Only Emission

Our next restriction is the opposite of the previous one. We now assume that the volume emits light, but the attenuation is negligible. With this assumption, defining the emission in terms of luminance, which is in units of per particle area, is not practical as the particle area is zero. Therefore, rather than use Equation 1.2 as the form for the volume rendering equation, we will instead use Equation 1.5 defined in Section 1.2.2 that uses a glow parameter instead of luminance. Setting $\tau (s) = 0$, Equation 1.5 reduces to

$$I(D) = I_0 + \int_0^D g(s) ds$$ (13)

The only-emission model has all the same advantages as the absorption-only model (defined in Section 1.3.1), but it shares all the same disadvantages also. In addition, the only-emission model suffers from color saturation. Equation 1.16 has no bounds on the intensity of the final light ray, and in practice, the intensity can easily soar beyond what a display device can handle. Therefore, real volume rendering systems seldom use the only-emission model.