Samuel Benge wrote:

> I understand this. But if we *can* get bevelled edges using higher
> accuracy values, why not let people use it that way? 
> 
Well... it would (again) be some sort of "art produced by numerical 
errors in the solver". 
I certainly won't keep you from doing such things and it's great if 
it works for you. 
However, don't yell at us if things break in the next version. Numerical 
algorithms typically give results which are only exact up to some certain 
error (i.e. accuracy). And it is no good idea to rely on a special behaviour 
of the numerical error. 

>> (3) It does greatly improve the actual surface accuracy. While the current
>>    code will have the real root depth within the specified accuracy, the
>>    patched code will have it within 1/10 to 1/100 (typ.) of the accuracy.
>>    This in turn means, that one can render equally-well images with smaller
>>    accuracy and hece faster. See below, too.
> 
> This is what's really frustrating me.... I know the patch is better when
> it comes to accuracy. With the 'official' isosurface, we have to use
> accuracy values which are MUCH SMALLER than the needed 'grain size', or
> resolution of the isosurface to rid ourselves of the black dots.
> 
The actual accuracy improvement depends on the function used. 
It seems you did not completely understand the grain size argument. 
If you look at the isosurface (mathematically!) of a function, this can be 
very smooth (like a billard ball) strongly grainy (like a sponge). 
In order to get a useful representation of the isosurface, you normally 
need an accuracy value which is smaller than the characteristic grain 
size. This is necessary to make sure that the first root along the 
light ray is actually found (and not any root). This is independent of 
my patch. 


landscape), are very smooth and hence the "grain size" is large and 
you could use a fairly large accuracy value. So, here we find the correct 
(i.e. first) root easily but the result for the ray depth of the root may 
be displaced from the real location by the amount of accuracy. And here 
comes the patch: By linearly interpolating, this guess for the actual 
location of the root between the last two evaluation points can be 
"greatly" improved. 
How much depends on the function used. For very smooth functions 
(especially the mars topo which is itself a linearly interpolated function), 
you can gain the mentiones factor 10 to 100. For other less smooth 
functions it will be less and for really rough ones we won't win anything 
but also not lose anything. 

> Can I give you an example isosurface statement, to test my theory on
> this? I've seen the problems; I believe your patch fixes them. If we
> break even on render time, so be it. A fix is a fix.
> 
Well, feel free to post or send me the SDL code in question. I'll give it a 
run (unless it takes ages :). 

> *Sigh* I wish I knew how to compile POV's source code... :c/
> 
tar xfj povray-3.6.1.tar.bz2
./configure COMPILED_BY=your_name
make

Wolfgang

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Christoph Hormann wrote:

> Samuel Benge wrote:
> 
>>
>> This is what's really frustrating me.... I know the patch is better 
>> when it comes to accuracy. With the 'official' isosurface, we have to 
>> use accuracy values which are MUCH SMALLER than the needed 'grain 
>> size', or resolution of the isosurface to rid ourselves of the black 
>> dots.
>>
> 
> You should really better show an example.  Black dots are nearly always 
> an indication of unsuccessful root finding and then this patch would not 
> help as explained.
> 
> Christoph


Okay, the example image has been uploaded to p.b.i. I gave three 
examples in png format, each one having a different accuracy value. 
Also, the isosurface has smooth and rough surfaces combined for obvious 
reasons. The code for the example is below. You might want to uncomment 
the max_gradient and use very high values, to conclude the artifacts 
shown are not related to max_gradient. Also, I encourage both you and 
Wolgang to try this in the patched version. I'd like to see the results.

