Assistance required with Rendering error

RADIANCE RENDERING ARTIFACTS PROBLEM - SEVERE GRAIN IN INDIRECT ILLUMINATION

Please can any Radiance users assist with a rendering error on an architectural illustration project.

Severe grain appears in a Radiance render where indirect illumination strikes an interior floor, similar to a moiré pattern. Some grain in Radiance images is easily corrected with PFILT using down-sampling and a radial blur filter, but the artifacts encountered were too severe for this to be effective.

Please visit my Internet Site :
www.daiservices.btinternet.co.uk/Radiance/Radiance_v3_5_UNIX/Render_Tests/Radiance_render_grain_error_01.htm
to assess images of the problem.

Kind regards

Digital Architectural Illustration Services

Hi DAI-crew,

the artifacts stem most likely from a special random generator used for all
kinds of ray jittering applied in the process of penumbra generation. I don't
really know why this is done, because pictures often look rather strange with
these patterns.

Luckily, however, Radiance provides a way to switch this off, just recompile
the whole stuff with the compiler flag -DMC, and the penumbras will look much
better.

You might experience problems in compiling with -DMC, at least I did (with
Linux). I had to change the following:

in ray/src/common/random.h :

put line 71 in #ifndef MC exclusion:

#ifndef MC /* <--- inserted */
extern int initurand(int size);
#endif /* <---- inserted */

in ray/src/rt/raycalls.c

the same with line 171ff:

#ifndef MC /* <--- inserted */
                                        /* initialize urand */
if (urperm == NULL)
                initurand(2048);
#endif /* <-----inserted */

Apart from this general treatment really strong artifacts may also appear due
some geometry error in the scene file (for example if a light source is
partly cut by some object).

-Carsten

Your website is not set up correctly to view the images -- I had to
go into the HTML source in order to get the correct URLs for the referenced
images.

Carsten's suggestion of using the -DMC build option will alter the
appearance of these artifacts, but will not eliminate them. My guess
is that the sampling noise you are seeing here arises from specular artifacts
rather than source sampling, which is why altering the -d* options did
not correct the problem. Unfortunately, there are no quick fixes for
specular sampling noise, but I can offer some suggestions:

1) Make sure only metallic surfaces have specular components greater than 0.06
2) Increase the basic sampling rate (i.e., use greater initial resolutions in
rpict and greater picture reductions in pfilt)
3) Decrease the value of the -sj parameter.
4) Increase the value of the -st parameter.

The use of low-discrepency sequences is a widely recognized means to
improved Monte Carlo convergence rates, and this is what Radiance
uses by default. The -DMC option switches over to a "pure" Monte
Carlo approach, which requires longer to converge to an average but
does not contain any visible patterns in its noise.

I hope this helps.
-Greg

···

------------ Original Message -----------------

RADIANCE RENDERING ARTIFACTS PROBLEM - SEVERE GRAIN IN INDIRECT =
ILLUMINATION

Please can any Radiance users assist with a rendering error on an =
architectural illustration project.

Severe grain appears in a Radiance render where indirect illumination =
strikes an interior floor, similar to a moir=E9 pattern. Some grain in =
Radiance images is easily corrected with PFILT using down-sampling and a =
radial blur filter, but the artifacts encountered were too severe for =
this to be effective.

Please visit my Internet Site :
www.daiservices.btinternet.co.uk/Radiance/Radiance_v3_5_UNIX/Render_Tests=
/Radiance_render_grain_error_01.htm
to assess images of the problem.

Kind regards

Digital Architectural Illustration Services