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Raytracing artifacts


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#1 Yours3!f   Members   -  Reputation: 1300

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Posted 11 February 2013 - 09:52 AM

Hi,

 

I'm writing a cpu raytracer using my math library based on this tutorial:

http://www.flipcode.com/archives/Raytracing_Topics_Techniques-Part_1_Introduction.shtml

 

I got through it up to chapter 6, where I noticed that I get subtle artifacts in the rendering (see pic), which gets even more pronounced when I enable fast mode (only diffuse lighting, 3 fps).

 

Any idea what am I doing wrong?

 

download here:

https://docs.google.com/file/d/0B33Sh832pOdOaE1LU3ZqMWZBUzQ/edit?usp=sharing

 

EDIT: to run the exe on linux, set the LD_LIBRARY_PATH to ../external/lib, ../external/lib/tbb

you'll also need to create a symlink from external/lib/libGLEW.so to libGLEW.so.1.6 (for some reason it links to libGLEW.so.1.6 too... even though I have 1.7)

you'll also need to install gsl (GNU scientific lib)

you can also build it using cmake. To disable the tbb (Intel thread building blocks) and gsl dependencies, set in globals.h USE_QRNG to 0, and COUNT_INTERSECTIONS to 1. Make sure you're not linking to it after this. Note that this will disable threading and usage of quasi random numbers.

 

Best regards,

Yours3lf

Attached Thumbnails

  • scrsht.png

Edited by Yours3!f, 11 February 2013 - 10:45 AM.


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#2 Álvaro   Crossbones+   -  Reputation: 12923

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Posted 11 February 2013 - 10:13 AM

"See pic"? See what pic?

#3 Yours3!f   Members   -  Reputation: 1300

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Posted 11 February 2013 - 10:46 AM

"See pic"? See what pic?

 

hmm I remember uploading it... nevermind now it should be there.



#4 Tasaq   Members   -  Reputation: 1218

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Posted 11 February 2013 - 12:19 PM

I remember having similar problem, it was gone after I added multisampling;)



#5 Yours3!f   Members   -  Reputation: 1300

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Posted 11 February 2013 - 12:38 PM

I remember having similar problem, it was gone after I added multisampling;)

 

well I did think that that is the solution, but in the original sample the artifacts are not there even without multisampling.

 

btw did you create a similer gamedev thread?



#6 Álvaro   Crossbones+   -  Reputation: 12923

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Posted 11 February 2013 - 12:43 PM

You have a bug. Using multisampling will only mask the problem. I would try to identify a pixel that gets assigned the wrong color and then step through the code, to see where the computations are going wrong. I have implemented a raytracer before and it didn't have this kind of issue.

#7 Tasaq   Members   -  Reputation: 1218

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Posted 11 February 2013 - 01:33 PM

btw did you create a similer gamedev thread?

Nope;)

And what Alvaro said is true that multisampling will only mask the problem, sorry for not mentioning that ;)

In my raytracer artifacts appeared in predictable places (like places with precision problem, for instance very far away from camera).



#8 GarFieldo   Members   -  Reputation: 130

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Posted 11 February 2013 - 02:33 PM

I have had a similar problem with my raytracer. When checking for intersections, I used to compare the factor for the direction with zero to determine whether the object is in front of the view plane:

...
t1 = object.intersect(ray);
if(t1 > 0 && ...)
...

It fixed once I compared to a value near zero, e.g. 0.000001 instead. I can only assume that some sort of floating point operation inaccuracy is the reason for the artifacts.


Edited by GarFieldo, 11 February 2013 - 03:43 PM.


#9 nfactorial   Members   -  Reputation: 729

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Posted 11 February 2013 - 03:17 PM

Artifacts like that can be caused by reflected vectors intersecting the object ifself, you can avoid them by moving the ray origin my a small epsilon ie. rayOrigin += rayDirection * EPSILON, or checking that the distance to the next intersected point is further away than the epsilon.

 

It could be something else too, but that would be my first guess looking at the picture.

 

n!


Edited by nfactorial, 11 February 2013 - 03:19 PM.


#10 Yours3!f   Members   -  Reputation: 1300

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Posted 12 February 2013 - 12:28 PM

You have a bug. Using multisampling will only mask the problem. I would try to identify a pixel that gets assigned the wrong color and then step through the code, to see where the computations are going wrong. I have implemented a raytracer before and it didn't have this kind of issue.

 I tried to, and fortunately the same pixels are wrong each frame, so I could debug it. I got to the conclusion yesterday that the shading is correct, however I think the intersections may be incorrect. (find_nearest() in main.cpp) I'll do further debugging, but I'm still learning the maths behind it...



#11 Yours3!f   Members   -  Reputation: 1300

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Posted 12 February 2013 - 12:31 PM

I have had a similar problem with my raytracer. When checking for intersections, I used to compare the factor for the direction with zero to determine whether the object is in front of the view plane:

...
t1 = object.intersect(ray);
if(t1 > 0 && ...)
...

It fixed once I compared to a value near zero, e.g. 0.000001 instead. I can only assume that some sort of floating point operation inaccuracy is the reason for the artifacts.

 

yeah I also noticed in the tutorials that it doesn't check for hitting something, but rather is it close enough?



#12 Yours3!f   Members   -  Reputation: 1300

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Posted 12 February 2013 - 12:37 PM

Artifacts like that can be caused by reflected vectors intersecting the object ifself, you can avoid them by moving the ray origin my a small epsilon ie. rayOrigin += rayDirection * EPSILON, or checking that the distance to the next intersected point is further away than the epsilon.

 

It could be something else too, but that would be my first guess looking at the picture.

 

n!

 

If you look at the code I'm already doing this:

main.cpp, 536:

result += color * vec3( p->mat.reflectivity ) * raytrace( ray( point + re * epsilon, re, ++curr_id ) ... );



#13 GarFieldo   Members   -  Reputation: 130

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Posted 12 February 2013 - 01:54 PM

Hm since this would be representative to 0.000001m (at least in my raytracer), this should be close enough.

I didn't notice that you already did the same thing by moving the ray away from the object as described by nfactorial (the result is the same as my approach). What is your value for epsilon?



#14 Yours3!f   Members   -  Reputation: 1300

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Posted 12 February 2013 - 04:38 PM

Hm since this would be representative to 0.000001m (at least in my raytracer), this should be close enough.

I didn't notice that you already did the same thing by moving the ray away from the object as described by nfactorial (the result is the same as my approach). What is your value for epsilon?

 

:)

globals.h, 24:

static const float fepsilon = 0.0001f;






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