3D graphics eBook - Course Materials Repository

Shadow mapping 180
Drawing the scene
Drawing the scene with shadows can be done in several different ways. If
programmable shaders are available, the depth map test may be performed by
a fragment shader which simply draws the object in shadow or lighted
depending on the result, drawing the scene in a single pass (after an initial
earlier pass to generate the shadow map).
If shaders are not available, performing the depth map test must usually be
implemented by some hardware extension (such as GL_ARB_shadow [1] ),
which usually do not allow a choice between two lighting models (lighted and
shadowed), and necessitate more rendering passes:
Final scene, rendered with ambient
shadows.
1. Render the entire scene in shadow. For the most common lighting models (see Phong reflection model) this
should technically be done using only the ambient component of the light, but this is usually adjusted to also
include a dim diffuse light to prevent curved surfaces from appearing flat in shadow.
2. Enable the depth map test, and render the scene lit. Areas where the depth map test fails will not be overwritten,
and remain shadowed.
3. An additional pass may be used for each additional light, using additive blending to combine their effect with the
lights already drawn. (Each of these passes requires an additional previous pass to generate the associated shadow
map.)
The example pictures in this article used the OpenGL extension GL_ARB_shadow_ambient [2] to accomplish the
shadow map process in two passes.
Shadow map real-time implementations
One of the key disadvantages of real time shadow mapping is that the size and depth of the shadow map determines
the quality of the final shadows. This is usually visible as aliasing or shadow continuity glitches. A simple way to
overcome this limitation is to increase the shadow map size, but due to memory, computational or hardware
constraints, it is not always possible. Commonly used techniques for real-time shadow mapping have been developed
to circumvent this limitation. These include Cascaded Shadow Maps, [3] Trapezoidal Shadow Maps, [4] Light Space
Perspective Shadow maps, [5] or Parallel-Split Shadow maps. [6]
Also notable is that generated shadows, even if aliasing free, have hard edges, which is not always desirable. In order
to emulate real world soft shadows, several solutions have been developed, either by doing several lookups on the
shadow map, generating geometry meant to emulate the soft edge or creating non standard depth shadow maps.
Notable examples of these are Percentage Closer Filtering, [7] Smoothies, [8] and Variance Shadow maps. [9]