.  As you may have noticed, I am
> > still in the process of understanding the different features.
> >
> > In the case of max_trace_level, I have put this all the way up to the max as you
> > suggested, but this did not resolve the issue from my original script.  I'll
> > have to keep digging for a solution in that regard.
> >
> > For the area_light, I was certainly not clear on the pov-ray documentation and
> > examples.  From what I understand, I am creating a box (light box) that is
> > parallel with the xy-plan, onto which I'm stretching an array of 100x100 light
> > sources.  And as you stated, by adding the adaptive, feature, this gets
> > increased when running (Great thing to know!  I wasn't completely clear on what
> > happened in the background).
> >
> > I am, however, still confused on why the area_light would be tilted.  As you
> > mentioned, I want a light parallel to the xy plane, and thought that this was
> > the case.
> >
> > Given this example below, my understanding is that my light_box is a box that is
> > 160x200x2 units (x,y,z) and collared white.  I then locate my light_source at
> > <light_loc_x, light_loc_y,light_loc_z>.  I then use the area_light feature to
> > define two points <-10, -10, 10>, <10, 10, 10> from the origin to indicate the
> > vectors over which to stretch the light.
> >
> > #declare light_box = box{ <-80,-100,0>, <80,100,2>
> >          pigment{color White}
> >         } //end light box
> > light_source {
> >      <light_loc_x, light_loc_y,light_loc_z>
> >      color White
> >      area_light <-10, -10, 10>, <10, 10, 10>, 100, 100
> >      adaptive 1
> >      jitter
> >      looks_like { light_box }
> >    }
> >
> >
> The size vectors of an area_light define the 2D dimentions of the
> area_light. A line going from the origin to one of them will be parallel
> to the side of the area_light.
>
> Simple length are used for a light array with sides parallel to the axis.
> Two non-zero component are used for an array parallel to one of the
> major planes but rotated/sheared.
> Three non-zero components will define an array that is not parallel to
> any of the planes.
>
> Normally, we use simple lengts parameters, each in line with one of the
> axis.
>
> In the present example, the vectors are diagonal. They pass through the
> origin <0,0,0> and the defined points, or <-10, -10, 10> and <10, 10, 10>.
>
> It goes from <-10, -10, 10> to <0, 0, 0> to <10, 10, 10>.

>
> If you want an area_light parallel to the X-Z plane, then, the vectors
> need their Y components to be zero.
>
> light_source {
>      <light_loc_x, light_loc_y,light_loc_z>
>      color White
>      area_light <-10, 0, 10>, <10, 0, 10>, 100, 100
>      adaptive 1
>      jitter
>      looks_like { light_box }
>    }

>
> light_source {
>      <light_loc_x, light_loc_y,light_loc_z>
>      color White
>      area_light <-10, -10, 0>, <10, 10, 0>, 100, 100
>      adaptive 1
>      jitter
>      looks_like { light_box }
>    }
> Defines a LINEAR light array as the two vectors are colinear. It won't,
> but should, generate at least a warning.

Hello Alain,

Thank you again for all of your input and help.  I was unexpectedly out of my
project for a bit, but have implemented a number of changes of the code that
have resolved the issues of my bubbles being opaque.  I have also declared
objects and materials to simplify the code.

The last issue that I'm having is this topic of the area_light.  Your
explanation is much clearer than the documentation online, and I think I now
have it set in my scene correctly.  I just wanted to bounce this off of you as a
sanity check:

What I want is an area_light acting as a diffuse backlight shining through my
tube setup into my camera.  I want this area_light parallel to the XY plane
with dimensions 160x200, centered along the Z-axis, and translated +100 units in
the Z direction.

If I were only defining an area light without attaching it to an object it would
be defined as:

light_source {
    <0, 0,100>
    color White
    area_light <160, 0, 0>, <, 200, 0>, 100, 100
    adaptive 1
    jitter
  }

When I do this, however, the background is all black, as I believe this only
produces rays from the defined area_light, but doesn't really simulate the
appearance of looking into a bright backlight.  To mimic realistic backlighting,
I can then use the command looks_like{ light_box } where I predefine a white
rectangle called "light_box."  My code then looks like this:

#declare light_box = box{ <-80,-100,0>, <80,100,2>
        pigment{color White}
       } //end light box

light_source {
    <0, 0,100>
    color White
    area_light <-10, -10, 0>, <10, 10, 0>, 100, 100
    adaptive 1
    jitter
    looks_like { light_box }
  }

Doing this, I have defined an area_light parallel to the XY, stretched it over
my box dimensions, and translated that box/area_light +100 units in the
Z-direction.

Is this correct?

Sincerely,

bubble_person

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