What's causing these artifacts???

radiance-general
1

Dear Group

In some of my recent renders, I have encountered some very specious artifacts. I generally specify a high number of ambient division (usually around 3072) and ambient super samples (500-1000) for my renders.

The first ones are these mysterious dark splotches:

I really can't figure out what's causing these. I am unsure if this is caused by poor interpolation. Also, occasionally I have come across the following bright spots:

No matter how many ambient divisions that I specify (I went up to 16,000 ambient divisions with 2000 ambient super samples), I can't get rid of these bright spots. The problem with these particular artifacts is that I can not predict when it will occur. What I need the most when performing renders is predictability. Does anyone have any ideas?

Thanks

Marcus

2

Dear Group

In some of my recent renders, I have encountered some very specious artifacts. I generally specify a high number of ambient division (usually around 3072) and ambient super samples (500-1000) for my renders.

The first ones are these mysterious dark splotches:

I really can't figure out what's causing these. I am unsure if this is caused by poor interpolation. Also, occasionally I have come across the following bright spots:

No matter how many ambient divisions that I specify (I went up to 16,000 ambient divisions with 2000 ambient super samples), I can't get rid of these bright spots. The problem with these particular artifacts is that I can not predict when it will occur. What I need the most when performing renders is predictability. Does anyone have any ideas?

Thanks

Marcus

3

What is your -aa ambient accuracy set at?
Also - does your rif have the correct size
ZONE= I
I find that I often forget to update this when I create another rif based on
a previous rif

Rob

···

-----Original Message-----
From: Marcus Jacobs
To: [email protected]
Sent: 12/5/2005 10:07 AM
Subject: [Radiance-general] What's causing these artifacts???

Dear Group

In some of my recent renders, I have encountered some very specious
artifacts. I generally specify a high number of ambient division
(usually
around 3072) and ambient super samples (500-1000) for my renders.

The first ones are these mysterious dark splotches:


g&.src=ph
g&.src=ph

I really can't figure out what's causing these. I am unsure if this is
caused by poor interpolation. Also, occasionally I have come across the
following bright spots:


g&.src=ph

No matter how many ambient divisions that I specify (I went up to 16,000

ambient divisions with 2000 ambient super samples), I can't get rid of
these bright spots. The problem with these particular artifacts is that
I
can not predict when it will occur. What I need the most when performing

renders is predictability. Does anyone have any ideas?

Thanks

Marcus

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

4

Hi Marcus,

Are you using rad at all for your renderings, or are you setting parameters by hand? You wouldn't be performing an overture calculation with different -a* settings (in particular -ab and/or -av), would you? The dark splotches look suspiciously like bad values in the ambient file at the start of the calculation. Is that possible? What are your rendering parameters? It helps a lot in debugging a problem. Your floor looks overly bright and even compared to the rest of the surfaces. How is it defined? Why aren't there any shadows under the chair legs? What is your overall scene size (run oconv -d on the octree) and your -ar setting?

The bright splotches in your last example could be caused by the bright, narrow opening surrounding the skylight. I would probably insert a rectangle flush with the ceiling and use mkillum to compute its effective output distribution to eliminate this particular artifact.

-Greg

···

From: "Marcus Jacobs" <[email protected]>
Date: December 5, 2005 10:07:11 AM PST

Dear Group

In some of my recent renders, I have encountered some very specious artifacts. I generally specify a high number of ambient division (usually around 3072) and ambient super samples (500-1000) for my renders.

The first ones are these mysterious dark splotches:

http://pg.photos.yahoo.com/ph/marcdevon/detail?.dir=b3e9&.dnm=4bcbre2.jpg&.src=ph

http://pg.photos.yahoo.com/ph/marcdevon/detail?.dir=b3e9&.dnm=c76fre2.jpg&.src=ph

I really can't figure out what's causing these. I am unsure if this is caused by poor interpolation. Also, occasionally I have come across the following bright spots:

http://pg.photos.yahoo.com/ph/marcdevon/detail?.dir=b3e9&.dnm=e02ere2.jpg&.src=ph

No matter how many ambient divisions that I specify (I went up to 16,000 ambient divisions with 2000 ambient super samples), I can't get rid of these bright spots. The problem with these particular artifacts is that I can not predict when it will occur. What I need the most when performing renders is predictability. Does anyone have any ideas?

Thanks

Marcus

5

Dear Group

Thanks for all of the suggestions regarding my rendering problems. I know its taken me a while to reply back about this issue to the group. I have spent some time investigating these issues that I am having. This way, I at least try to solve my problems myself before bringing to this group.

First, the dark splotches:


I think I have some idea as to what is causing this issue. Generally when I render, I perform a small overture calculation at a 100 x 100 pixel resolution. The ambient settings are generally at -aa 0.1 and -ar 0. The next step is to take a medium resolution overture usually at 500 x 500 pixels. The settings are generally set at -aa (.15 - .2) -ar = maximum scene size as determined by getinfo -d in inches (basically a 1 inch grid). The final render is performed at -aa (.25 - .3) and -ar = maximum scene size as determined by getinfo -d in inches/(2 to 10) (basically a 2 - 10 inch grid) depending on the illumination contrast. The -ad and -as settings are generally set based on the illumination contrast in the scene. For these renderings, I set -ar very low since I had already rendered it at the same resolution with higher -ar settings. Essentially, I wanted to force it to interpolate using the existing ambient data. The splotches usually come in as the -ar parameter degrades. The number of splotches depends on how low this parameter has been set. The reason why I ask is because this seems to me to have started occuring recently even though I have used the aforementioned technique for some time now.

