This note writes about casting lights and casting shadows, i.e. projections.
We discuss the principles of casting shadows, especially by point lights (e.g. bulbs, lamps, etc.) and directional lights (e.g. sunlight, moonlight).
Point lights
Directional lights
Example 1 Simple geometry
Let's just follow the steps below to understand the rules of casting shadows.
[ Point Light ]
Step 1 Build up the perspective reference. Settling down the vanishing points and the horizon. For convenience I will just use 2-VP perspective here.
Step 2 Draw a small house and put several boxes beside. Keep the reference lines.
Step 3 Also let's get our plans and sections prepared.
Step 4 Now we need to set our point light. Remember that in order to tell the exact position of the point light, we also need to know the position of the foot of the light source. The foot lands on the ground, so by using the perspective reference, we can
Step 5 Now connect the airborne points with the lighting source. The points on the ground will not cast shadow, they will only form occlusion shadows.
Step 6 Connect the foot with the corresponding projection of the airborne points on the ground. Since we are using cubes, we don't need to draw any additional points here. Notice how the shadow borders follow the perspective.
Step 7 Clean up the lines and we finished the shading for point light.
[ Directional Lights ]
Different from point lights whose lightrays are radiated from the light source, the lights for directional lights are all parallel.
Step 1 Setting up the directions for light and shadow. Since sunlight changes its direction frequently, the directions can be arbitrary.
Step 2 Repeat the previous step until we can form the shadow on the ground.
The rest are similar, so I will not repeat and finish the shadows for the cubes beside here.
Example 2 Indoor Scene
Example 3 Character