# What are the meanings of theta and gamma in this equation?

I'm having a bit of trouble understanding the notation from this paper: http://graphics.cs.aueb.gr/graphics/docs/papers/GraphiCon09_PapadopoulosPapaioannou.pdf

float3 surface = exp(dot(-y, d)) * fromSurface;
float3 viewer = exp(dot(-y, d)) * fromViewer;
float3 final = dot(photonIntensity, dot(mie(0), dot(viewer, surface));


I know I = intensity, mie is mie scattering, but what does the wierd 0 stand for? And what is y? Is d = distance?

Second go at it:

float Attenuation = 130.0;
float MieAnisotropy = 0.7;

float MiePhase(float CosTheta, float Anisotropy)
{
const float F = 1.0 / (4.0 * PI);
return F * ((1.0 - pow(Anisotropy, 2.0)) / pow(1.0 - 2.0 * Anisotropy * CosTheta + pow(Anisotropy, 2.0), 1.5));
}

float3 ViewPosition = normalize(WorldPosition - CameraPosition);
float CosViewSunAngle = dot(ViewPosition, SunDirection);
float Mie = MiePhase(CosViewSunAngle, MieAnisotropy);

float FromSurface = exp(dot(-Attenuation, DistanceFromSurface));
float FromViewer = exp(dot(-Attenuation, DistanceFromViewer));
float PhotonIntensity = 1.0;
float Final = dot(PhotonIntensity, dot(Mie, dot(FromViewer, FromSurface));

• The weird 0 is a Greek letter theta. The weird y is a Greek letter gamma. The paper should tell you what they are but I'm not going to read it.... – immibis Jan 30 '18 at 2:17

Immediately after the first appearance of the equation in the paper, the use of $\theta$ is described as
The use of $\gamma$ is first described just above; it
"Medium" in this sense means the substance through which the photon is traveling (not "medium" as in "low, medium, high"), so this is basically a constant for tweaking. Later in the paper, an example value for $\gamma$ is given (in the "Implementation Details" section).
And yes, $d$ is distance from the respective points.