This page describes details of Forward rendering path.
Forward Rendering path renders each object in one or more passes, depending on lights that affect the object. Lights themselves are also treated differently by Forward Rendering, depending on their settings and intensity.
In Forward Rendering, some number of brightest lights that affect each object are rendered in fully per-pixel lit mode. Then, up to 4 point lights are calculated per-vertex. The other lights are computed as Spherical Harmonics (SH), which is much faster but is only an approximation. Whether a light will be a per-pixel light or not is dependent on this:
Rendering of each object happens as follows:
For example, if there is some object that’s affected by a number of lights (a circle in a picture below, affected by lights A to H):
Let’s assume lights A to H have the same color and intensity and all of them have Auto rendering mode, so they would be sorted in exactly this order for this object. The brightest lights will be rendered in per-pixel lit mode (A to D), then up to 4 lights in per-vertex lit mode (D to G), and finally the rest of lights in SH (G to H):
Note that light groups overlap; for example last per-pixel light blends into per-vertex lit mode so there are less “light popping” as objects and lights move around.
Base pass renders object with one per-pixel directional light and all SH lights. This pass also adds any lightmaps, ambient and emissive lighting from the shader. Directional light rendered in this pass can have Shadows. Note that Lightmapped objects do not get illumination from SH lights.
Additional passes are rendered for each additional per-pixel light that affect this object. Lights in these passes can’t have shadows (so in result, Forward Rendering supports one directional light with shadows).
Spherical Harmonics lights are very fast to render. They have a tiny cost on the CPU, and are actually free for the GPU to apply (that is, base pass always computes SH lighting; but due to the way SH lights work, the cost is exactly the same no matter how many SH lights are there).
The downsides of SH lights are:
In summary, SH lights are often good enough for small dynamic objects.