For the bright splotches here:

it looks like mkillum saves the day again. I tried Greg's suggestion of setting an illum at skylight openings at the surface level of the ceiling. This seems to have done the trick. It's funny but I ran dozens upon dozens of renderings and for some reason I thought these artifact were due to the bright spots on the floor rather than the skylight openings. Here are the results:


May I ask, why is the brightness of the skylight openings more prone to causing these artifacts rather than a bright spot on the floor or wall?

When initially attempting to find a solution to this problem, I tried to go back to issues that Rob Guglielmetti was experiencing a couple of years back with a museum that he was modeling. At one time he documented his progression on his blog but the page has since been deleted. I did remember a couple of threads from July/2004 which discussed whether to place illums inside or outside the scene:

http://www.radiance-online.org/pipermail/radiance-general/2004-July/001871.html
http://www.radiance-online.org/pipermail/radiance-general/2004-July/001953.html

The reason why I have been hesitant to using illums inside is because I thought that mkillum could only simulate illumination that's a smooth function with no discontinuities. I thought that this may be an issue when windows have lites. I did a little experiment with another old scene of mine. It is a window that I knew would receive direct illumination from the sun. I wanted to place the illums on the inside to see if mkillum would pick up the lites. Here is the result:

I am impressed that mkillum has able to detect the presence of lites in the windows as you can see their occluding effect by the direct sunlight that is on the floor. By placing the illums on the inside of this room, I was also able to eliminate some bright splotches (although less conspicuous than the previous scene) from this room.

In my investigation of course there are more questions/issues that has arisen. From another test room, I wanted to see how much the smoothness the indirect calculation would improve by placing an illum inside its skylight. This is what's happened:




The indirect illumination in the corner I do not believe is correct. I know that this problem isn't due the presence of the illum in the skylight at the surface level of the vaulted ceiling. This seems to occur sort of haphazardly. Here is the same room with the same day/time settings, albeit with different materials, and this problem didn't occur:



This problem that I am receiving looks similar to issues that I am having with this rendering (right side near the corner):

Also, for the previous rendering and the next, I do not know what's causing the gray artifacts on the floor nearby the dishwasher and lower cabinets:

Generally, what I wish to be able to acquire is achieving a higher level of predictability so that I may take a scientific approach to rendering rather than constantly tinkering. Sometimes when I think I got it, I later realize that I don't. That's pretty much it in a nutshell.

Thanks everyone for all of your help.

Marcus

6

Marcus Jacobs wrote:

When initially attempting to find a solution to this problem, I tried to go back to issues that Rob Guglielmetti was experiencing a couple of years back with a museum that he was modeling. At one time he documented his progression on his blog but the page has since been deleted.

Hi Marcus,

Thanks for reminding me that I have yet to fully restore all the stuff questions, but in lieu of that, I at least restored the page you were looking for, albeit to a different (and temporary) location:

http://www.rumblestrip.org/static/rad-mkillum/rad_01.html

It's basically a long-winded journey toward mkillum enlightenment. One of these days, I'll get totally organized, but let's just say I'm not making it a new year's resolution. :wink:

Now's a good time to also wish the entire list a happy end-of-the-year. May you all have holidays filled with light (and if the light source is small relative to the scene, use mkillum to render it).

- Rob

···

from my old site. I wish I could answer some of your interesting

7

Hi Marcus,

There's rather a lot to respond to here, but I'll give it a go...

From: "Marcus Jacobs" <[email protected]>
Date: December 23, 2005 10:25:01 AM PST

Dear Group

Thanks for all of the suggestions regarding my rendering problems. I know its taken me a while to reply back about this issue to the group. I have spent some time investigating these issues that I am having. This way, I at least try to solve my problems myself before bringing to this group.

First, the dark splotches:

http://i35.photobucket.com/albums/d178/marcdevon/AFrame2_Camera6_final.jpg
http://i35.photobucket.com/albums/d178/marcdevon/AFrame2_Camera03_final.jpg

I think I have some idea as to what is causing this issue. Generally when I render, I perform a small overture calculation at a 100 x 100 pixel resolution. The ambient settings are generally at -aa 0.1 and -ar 0. The next step is to take a medium resolution overture usually at 500 x 500 pixels. The settings are generally set at -aa (.15 - .2) -ar = maximum scene size as determined by getinfo -d in inches (basically a 1 inch grid). The final render is performed at -aa (.25 - .3) and -ar = maximum scene size as determined by getinfo -d in inches/(2 to 10) (basically a 2 - 10 inch grid) depending on the illumination contrast. The -ad and -as settings are generally set based on the illumination contrast in the scene. For these renderings, I set -ar very low since I had already rendered it at the same resolution with higher -ar settings. Essentially, I wanted to force it to interpolate using the existing ambient data. The splotches usually come in as the -ar parameter degrades. The number of splotches depends on how low this parameter has been set. The reason why I ask is because this seems to me to have started occuring recently even though I have used the aforementioned technique for some time now.

