[Radiance-general] Reflectance meausrement using macbethcal utility

[Greg Ward]

Is the illumination of your scene uniform?  Unless you are in direct sunlight with everything in the same plane and far away from obstructions, this is very difficult to achieve in practice.  Reflectance measurements from anything other than a scanner will give large errors of 20-50% in most cases.  If this is good enough for you, then go ahead.

-Greg

> From: Jia Hu <hujia06 at gmail.com>
> Subject: Re: [Radiance-general] Reflectance meausrement using macbethcal utility
> Date: July 30, 2014 4:48:36 PM PDT
> 
> Hi Greg,
> 
>  I do not require a high accuracy of reflectance. How about If I use pvalue to calculate the average of RGB values for a surface of interest in the HDR image, and then use the average to estimate the surface reflectance? 
> 
> Thanks :)
> Jia  
> 
> 
> On Wed, Jul 30, 2014 at 12:09 PM, Greg Ward <gregoryjward at gmail.com> wrote:
> It's all explained (as best I can) in the macbethcal man page:
> 
>         http://radsite.lbl.gov/radiance/man_html/macbethcal.1.html
> 
> You give it an HDR file of a Macbeth color checker chart under uniform illumination, and identify the chart corners in the image.  From that, it creates a color mapping file you can use with pcomb to correct the image.
> 
> However, it does not work very well for digital cameras.  You are much better off converting the RAW files directly using Dave Coffin's dcraw, which you can find on the web.  Getting to reflectance is more difficult, and depends on your situation.
> 
> Check out the following if you haven't already:
> 
>         http://radsite.lbl.gov/radiance/refer/sg96crs.pdf
> 
> Cheers,
> -Greg
> 
> > From: Jia Hu <hujia06 at gmail.com>
> > Subject: [Radiance-general] Reflectance meausrement using macbethcal utility
> > Date: July 29, 2014 8:22:08 PM PDT
> >
> > Hello,
> >
> > I am using Radiance macbethcal to estimate the material reflectance.  If I get the calibrated HDR image, how could I determine the RGB/reflectance?   Should  I pick a point in the generated HDR image to retrieve the RGB values, and then directly set the RGB to material RGB in Radiance?
> >  Is there any Radiance tool to directly obtain RGB value of a given pixel?
> >
> > Thanks,
> > Jia
> 
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