[HDRI] calibrating HDR pictures with real world luminance

[Pierre-Felix Breton]

Hi!
 
I am experimenting with HDR images and a luminance meter to see how tone
mappers can actually help reproducing human perception.
 
Here is what I  am currently  doing:
 
a)capture a serie of LDR images using a Canon Rebel XT, only changing the
shutter speed, with constant Fstop,  saved as JPG with sRGB profile
b)creating a calibration curve in HDR Shop
c)measuring a few points in cd/m2 with a luminance meter (LS-110)
d)making an HDR image out of the LDR images using the calibration curve
e)experimenting different tone mappers available on the DVD of the HDRI book
from our favorite well known HDRI gurus.
 
Here are a few questions that puzzles me still:
 
1)  I am trying to scale my HDR image to the measured physical  values.  I
found somewhere that luminance = 179 * (R*0.265 + G*0.067 + B*0.065).  Can I
assume that this equation is the one to use to scale my HDR file properly?
 
2) In some references, it is mentionned that I should always capture LDRs
with a Daylight white balance.  Is this the general agreement?  Even if my
scene is lit with tungsten lights?
 
3)I found limited explanation on how the camera calibration curve would
affect the results in HDR Shop.  I would have to think  that this is to
linearize the color values (remove the effect of Gamma + CMOS response) so
luminance measurements can be actually applied to the colors later on.
 
4)HDR Shops refers to Fstops in the UI.  However,  I read that you want to
maintain Fstops constant and only change the shutter speed.  The problem I
have is that the interpreted Fstops of HDR Shop turn into some "arbitrary"
numbers and I can't really type-in the values used by my camera.  
 
I am not too sure if I am using the right methods.   Some "cook  book" would
be handy.
 
Best regards,
 
Pierre-Félix Breton
www.pfbreton.com <http://www.pfbreton.com/> 

  
 
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