Chaos Corona Forum

Chaos Corona for 3ds Max => [Max] I need help! => Topic started by: GRouslan on 2014-10-10, 16:00:07

Title: Calibrated hdri
Post by: GRouslan on 2014-10-10, 16:00:07

Hello, is there a way to use calibrated to cd/m2  HDRI maps correctly?

Title: Re: Calibrated hdri
Post by: GRouslan on 2014-10-13, 20:49:44

Here is the example, how I can do it in Mental ray. (See attached hdri map)
(https://forum.corona-renderer.com/index.php?action=dlattach;topic=5486.0;attach=22436;image)

Title: Re: Calibrated hdri
Post by: Ondra on 2014-10-13, 21:11:54

corona interprets the input as W/m2/sr

Title: Re: Calibrated hdri
Post by: GRouslan on 2014-10-13, 21:43:20

1cd/m2 = 1 lumen/m2/sr
lm = W/ (lumen per watt)

How many lumens in 1 watt?

Title: Re: Calibrated hdri
Post by: Ondra on 2014-10-13, 21:50:06

it is not that simple and I dont have time to study details now.. but try multiplying or dividign with 683

Title: Re: Calibrated hdri
Post by: GRouslan on 2014-10-13, 22:02:02

Probably they are equal. Thank you.

Title: Re: Calibrated hdri
Post by: borisquezadaa on 2014-10-14, 00:02:44

It depends on the efficiency of the lamp. But beacuse its already calibrated, i would asume there is no "loss" in the transformation cause its not a "ligth bulb" just a measure of that ligth.
So in this case efficiency would be 100%, and therefore are equivalent.

Check this:

http://www.rapidtables.com/calc/light/lumen-to-watt-calculator.htm

Title: Re: Calibrated hdri
Post by: GRouslan on 2014-10-14, 11:25:47

I don't quite understand you, but I have already found http://en.wikipedia.org/wiki/Luminous_efficacy  -


source                                                                      |              Luminous efficacy of radiation     (lm/W)         |      Luminous efficiency
 
 Ideal monochromatic 555 nm                                  |             683                                                                   |      100%
 Ideal monochromatic 507 nm                                  |             1699 lm/W[12] or 1700 lm/W[13]                     |      100%

Title: Re: Calibrated hdri
Post by: borisquezadaa on 2014-10-14, 15:52:33

I was saying that conversion from W to Lumen depends on the "eficiency" of that energy conversión in the real world.
The ideal monochromatic 555 nm source has a Luminous efficacy of 683 lm/W with respect to máximum response of the human eye means something as for each watt applied to the ideal ligth source (some kind of green laser ligth i guess) they produces an output of 683 lumens at 100% eficiency (respect to the response of a human eye).
But for an hdri the encodes ligth in form of a number (a measured number) there is no efficiency involved cause you just have that measured ligth straight away. There is no Watt input to a lamp to see how much that lamp disipates in other forms than visible ligth so you can have a W to Lumen ratio and cause HDRI's are linear encoded in nature i would asume that units are equivalent.