-Sam

// Begin Code

global_settings{
  assumed_gamma 1
}

#default{ finish{ ambient 0 } }

camera{
  fisheye
  right x*.5 up y*.5
  //right x*.5*1.33 up y*.5
  location< 0, 20, -30 >
  look_at 0
  angle 14
}

background{ rgb<.3 .4 .5> }

light_source{<1,2,-.95>*100000,<1 1 .8>*2 }

#declare tex=
function{
  pattern{
   granite
  }
}

isosurface {
  function{
   min(
    max( // cube
     abs(x),
     abs(y),
     abs(z)
    )-1,
    max(
     sqrt(y*y+z*z)-.75, // cylinder
     abs(x)-2
    )+tex(x,y,z)/10 // rough surface
   )
  }
  accuracy .05 // .01 // .001
  //max_gradient 100
  evaluate 0.6, 1.8, 0.95
  contained_by{
   box{
    <-2.1,-1.1,-1.1>,
    <2.1,1.1,1.1>
   }
  }
  pigment{ rgb 1 }
  rotate y*35
}

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Samuel Benge wrote:
> 
> Okay, the example image has been uploaded to p.b.i. I gave three 
> examples in png format, each one having a different accuracy value. 
> Also, the isosurface has smooth and rough surfaces combined for obvious 
> reasons. The code for the example is below. You might want to uncomment 
> the max_gradient and use very high values, to conclude the artifacts 
> shown are not related to max_gradient. Also, I encourage both you and 
> Wolgang to try this in the patched version. I'd like to see the results.
> 

What you see on the front face of the box are exactly the explained 
shadow ray noise effects you also see on Wolfgang's samples caused by 
tangential lighting.  The surface is always found reliably, just if it 
is in shadow or not varies.  Since these surfaces result from a 
precisely linear function the linear interpolation leads to perfect 
results here for arbitrary accuracy values (on the faces, not near the 
edges and only within the accuracy of floating point numbers of course).

Since you usually won't use tangential lighting in real scenes for such 
flat surfaces i moved the light source a bit.  I also changed 
max_gradient to 4 (the real maximum gradient of the function seems just 
above 3).  Here are the results:

no interpolation:

http://www.tu-bs.de/~y0013390/files/sam_test1_o05.png
http://www.tu-bs.de/~y0013390/files/sam_test1_o01.png
http://www.tu-bs.de/~y0013390/files/sam_test1_o001.png

with Wolfgang's patch:

http://www.tu-bs.de/~y0013390/files/sam_test1_i05.png
http://www.tu-bs.de/~y0013390/files/sam_test1_i01.png
http://www.tu-bs.de/~y0013390/files/sam_test1_i001.png

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 06 Jul. 2004 _____./\/^>_*_<^\/\.______

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Christoph Hormann wrote:

 >What you see on the front face of the box are exactly the explained 
 >shadow ray noise effects you also see on Wolfgang's samples caused by 
 >tangential lighting.  The surface is always found reliably, just if it 
 >is in shadow or not varies.  Since these surfaces result from a 
 >precisely linear function the linear interpolation leads to perfect 
 >results here for arbitrary accuracy values (on the faces, not near the 
 >edges and only within the accuracy of floating point numbers of >course).

 >Since you usually won't use tangential lighting in real scenes for 
such >flat surfaces i moved the light source a bit.


Umm, I see this a lot, actually. The more complex a shape becomes, the 
more these 'tangental lighting' artifacts appear. They happen on curved 
surfaces, too, but with less obvious striation. I hate having to move my 
light_source so often, but at least it's a fix for the flat surfaces...

 >I also changed max_gradient to 4 (the real maximum gradient of the 
 >function seems just above 3).  Here are the results:

 >no interpolation:

 >http://www.tu-bs.de/~y0013390/files/sam_test1_o05.png
 >http://www.tu-bs.de/~y0013390/files/sam_test1_o01.png
 >http://www.tu-bs.de/~y0013390/files/sam_test1_o001.png

 >with Wolfgang's patch:

 >http://www.tu-bs.de/~y0013390/files/sam_test1_i05.png
 >http://www.tu-bs.de/~y0013390/files/sam_test1_i01.png
 >http://www.tu-bs.de/~y0013390/files/sam_test1_i001.png

 >Christoph


Wow. Those results look encouraging. It's much like I suspected: edges 
no longer have that jagged look. The chopped-up surface is now smooth. 
Some spots still appear, though. I'm sure in time all such artifacts 
will be resolved.

Have you tried both patches (yours and Wolfgang's) combined yet? With AA?


