MCPov

General description
    The ray tracing algorithm
    MCPov modifications
Diffuse contribution
    Cosine weighted sampling
    Russian Roulette

Portals
        Triangular portals
        Spherical portals
        Bounding sphere portals
        Portal overlap
        Portal speedup
Reflective contribution
Refractive contribution
Global settings
Multi core/CPU support
Sample scenes
Limitations - bugs
Release notes
Download
Links
Contact

    test scene : test_01.01a.pov, 30 seconds rendering.

The use of the keyword mc_use_cosine_distrib activate the cosine weighted hemisphere sampling.
It should reduce the noise quickly than a uniform sampling.

 

When a sky is define with a hdr bitmap, the sampling of the brighter parts of the picture reduce the noise.
The use of the keyword mc_sky activate the sky importance sampling :

    finish { montecarlo { mc_sky { sky_grid_dimension } } }
    sky_grid_dimension : define the grid dimension for the build of the importance sampling distribution. Should be lower or equal to 1000.

    example : finish { montecarlo { mc_sky { 1000 } } }

If both the sky importance sampling and the cosine weighted sampling is used, sky importance sampling is used.

test scene : sky.pov (hdr file available here), 1 minute rendering without the use of the mc_sky keyword :

test scene : sky.pov (hdr file available here), 1 minute rendering with the use of the mc_sky keyword :

    Portals

As shown in the previous pictures, the contribution of an object with ambient term is difficult to catch if the emitting object is small or far from the hit point (most of the ray miss the object).
A portal define an area on the hemisphere that is sampled in priority with a latin hypercube sampling pattern.
A portal is an object. This object may be visible or not.
For instance, a portal may be (or surround) :
- a small object with ambient term.
- a refractive object.
In the first case, it reduces the noise and in the second case it helps the caustic resolution.

There are 3 types of portals :

    Spherical portals
        It is a sphere object define as follow :
            sphere { c, r
                montecarlo  {  mc_portal { n1_max n2_max n1_min n2_min }  }
                ...
            }
        The montecarlo block must be just after the sphere radius.
        With :
            n1_max : the maximum number of rays when the hitting ray is a primary ray (origin from the camera).
            n2_max : the maximum number of rays for the other hitting rays.
            n1_min : the minimum number of rays when the hitting ray is a primary ray (origin from the camera).
            n2_min : the minimum number of rays for the other hitting rays.

        test scene : test_01.02.pov, 30 seconds rendering :

   Triangular portals
        It is a triangle object define as follow :
            triangle { p1, p2, p3
                montecarlo  {  mc_portal { n1_max n2_max n1_min n2_min }  }
                ...
            }
        The montecarlo block must be just after the last triangle point.
        n1_max, n2_max, n1_min and n2_min have the same meaning than for a spherical portal.
      
        test scene : test_02.pov, 30 seconds rendering :

    Bounding sphere portals
        A bounding spherical portal is defined around an object as follow (with the example of a box) :
            box { p1, p2
                ...
                montecarlo  {  mc_portal { n1_max n2_max n1_min n2_min }  }
                ...
            }
        The montecarlo block may be place anywhere in the object block description.
        n1_max, n2_max, n1_min and n2_min have the same meaning than for a spherical portal.

                  Caution : If a bounding sphere portal is defined on a triangular or spherical portal, 2 portals are defined for the same object.

                  test scene : test_03.pov, 2 minutes rendering :

Portal overlap
In case of portal overlap, the standard algorithm introduce errors (some points received n times the same contribution).
The use of the keyword mc_check_portal_overlap solve this problem.

Test scene : test_04.pov, 30 seconds rendering. We see that the points outside the 2 portals are too bright when comparing to the inside ones :

global_settings {
...
  montecarlo
  {
    mc_rand_seed                                                                  // the C random sequence is seed with the current time
    mc_max_diffuse_bounce 3                                              // max number of diffuse bounce
    mc_pixel_refresh_ratio 0.5                                              // ratio of pixels computed at each iteration (try to reduce noise first)
    mc_dof { mc_focal_distance 20.0 mc_lens_radius 0.5 } // depth of  field
    mc_max_diff_stop_value 0.05                                        // stop criteria
    mc_max_pass 1000                                                         // max iteration
    mc_colour_clip 1.0                                                          // max rgb color  for clipping
    mc_check_portal_overlap                                               // handle portal overlap
    mc_sort_portals                                                               // sort portal before use
    mc_russian_roulette_prob 0.5                                         // probability of path stopping
    mc_russian_roulette_threshold 0.1                                 // threshold on pixel contribution to start russian roulette
  }
...
}

