Rendering a shadowed scene involves two major drawing steps. The first produces the shadow map itself, and the second applies it to the scene. Depending on the implementation (and the number of lights), this may require two or more drawing passes.
Creating the shadow map The first step renders the scene from the light's point of view. For a point light source, the view should be a
perspective projection as wide as its desired angle of effect (it will be a sort of square spotlight). For directional light (e.g., that from the
Sun), an
orthographic projection should be used. From this rendering, the depth buffer is extracted and saved. Because only the depth information is relevant, it is common to avoid updating the color buffers and disable all lighting and texture calculations for this rendering, to save drawing time. This
depth map is often stored as a texture in graphics memory. This depth map must be updated any time there are changes to either the light or the objects in the scene, but can be reused in other situations, such as those where only the viewing camera moves. (If there are multiple lights, a separate depth map must be used for each light.) In many implementations, it is practical to render only a subset of the objects in the scene to the shadow map to save some of the time it takes to redraw the map. Also, a depth offset which shifts the objects away from the light may be applied to the shadow map rendering in an attempt to resolve
stitching problems where the depth map value is close to the depth of a surface being drawn (i.e., the shadow-casting surface) in the next step. Alternatively, culling front faces and only rendering the back of objects to the shadow map is sometimes used for a similar result.
Shading the scene The second step is to draw the scene from the usual
camera viewpoint, applying the shadow map. This process has three major components. The first step is to find the coordinates of the object as seen from the light, as a
3D object only uses
2D coordinates with axis X and Y to represent its geometric shape on screen, these vertex coordinates will match up with the corresponding edges of the shadow parts within the shadow map (depth map) itself. The second step is the depth test which compares the object z values against the z values from the depth map, and finally, once accomplished, the object must be drawn either in shadow or in light.
Light space coordinates To test a point against the depth map, its position in the scene coordinates must be transformed into the equivalent position as seen by the light. This is accomplished by a
matrix multiplication. The location of the object on the screen is determined by the usual
coordinate transformation, but a second set of coordinates must be generated to locate the object in light space. The matrix used to transform the world coordinates into the light's viewing coordinates is the same as the one used to render the shadow map in the first step (under
OpenGL this is the product of the model, view and projection matrices). This will produce a set of
homogeneous coordinates that need a perspective division (
see 3D projection) to become
normalized device coordinates, in which each component (
x,
y, or
z) falls between −1 and 1 (if it is visible from the light view). Many implementations (such as OpenGL and
Direct3D) require an additional
scale and bias matrix multiplication to map those −1 to 1 values to 0 to 1, which are more usual coordinates for depth map (texture map) lookup. This scaling can be done before the perspective division, and is easily folded into the previous transformation calculation by multiplying that matrix with the following: \begin{bmatrix} 0.5 & 0 & 0 & 0.5 \\ 0 & 0.5 & 0 & 0.5 \\ 0 & 0 & 0.5 & 0.5 \\ 0 & 0 & 0 & 1 \end{bmatrix} If done with a
shader, or other graphics hardware extension, this transformation is usually applied at the vertex level, and the generated value is interpolated between other vertices and passed to the fragment level.
Depth map test Once the light-space coordinates are found, the
x and
y values usually correspond to a location in the depth map texture, and the
z value corresponds to its associated depth, which can now be tested against the depth map. If the
z value is greater than the value stored in the depth map at the appropriate (
x,
y) location, the object is considered to be behind an occluding object and should be marked as a
failure, to be drawn in shadow by the drawing process. Otherwise, it should be drawn lit. If the (
x,
y) location falls outside the depth map, the programmer must either decide that the surface should be lit or shadowed by default (usually lit). In a
shader implementation, this test would be done at the fragment level. Also, care needs to be taken when selecting the type of texture map storage to be used by the hardware: if interpolation cannot be done, the shadow will appear to have a sharp, jagged edge (an effect that can be reduced with greater shadow map resolution). It is possible to modify the depth map test to produce shadows with a soft edge by using a range of values (based on the proximity to the edge of the shadow) rather than simply pass or fail. The shadow mapping technique can also be modified to draw a texture onto the lit regions, simulating the effect of a
projector. The picture above captioned "visualization of the depth map projected onto the scene" is an example of such a process.
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), which usually does not allow a choice between two lighting models (lit and shadowed), and necessitate more rendering passes: • 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. • Enable the depth map test and render the scene lit. Areas where the depth map test fails will not be overwritten and will remain shadowed. • 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 to accomplish the shadow map process in two passes. == Shadow map real-time implementations ==