HDR light map photometric units in fp texture

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  04 April 2013
HDR light map photometric units in fp texture

What units does max store in an 32 bit floating point texture when rendering HDR light map

My tests show that for an expected illuminance of 10000 lux max stores 6.807
if I increase the light intensity by 100x max stores 680.7

So the unit is definitely proportional to illuminance, but what unit is it?

The big question is

How can I combine HDR textures from different sources without knowing what units they use?

Nowhere can I find what units Paul Debevec's light probes use (proportional to luminance, but what?)

Is this the new gamma?
 
  04 April 2013
HDR Images stores luminance data (cd/m).
For HDR images, you have to use linear exposure control and you need the correct physical scale (in 3dsmax 2012 it is the value of 562)

maybe there is also a relation between the values of the framebuffer (65k * value) and the real values ..dunno. ..you will maybe find it in the 3dsmax documentation.
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  04 April 2013
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Yes!

Using Linear Exposure Control seems to fix the problem

But for me using a Physical Scale of 1.0 gives me a correct value of 10000 lux in the baked texture (I am using System Unit = 1.0 metre)

(I am rendering traditional light maps in HDR which capture Illuminance in lux)

I even tried this previously but my System Units had changed (didn't leave "Respect Units" turned on so I didn't notice ...)

The 562 is a mystery to me now ... what System Units are you using?

Thanks seismograph
 
  04 April 2013
I don't know exactly why 562, but i think it was a setup with a software we use here at work and we needed correct values out of 3dsmax.
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  04 April 2013
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