So Bidirectional essentially means "two directions", right? But what are these two direction (left-right? up-down? north-south?) we are talking about in regards to bidirectional reflectance distribution function (BRDF), bidirectional scattering distribution function (BSDF), or bidirectional transmittance distribution function (BTDF) in computer graphics?
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asked May 21, 2017 at 13:53
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$\begingroup$ It means their values depend on two directions (usually incoming and outgoing light). With a direction being a direction in 3-space, of course, not something like "up-down". $\endgroup$– Christian RauMay 21, 2017 at 16:37
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2 Answers
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In general the two directions in BxDF are incoming $\omega_i$ and outgoing $\omega_o$ radiance directions, often defined in spherical coordinates $[\theta, \phi]$ or as a 3D unit vector in Cartesian coordinates $[x, y, z]$. The BxDF $f(\omega_i, \omega_o)$ defines how much of the incident radiance $L_i$ from direction $\omega_i$ hitting the surface is scattered towards direction $\omega_o$. What $\omega_i$ and $\omega_o$ exactly represent depends on the context.
Often $\omega_i$ represents the direction of a light source, but it could also be the direction of a surfel reflecting light or some other light emitting/scattering element in space. $\omega_o$ often represent the direction of the viewpoint (camera/eye), but it could as well be the direction of a surfel or other point in space whose incident radiance we are interested in calculating.
While $\omega_o$ is valid for normal oriented hemisphere above the surface, there are differences in valid $\omega_i$ domain between BxDF's. Because BRDF defines the amount of reflected light $\omega_i$ is valid for the normal oriented hemisphere above the surface. On the other hand, BTDF defines the amount of light transmitted through the surface so $\omega_i$ is valid for the normal oriented hemisphere below the surface. BSDF is the sum of the two so $\omega_i$ is valid for the entire sphere.
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BRDFs are functions the define the distribution of the way light reflects off of a surface. They effectively answer the question, "given a light source here, how much light is reflected to me?" Well, that question has the two directions right there: the direction from the surface point to the light source, and the direction from the surface point to "me": the viewer.
BSDFs and BTDFs work in the same way, just asking slightly different questions. But the givens to those questions are the same: a light source and a viewer. And thus, they use the same directions.
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2$\begingroup$ So the "two directions" are simply the incidence ray and the reflection/transmission ray? :) $\endgroup$ May 21, 2017 at 19:08