keywords	default values	Explanation
mc_rand_seed	deactivated	Initiate the random sequence used by the montecarlo algorithm with the current date.
mc_max_diffuse_bounce	3	Max number of diffuse rays in  the path. The path may also include reflected and refracted rays. The total max path depth is limited by the max_trace_level keyword. Maximum value : 125.
mc_pixel_refresh_ratio	1.0	Define the ratio of the pixels to refresh at each iteration (1.0 -> 100%, 0.5 -> 50%). Every 5 iterations, all the pixels are refreshed.
mc_dof       mc_focal_distance       mc_lens_radius	0.0 0.0	To activate depth of field effect computation.       Gives the distance from the camera to the point in focus. Should be greater than 0.0.       Gives the radius of the lens (to increase in order to increase the effect). Should be greater than 0.0.
mc_max_diff_stop_value	-1.0	Define a rendering stop criteria based on the noise level. Dependant of mc_pixel_refresh_ratio.
mc_max_pass	0	Define the maximum number of pass before the rendering stops. The rendering never stops if null or negative.
mc_colour_clip	0.0	Define the max value for RGB components. 0.0 deactivate the clipping. Introduce a bias in the rendering. Useful with scenes producing bright spots.
mc_check_portal_overlap	deactivated	See more explanation here.
mc_sort_portals	deactivated	Experimental...
mc_russian_roulette_prob	0.5	See more explanation here.
mc_russian_roulette_threshold	0.1	See more explanation here.

MegaPov 1.2.1 doesn't support natively a multi core/CPU rendering.
Nevertheless, the montecarlo integration make easy the use of multi cores/CPUs.
Follow two possible solutions :
- SMPOV : the picture is split and rendered on the different cores. The disaventage is that if some parts are longer to render, the noise doesn't reduce at the same rate. It may be visible when the picture is reassembled.
- Run an instance of  MCPov on each available core/CPU and render different times the same scene. Include in your scene the keyword mc_rand_seed in order to initiate the random number generator differently. Reassemble the different computed pictures in one picture by merging them in any picture manipulation tool (for instance Paint.NET). The disaventage of the method is that the final noise level is not visible without a merging of the pictures (but it is possible to make picture copies and a merging without stopping the rendering).

All scenes rendered on 1 core of a Intel Q6600 2.4 GHz, 4 GB of RAM.

This patch is experimental and has some limitations, bugs and some features of MegaPov 1.2.1 may have been changed or are no more working correctly :
    - Maximum number of portals per scene : 10.
    - Maximum number of samples per portal : 256.
    - Radiosity : should not be used (unnecessary).
    - Antialiasing : should not be used (unnecessary). MCPov do it for all rendered scenes.
    - Photon : incompatible in its current implementation.
    - Media : seems no more working correctly.
    - Object with ambient term : if the patch is not activated in a scene, the ambient term is not taken into account.
    - The PovRay light models (point light, etc.) are not supported.
    - Continuing a trace ( +c ) doesn't work.

    12 may 2008 : version 0.0.1
        initial release.
    15 may 2008 : version 0.0.2
        - compile now correctly under Linux.
        - use of  a fast high-quality RNG as provided by Warp (the original RNG algorithm and implementation is by Bob Jenkins).
    18 may 2008 : version 0.0.3
        - bugfix : sometimes wrong results when depth of field is activated.
        - bugfix : wrong results if a CSG object is defined as portal.
        - bugfix : segmentation fault if the mc_sky keyword is used in a CSG object.
        - bugfix : wrong results with mc_sky.
        - Max grid dimension for sky is now 1000 (instead of 100).

PovRay raytracer
MegaPov raytracer : introduce valuable extension to PovRay (hdr support, etc.).
PovRay Newsgroup : Newsgroup of the PovRay community. Good entry point for help with PovRay use.

Luxrenderer : Open-source free unbiased renderer.
Indigo : Free unbiased renderer.
Monte Carlo Siggraph course : gives math background for implementation.