[Radiance-general] Luminance contrast calculation

[Jan Wienold]

Hi Marija,

there are two issues:
1. metric
2. how to calculate technically

regarding metrics, the whole thing is not as easy. Definitely the DGP 
doesn't catch contrast related glare, when the "contrast" is located 
directly besides the task. For example, if you are investigating a 
computer workplace in an open plan office and just above your computer 
screen is located in the far a window. In this case users might 
experience glare, even for very low luminances, which are also no 
problem regarding reflectances on the computer screen. In that case, a 
ratio 1:3 seems reasonable, but is also not proven by any user 
assessments so far (at least to my knowledge). But what happens, when 
this contrast is more displaced from viewing position? This is  not 
clear to me how to handle. Of course you could evaluate on the "safe 
side", saying in a 30° cone you allow only 1:3, outside that cone 1:10.
Regarding using max or mean of a surface, I would say it depends. If you 
have really a peak (lets say a specular reflection of the sun), then you 
should take it into account as glare source (and weight it with its 
solid angle). This glare is captured pretty good by the DGP, but I 
wouldn't use the luminance of it for a contrast evaluation. For the 
contrast I would use the average luminance of the surface of your interest.

how to calculate contrast for your surfaces: technically you can use 
evalglare for this. I'm currently on a plane and can't check the 
web-site, but as far as I can remember I had a tutorial for that in my 
presentation at the radiance workshop in 2009 in Boston (seminar in 
advance to the workshop). The trick is using masks. basically you have 
to generate a image only containing your surfaces of interest, having 
the correct luminances. then run evalglare in a way, that each surface 
will be one glare source and evaluate them with the -d option. You may 
reduce the search radius (-r option).
Your mask should contain only the surfaces, which you want to 
investigate (=calculate the average luminance of this surface). In the 
case of HDR images, I used simple photoshop to mark the area of interest 
(-> make them black). In case of simulations, it is easier, since you 
can just change the reflectance of your surfaces of interest to zero and 
generate an masking image. If the surfaces are not touching each other, 
you can evaluate all the surfaces also at once. The masking command I 
used was pcomb /input.pic/--s -1000 /mask.pic/> /eval.pic./ Using the -d 
option of evalglare (and switching off the peak extraction by -x) you 
get provided all found glare sources, including position in x-y 
coordinates, solid angle and average luminance. You should check by the 
-c option, if evalglare detects the surfaces properly. Each found glare 
source should correspond then your surfaces of interest. Don't use any 
of the glare metrics then, just the information of the glare sources.

Good luck!Best,

Jan


On 06/21/2013 08:58 PM, Greg Ward wrote:
> 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 
>> <mailto: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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-- 
Dr.-Ing. Jan Wienold
Head of Team Passive Systems and Daylighting
Fraunhofer-Institut für Solare Energiesysteme
Thermal Systems and Buildings
Heidenhofstr. 2, 79110 Freiburg, Germany
Phone: +49(0)761 4588 5133 Fax:+49(0)761 4588 9133
jan.wienold at ise.fraunhofer.de
http://www.ise.fraunhofer.de

In office:
Mo,Tue: 8:30-18:00
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