Shadow mapping

Last updated
Scene with shadow mapping 7fin.png
Scene with shadow mapping
Scene with no shadows 3noshadow.png
Scene with no shadows

Shadow mapping or shadowing projection is a process by which shadows are added to 3D computer graphics. This concept was introduced by Lance Williams in 1978, in a paper entitled "Casting curved shadows on curved surfaces." [1] Since then, it has been used both in pre-rendered and realtime scenes in many console and PC games.

Contents

Shadows are created by testing whether a pixel is visible from the light source, by comparing the pixel to a z-buffer [2] or depth image of the light source's view, stored in the form of a texture.

Principle of a shadow and a shadow map

If you looked out from a source of light, all the objects you can see would appear in light. Anything behind those objects, however, would be in shadow. This is the basic principle used to create a shadow map. The light's view is rendered, storing the depth of every surface it sees (the shadow map). Next, the regular scene is rendered comparing the depth of every point drawn (as if it were being seen by the light, rather than the eye) to this depth map.

This technique is less accurate than shadow volumes, but the shadow map can be a faster alternative depending on how much fill time is required for either technique in a particular application and therefore may be more suitable to real-time applications. In addition, shadow maps do not require the use of an additional stencil buffer and can be modified to produce shadows with a soft edge. Unlike shadow volumes, however, the accuracy of a shadow map is limited by its resolution.

Algorithm overview

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

Scene rendered from the light view 1light.png
Scene rendered from the light view
Scene from the light view, depth map 2shadowmap.png
Scene from the light view, depth 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

Visualization of the depth map projected onto the scene 4overmap.png
Visualization of the depth map projected onto the scene

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 modelview 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:

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

Depth map test failures 5failed.png
Depth map test failures

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

Final scene, rendered with ambient shadows 7fin.png
Final scene, rendered with ambient shadows

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:

  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 will 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 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 determine 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]

Shadow mapping techniques

Simple

Splitting

Warping

Smoothing

Filtering

Soft Shadows

Assorted

Miscellaneous

See also

Further reading

Related Research Articles

<span class="mw-page-title-main">Rendering (computer graphics)</span> Process of generating an image from a model

Rendering or image synthesis is the process of generating a photorealistic or non-photorealistic image from a 2D or 3D model by means of a computer program. The resulting image is referred to as a rendering. Multiple models can be defined in a scene file containing objects in a strictly defined language or data structure. The scene file contains geometry, viewpoint, textures, lighting, and shading information describing the virtual scene. The data contained in the scene file is then passed to a rendering program to be processed and output to a digital image or raster graphics image file. The term "rendering" is analogous to the concept of an artist's impression of a scene. The term "rendering" is also used to describe the process of calculating effects in a video editing program to produce the final video output.

<span class="mw-page-title-main">Texture mapping</span> Method of defining surface detail on a computer-generated graphic or 3D model

Texture mapping is a method for mapping a texture on a computer-generated graphic. "Texture" in this context can be high frequency detail, surface texture, or color.

In computer graphics, photon mapping is a two-pass global illumination rendering algorithm developed by Henrik Wann Jensen between 1995 and 2001 that approximately solves the rendering equation for integrating light radiance at a given point in space. Rays from the light source and rays from the camera are traced independently until some termination criterion is met, then they are connected in a second step to produce a radiance value. The algorithm is used to realistically simulate the interaction of light with different types of objects. Specifically, it is capable of simulating the refraction of light through a transparent substance such as glass or water, diffuse interreflection between illuminated objects, the subsurface scattering of light in translucent materials, and some of the effects caused by particulate matter such as smoke or water vapor. Photon mapping can also be extended to more accurate simulations of light, such as spectral rendering. Progressive photon mapping (PPM) starts with ray tracing and then adds more and more photon mapping passes to provide a progressively more accurate render.

<span class="mw-page-title-main">Normal mapping</span> Texture mapping technique

In 3D computer graphics, normal mapping, or Dot3 bump mapping, is a texture mapping technique used for faking the lighting of bumps and dents – an implementation of bump mapping. It is used to add details without using more polygons. A common use of this technique is to greatly enhance the appearance and details of a low polygon model by generating a normal map from a high polygon model or height map.

