[Radiance-general] Radiance Vs Viz

[Martin Moeck]

Hi Mark,
 
see below.
 
Martin Moeck

	-----Original Message----- 
	From: Mark de la Fuente [mailto:MdelaFuente at wmtao.com] 
	Sent: Tue 5/18/2004 7:07 PM 
	To: radiance-general at radiance-online.org 
	Cc: 
	Subject: [Radiance-general] Radiance Vs Viz
	
	 
	 
	Even though we are specifically talking about Radiance Vs Lightscape & Viz, I think this is in part more a comparison about radiosity Vs raytracing.  From what I know, the big problem with radiosity is that it simplifies things and assumes lambertian reflections.  This is obviously a deviation from reality, but I assume it's still possible to achieve a pretty accurate calculation of some sort depending on the situation.  (I know the person I'm having this discussion with is certainly convinced you can).  Therefore what I am more interested in, is knowing what type of calculation you really CAN'T do with radiosity (Viz or Lightscape) that you could do with Radiance.
	 
	 
	1) Venetian blinds: If you cannot subdivide surfaces like Venetian blinds into very small patches and override the automatic triangular subdivision (i.e., Lumen Designer), the calculation is meaningless. Exception: The new AGI32
	 
	2) Specular highlights are not considered in the hybrid raytracing method in AGI32, Lumen Designer and VIZ4. You might see a small reflection of a light source on a varnished oak floor. If you increase -ad to a large number, i.e., 5000 - 10,000, Radiance will consider the contribution of that specular highlight. The luminance of this highlight is typically very significant, even if the specular reflectance of the varnished floor is 1% or less. A sun beam reflected off that floor can create an additional 200 Lux on the ceiling in the specular direction, which the radiosity calculation  never catches. This error is very significant. 
	 
	3) You can combine Radiance plastic, metal and glass with the mirror material and turn any Radiance surface that has a slight or large specular component into a virtual light source. Again, the error is very significant in radiosity since those specuar reflections are ignored. 
	 
	4) The important underlying assumption in using radiosity is that building materials are mostly diffuse, and might only have a specularity of 0.1% or so. Assume even a very low specularity of 0.1%, times the luminance of the sun or an HID lamp reflected in that "matte" material. This is an extremely bright virtual light source, no matter if the material is black or white. That contribution cannot be ignored. If you have an aluminum ceiling, aluminum lightshelf and some blinds, radiosity cannot be used at all, since the specularity of most aluminum materials is 50% to 902%. 
	 
	 
	 
	 
	 
	 Like if I am trying to evaluate the performance of a light shelf, can I use a radiosity based engine to compare the differences from one light shelf of material A with the same light shelf made out of material B?  I mean if material A is matte white paint and ! material B is chrome, how could you compare the two scenarios if the software assumes lambertian reflections?
	 
	 
	Not at all. You would have to do it by hand and it would be much more accurate. The diffuse and specular reflectance of both materials is known, and all you need is Excel to calculate the specular and diffuse light transfer. Then add 15% indirect and you got it. 
	 

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