[Radiance-general] Luminance contrast calculation

[Greg Ward]

Did no one respond to this?

There isn't really such a thing as "average luminance" for a surface.  It has to be "average luminance for a surface from a specific view," which sounds like what you are discussing.  In this context, I don't think it makes much difference whether you average by pixel solid angle or not, as it tends to change slowly over an image.  Similarly, the view type does not matter very much to the result.

If you actually want to know the significance of an object's luminance to glare or the like, then you don't want to average luminance at all -- you want the contribution to vertical illuminance as a sum of pixels times projected solid angle.  For this, simply adding up pixels from a hemispherical fisheye is the easiest method.

The whole notion of luminance contrasts in a viewing environment is ill-conceived in my opinion.  Originally, the rule of thumb was a maximum contrast ratio of 3:1, which no reasonable space actually achieves.  One specular highlight will ruin your whole metric!

Cheers,
-Greg

> From: Marija Velickovic <maricanis at gmail.com>
> Date: June 18, 2013 3:16:21 AM PDT
> 
> Hi,
> 
> I'm trying to calculate luminance contrast between various surfaces or Radiance image, and compare it to recommended values.
> Idea is to calculate for each suface (eg. paper, desk, walls, floor etc) average luminance, and obtain contrast as ratio between these luminances
> 
> 
> Question: How to calculate average luminance for a surface on Radiance image?
> My first idea was to identify pixels for each surface, and calculate their average luminance as simple average value:
> Average_lum = sum(pixel_luminance)/num_of_surface_pixels,
> where only pixels for specific surface are taken into account.
> 
> But after some thinking, maybe that wouldn't be correct approach. 
> Maybe solid angle of each pixel should be taken into account to have physically correct value 
> Average_lum = sum(pixel_luminance*pixel_solid_angle)/sum(pixel_solid_angle), 
> where only pixels for specific surface are taken into account.
> 
> So my real question would be: if we calculate average luminance for some part of radiance image, should we take into account size of each pixel (solid angle) or not?
> 
> ###
> Related to contrast calculation I'm still thinking if images I'm rendering and analysing should be hemispherical (like for evalglare) or have some human-like field of vision (-vtv -vh 110 -vv 70 for example)
> Suggestions?
> 
> ###
> Another question is more theoretical - when we are talking about contrasts, should we take into account average values (for work task, environment surfaces etc), or extreme values like maximal luminance.
> 
> I know this question is somewhat close to glare calculation, so maybe Jan has some experience with luminance calculation for various surfaces on the scene.
> 
> Thanks in advance,
> Marija
> 
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