-Sam

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Samuel Benge wrote:
> 
> Wow. Those results look encouraging. It's much like I suspected: edges 
> no longer have that jagged look. The chopped-up surface is now smooth. 
> Some spots still appear, though. I'm sure in time all such artifacts 
> will be resolved.

???

To me the difference seems very minor, not even close to 10x the 
accuracy value leading to the same result quality.

Just in case this was not clear: the 'i' samples are with the patch, the 
'o' ones without (like with official POV-Ray 3.6).

> Have you tried both patches (yours and Wolfgang's) combined yet? With AA?

What patch of mine are you referring to?  The isosurface bounding tree 
patch has nothing to do with this.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 06 Jul. 2004 _____./\/^>_*_<^\/\.______

Post a reply to this message

Christoph Hormann wrote:

> Samuel Benge wrote:
> 
>>
>> Wow. Those results look encouraging. It's much like I suspected: edges 
>> no longer have that jagged look. The chopped-up surface is now smooth. 
>> Some spots still appear, though. I'm sure in time all such artifacts 
>> will be resolved.
> 
> 
> ???
> 
> To me the difference seems very minor, not even close to 10x the 
> accuracy value leading to the same result quality.


That's because you seem to be bent on using really low accuracy 
settings, or something. All I'm implying is that it's not necessary to 
use such low accuracy values all the time... it actually seems 'safe' 
not to now, with the new patch.

 
> Just in case this was not clear: the 'i' samples are with the patch, the 
> 'o' ones without (like with official POV-Ray 3.6).


It was clear, thanks.

 
>> Have you tried both patches (yours and Wolfgang's) combined yet? With AA?
> 
> 
> What patch of mine are you referring to?  The isosurface bounding tree 
> patch has nothing to do with this.
> 
> Christoph


I guess if you believe linear interpolation patch won't help speed up 
isosurface rendering significantly, you might say that.

-Sam

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Samuel Benge wrote:

> 
> I guess if you believe (the) linear interpolation patch won't help speed up 
> isosurface rendering
>

Post a reply to this message

Samuel Benge wrote:
>>
>> To me the difference seems very minor, not even close to 10x the 
>> accuracy value leading to the same result quality.
> 
> That's because you seem to be bent on using really low accuracy 
> settings, or something. All I'm implying is that it's not necessary to 
> use such low accuracy values all the time... it actually seems 'safe' 
> not to now, with the new patch.

I have no idea how you come to this conclusion, to me

http://www.tu-bs.de/~y0013390/files/sam_test1_o01.png

looks much better than:

http://www.tu-bs.de/~y0013390/files/sam_test1_i05.png

and it only uses 5 times the accuracy value.  I am using exactly the 
accuracy values you proposed (x05 meaning accuracy 0.05).

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 06 Jul. 2004 _____./\/^>_*_<^\/\.______

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Christoph Hormann wrote:
> with Wolfgang's patch:
> 
It's nice to see that you're actually using it (sometimes at least) ;))

Wolfgang

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Christoph Hormann wrote:
> I have no idea how you come to this conclusion, to me
> 
> http://www.tu-bs.de/~y0013390/files/sam_test1_o01.png
> 
> looks much better than:
> 
> http://www.tu-bs.de/~y0013390/files/sam_test1_i05.png
> 
...but 
  http://www.tu-bs.de/~y0013390/files/sam_test1_i01.png
looks better than 
  http://www.tu-bs.de/~y0013390/files/sam_test1_o01.png
doesn't it?

BTW, we're not talking about what _looks_ better but what matches 
the actual surface better. 
I'm saying this because of the different looks of the grainy cylinder. 
Seems the grain size is in the order of the accuracy. 

>Since you usually won't use tangential lighting in real scenes for such 
>flat surfaces 
>
Well, I think this argument won't help. Imagine an animation where 
the object is moved relative to the light (rotation e.g.)...

When I think that we would not have all that discussion if R. Suzuki had 
thought of the linear interpolation himself, then it's probably a good time 
to say good night and go to bed... :)

Wolfgang

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