Re: [Openexr-user] retrieving light informations from an exr image

[Florian Kainz]

Enrico Borrione wrote:
> 
> hi all,
> new mail, old problems...
> good enough my cubemap is working... just it is 8 bit converted. I think
> it isn't possible to take advantage of 16 bit framebuffers 4 the
> visualization part yet.
> Anyway, i'd like to extract the actual light infos from the image
> itself... somehow. I think my target is to get the Y part out of the rgb
> half values. Is it right? the formula is Y=R*w+G*w+B*w?? Y is the light
> intensity radiated from that direction?
> 
> Difficult subject for my limited brain :)
> 
> thx all!!
> 


A pixel's luminance, Y, is a weighted sum of the pixel's R, G and B
values.  The proper weighting factors can be derived from the 
chromaticities of R, G, B and white.

Given an OpenEXR file's "chromaticities" and "whiteLuminance" attributes, 
the code snippet below computes a pixel's absolute luminance, in candelas 
per square meter.  (If one or both of the attributes are missing, the code 
falls back to reasonable default values.)

    RgbaInputFile in (...)

    Chromaticities cr;
    float wl = 1;

    if (hasChromaticities (in.header()))
        cr = chromaticities (in.header());

    if (hasWhiteLuminance (in.header()))
        wl = whiteLuminance (in.header());

    M44f m = RGBtoXYZ (cr, wl);

    Rgba &pixel = ...

    float Y = pixel.r * m[0][1] + pixel.g * m[1][1] + pixel.b * m[2][1];

(Note that "luminance" is not the same as "intensity".  For definitions
of the terms, see http://www.schorsch.com/kbase/glossary/luminance.html,
http://www.schorsch.com/kbase/glossary/luminous_intensity.html, and
http://www.schorsch.com/kbase/glossary/radiant_intensity.html.)