To handle transparent objects, we can add a 4th channel to color, $\alpha$ = opacity, range [0,1]
Transparency = 1 - Opacity
Applications: Image compositing, particle rendering, volume rendering
Two ways to represent colors: pre-multiplied and straight alpha
Operators: over, in , out , atop, xor
Straight:
$$ c_0 = \frac{c_f\alpha_f + c_b\alpha_b(1-\alpha_f)}{\alpha_f + \alpha_b(1-\alpha_f)} $$
Pre-multiplied
$c_0 = c_f + c_b(1-\alpha_f)$ ($c_f$ → color of front, $c_b$ → color of back )
$\alpha_0 = \alpha_f + \alpha_b(1-\alpha_f)$
For modeling cloud, smoke, water, crowd, flock: using behavior
This model describes objects in an algorithmic manner (spheres and ellipsoids), generate polys only when necessary. It also combine computer graphics with physical laws or for modeling solid objects
Sprites vs Billboards (always facing toward the camera)
2D projection of 3D sampled dataset
Volume rendering algorithms:
Usually no illumination or shadows, just compositing
Therefore only ray-casting, no recursive ray-tracing
No perspective, only parallel projection