Your analysis makes sense. As noted in discussions earlier this week, it is probably a bad idea to change the -ar setting between runs. This is most likely the source of your problems.

For the bright splotches here:

http://photobucket.com/albums/d178/marcdevon/th_room2a_view2_render1.jpg

it looks like mkillum saves the day again. I tried Greg's suggestion of setting an illum at skylight openings at the surface level of the ceiling. This seems to have done the trick. It's funny but I ran dozens upon dozens of renderings and for some reason I thought these artifact were due to the bright spots on the floor rather than the skylight openings. Here are the results:

http://i35.photobucket.com/albums/d178/marcdevon/room2_view3.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room2_view2.jpg

May I ask, why is the brightness of the skylight openings more prone to causing these artifacts rather than a bright spot on the floor or wall?

It's difficult to say for certain, but it probably has to do with the solid angles visible from the problem areas. Also, if the wall surface on the coves of your skylight are smooth and specular, this could greatly amplify their indirect contribution. I believe specularities are at play in some of the artifacts you mention below.

When initially attempting to find a solution to this problem, I tried to go back to issues that Rob Guglielmetti was experiencing a couple of years back with a museum that he was modeling. At one time he documented his progression on his blog but the page has since been deleted. I did remember a couple of threads from July/2004 which discussed whether to place illums inside or outside the scene:

http://www.radiance-online.org/pipermail/radiance-general/2004-July/001871.html
http://www.radiance-online.org/pipermail/radiance-general/2004-July/001953.html

The reason why I have been hesitant to using illums inside is because I thought that mkillum could only simulate illumination that's a smooth function with no discontinuities. I thought that this may be an issue when windows have lites. I did a little experiment with another old scene of mine. It is a window that I knew would receive direct illumination from the sun. I wanted to place the illums on the inside to see if mkillum would pick up the lites. Here is the result:

http://i35.photobucket.com/albums/d178/marcdevon/room2_camera1_tm.jpg

I am impressed that mkillum has able to detect the presence of lites in the windows as you can see their occluding effect by the direct sunlight that is on the floor. By placing the illums on the inside of this room, I was also able to eliminate some bright splotches (although less conspicuous than the previous scene) from this room.

I am a little confused by the term "lite," which my dictionary defines as "beer with relatively few calories." I assume you mean the mullions. Bear in mind that mkillum does not attempt to model light sources already in the scene, such as the sun, and this light passes without intervention. Mkillum only takes over for ambient contributions and glow's, such as the sky.

In my investigation of course there are more questions/issues that has arisen. From another test room, I wanted to see how much the smoothness the indirect calculation would improve by placing an illum inside its skylight. This is what's happened:

http://i35.photobucket.com/albums/d178/marcdevon/room1_12_amb_view4.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_12_amb_view5.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_12_amb_view6.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_12_amb_view8.jpg

The indirect illumination in the corner I do not believe is correct. I know that this problem isn't due the presence of the illum in the skylight at the surface level of the vaulted ceiling. This seems to occur sort of haphazardly. Here is the same room with the same day/time settings, albeit with different materials, and this problem didn't occur:

http://i35.photobucket.com/albums/d178/marcdevon/room1_b_view4_render2.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_b_view5_2.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_b_view6.jpg

What are the specularities and roughness values of your floor surfaces? And which version of Radiance are you running? This looks like it could be a bug, or it could just be mirror-like specular reflections off your floor. I suspect you have a low roughness value -- do you?

This problem that I am receiving looks similar to issues that I am having with this rendering (right side near the corner):

http://i35.photobucket.com/albums/d178/marcdevon/KountryKitchen_view2_render3_3.jpg

This looks like mirror reflections off your counter top, which could be real but are poorly rendered by the indirect calculation. (You could use the "mirror" type to correct this, though I'm not sure it's worth it.)

Also, for the previous rendering and the next, I do not know what's causing the gray artifacts on the floor nearby the dishwasher and lower cabinets:

http://i35.photobucket.com/albums/d178/marcdevon/KountryKitchen_view1_render3_99.jpg

These artifacts are more worrisome. I'm not sure what's causing them, unless it's more of the low-angle specular reflections off your skylight cove. Are these surfaces purely diffuse, or do they have a small specular component that might get blown out of proportion at low angles? (This is a problem that was discussed earlier this month under the heading "How to increase the rendering speed?" and fixed in the latest HEAD release.)

Generally, what I wish to be able to acquire is achieving a higher level of predictability so that I may take a scientific approach to rendering rather than constantly tinkering. Sometimes when I think I got it, I later realize that I don't. That's pretty much it in a nutshell.

A worthy goal. I think we can all benefit from what we learn in this exploration, and I appreciate your sharing your results.

Happy Holidays!
-Greg