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{
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Glass Bead Retroreflector BRDF
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4/25/2002 Greg Ward
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Use with metfunc type, where A4 is estimated total retroreflection
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(doesn't need to be accurate) and A5 is the specific intensity
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(candelas per lux per square meter) at normal incidence and a
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0.5 degree observation angle. If you have a value at some
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other angle, simply divide it by obs_mult1(obs_meas_deg)
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to get the equivalent value at 0.5 degrees.
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Do not use this file with prismatic retroreflectors!
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}
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ldot(x,y,z) = max(cos(89.5*DEGREE), Nx*x + Ny*y + Nz*z);
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normk = A5 / grey(A1,A2,A3) / A4;
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normf(x,y,z) = normk / ldot(x,y,z);
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{ fitted function of observation angle }
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{ based on ray simulation of glass bead }
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obs_mult1(a) = if(.5-a,
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cos(PI/2/.5*a)*3 + 1,
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if(20-a,
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.07/sin(7.6*PI/180*a) + .02,
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.157/(a-19)
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)
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);
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obs_mult(x,y,z) = obs_mult1(Acos(-Dx*x-Dy*y-Dz*z)/DEGREE);
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{ use the following when normal value is unknown }
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retro_paint(x,y,z) = normf(x,y,z) * obs_mult(x,y,z);
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{ fitted function of entrance angle }
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{ based on 3M sheet reflector }
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ent_mult1(a) = exp(-a*a/2220);
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ent_mult(x,y,z) = ent_mult1(Acos(ldot(x,y,z))/DEGREE);
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{ use the following for sheet reflectors }
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retro_sheet(x,y,z) = normf(x,y,z) * obs_mult(x,y,z) * ent_mult(x,y,z);
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