Radiance Parameters

The user can use the Quality, Detail and Variability settings to control all the Radiance parameters or change each parameter individually. For most cases, the default settings of (M)edium M M, will give results accurate to +/-20% and is good for initial evaluation of a space. However, for accurate reports and design comparisons, a Variability setting of (H)igh is recommended and required for valid reports. The renderings with these settings might still be a bit splotchy but will give a sense of the daylight distribution. Setting Quality and Variability to H will result in better renderings and more accuracy overall at the cost of increased time. A Detail setting of M is usually sufficient for most spaces. However, if there is a lot of detail due to many smaller windows or many small or skinny block objects, then Detail should be set to H for better accuracy and renderings. Use caution whenever changing Radiance parameters, they can greatly influence the simulations time and accuracy.

Note, any time BSDF files are used and represent a significant source of light into the space, higher Radiance ambient parameters are typically required to maintain accuracy: particularly the ambient division (-ad) parameter should be set rather high at 4096 or 8192. The program will detect when these are used and automatically set -ad to 4096.

Copied from the Radiance manual for rpict, below are descripitions of the various Radiance parameters:

-dj frac Set the direct jittering to frac. A value of zero samples each source at specific sample points (see the −ds option below), giving a smoother but somewhat less accurate rendering. A positive value causes rays to be distributed over each source sample according to its size, resulting in more accurate penumbras. This option should never be greater than 1, and may even cause problems (such as speckle) when the value is smaller. A warning about aiming failure will issued if frac is too large.

-ds frac Set the direct sampling ratio to frac. A light source will be subdivided until the width of each sample area divided by the distance to the illuminated point is below this ratio. This assures accuracy in regions close to large area sources at a slight computational expense. A value of zero turns source subdivision off, sending at most one shadow ray to each light source.

-dt frac Set the direct threshold to frac. Shadow testing will stop when the potential contribution of at least the next and at most all remaining light sources is less than this fraction of the accumulated value. (See the −dc option below.) The remaining light source contributions are approximated statistically. A value of zero means that all light sources will be tested for shadow.

−dc frac Set the direct certainty to frac. A value of one guarantees that the absolute accuracy of the direct calculation will be equal to or better than that given in the −dt specification. A value of zero only insures that all shadow lines resulting in a contrast change greater than the −dt specification will be calculated.

-dr N Set the number of relays for secondary sources to N. A value of 0 means that secondary sources will be ignored. A value of 1 means that sources will be made into first generation secondary sources; a value of 2 means that first generation secondary sources will also be made into second generation secondary sources, and so on.

-dp D Set the secondary source presampling density to D. This is the number of samples per steradian that will be used to determine ahead of time whether or not it is worth following shadow rays through all the reflections and/or transmissions associated with a secondary source path. A value of 0 means that the full secondary source path will always be tested for shadows if it is tested at all.

-ss samp Set the specular sampling to samp. For values less than 1, this is the degree to which the highlights are sampled for rough specular materials. A value greater than one causes multiple ray samples to be sent to reduce noise at a commmesurate cost. A value of zero means that no jittering will take place, and all reflections will appear sharp even when they should be diffuse.

-st frac Set the specular sampling threshold to frac. This is the minimum fraction of reflection or transmission, under which no specular sampling is performed. A value of zero means that highlights will always be sampled by tracing reflected or transmitted rays. A value of one means that specular sampling is never used. Highlights from light sources will always be correct, but reflections from other surfaces will be approximated using an ambient value. A sampling threshold between zero and one offers a compromise between image accuracy and rendering time.

-av red grn blu Set the ambient value to a radiance of red grn blu . This is the final value used in place of an indirect light calculation. If the number of ambient bounces is one or greater and the ambient value weight is non-zero (see -aw and -ab below), this value may be modified by the computed indirect values to improve overall accuracy.

-aw N Set the relative weight of the ambient value given with the -av option to N. As new indirect irradiances are computed, they will modify the default ambient value in a moving average, with the specified weight assigned to the initial value given on the command and all other weights set to 1. If a value of 0 is given with this option, then the initial ambient value is never modified. This is the safest value for scenes with large differences in indirect contributions, such as when both indoor and outdoor (daylight) areas are visible.

-ab N Set the number of ambient bounces to N. This is the maximum number of diffuse bounces computed by the indirect calculation. A value of zero implies no indirect calculation.

-ar res Set the ambient resolution to res. This number will determine the maximum density of ambient values used in interpolation. Error will start to increase on surfaces spaced closer than the scene size divided by the ambient resolution. The maximum ambient value density is the scene size times the ambient accuracy (see the −aa option below) divided by the ambient resolution. The scene size can be determined using getinfo(1) with the −d option on the input octree.

-aa acc Set the ambient accuracy to acc. This value will approximately equal the error from indirect illuminance interpolation. A value of zero implies no interpolation.

-ad N Set the number of ambient divisions to N. The error in the Monte Carlo calculation of indirect illuminance will be inversely proportional to the square root of this number. A value of zero implies no indirect calculation.

-as N Set the number of ambient super-samples to N. Super-samples are applied only to the ambient divisions which show a significant change.