<span class="mw-page-title-main">Shadow volume</span> Computer graphics technique

Shadow volume is a technique used in 3D computer graphics to add shadows to a rendered scene. They were first proposed by Frank Crow in 1977 as the geometry describing the 3D shape of the region occluded from a light source. A shadow volume divides the virtual world in two: areas that are in shadow and areas that are not.

2.5D perspective refers to gameplay or movement in a video game or virtual reality environment that is restricted to a two-dimensional (2D) plane with little or no access to a third dimension in a space that otherwise appears to be three-dimensional and is often simulated and rendered in a 3D digital environment.

In computer graphics, texture filtering or texture smoothing is the method used to determine the texture color for a texture mapped pixel, using the colors of nearby texels.

<span class="mw-page-title-main">Shader</span> Type of program in a graphical processing unit (GPU)

In computer graphics, a shader is a computer program that calculates the appropriate levels of light, darkness, and color during the rendering of a 3D scene—a process known as shading. Shaders have evolved to perform a variety of specialized functions in computer graphics special effects and video post-processing, as well as general-purpose computing on graphics processing units.

<span class="mw-page-title-main">Lightmap</span> Data structure used in lightmapping

A lightmap is a data structure used in lightmapping, a form of surface caching in which the brightness of surfaces in a virtual scene is pre-calculated and stored in texture maps for later use. Lightmaps are most commonly applied to static objects in applications that use real-time 3D computer graphics, such as video games, in order to provide lighting effects such as global illumination at a relatively low computational cost.

<span class="mw-page-title-main">Real-time computer graphics</span> Sub-field of computer graphics

Real-time computer graphics or real-time rendering is the sub-field of computer graphics focused on producing and analyzing images in real time. The term can refer to anything from rendering an application's graphical user interface (GUI) to real-time image analysis, but is most often used in reference to interactive 3D computer graphics, typically using a graphics processing unit (GPU). One example of this concept is a video game that rapidly renders changing 3D environments to produce an illusion of motion.

<span class="mw-page-title-main">Subsurface scattering</span> Mechanism of light transport

Subsurface scattering (SSS), also known as subsurface light transport (SSLT), is a mechanism of light transport in which light that penetrates the surface of a translucent object is scattered by interacting with the material and exits the surface potentially at a different point. Light generally penetrates the surface and gets scattered a number of times at irregular angles inside the material before passing back out of the material at a different angle than it would have had if it had been reflected directly off the surface.

<span class="mw-page-title-main">Reflection mapping</span>

In computer graphics, reflection mapping or environment mapping is an efficient image-based lighting technique for approximating the appearance of a reflective surface by means of a precomputed texture. The texture is used to store the image of the distant environment surrounding the rendered object.

Clipping, in the context of computer graphics, is a method to selectively enable or disable rendering operations within a defined region of interest. Mathematically, clipping can be described using the terminology of constructive geometry. A rendering algorithm only draws pixels in the intersection between the clip region and the scene model. Lines and surfaces outside the view volume are removed.

<span class="mw-page-title-main">High-dynamic-range rendering</span> Rendering of computer graphics scenes by using lighting calculations done in high-dynamic-range

High-dynamic-range rendering, also known as high-dynamic-range lighting, is the rendering of computer graphics scenes by using lighting calculations done in high dynamic range (HDR). This allows preservation of details that may be lost due to limiting contrast ratios. Video games and computer-generated movies and special effects benefit from this as it creates more realistic scenes than with more simplistic lighting models.

<span class="mw-page-title-main">Cube mapping</span> Method of environment mapping in computer graphics

In computer graphics, cube mapping is a method of environment mapping that uses the six faces of a cube as the map shape. The environment is projected onto the sides of a cube and stored as six square textures, or unfolded into six regions of a single texture.

<span class="mw-page-title-main">Stencil buffer</span>

A stencil buffer is an extra data buffer, in addition to the color buffer and Z-buffer, found on modern graphics hardware. The buffer is per pixel and works on integer values, usually with a depth of one byte per pixel. The Z-buffer and stencil buffer often share the same area in the RAM of the graphics hardware.

In computer graphics, per-pixel lighting refers to any technique for lighting an image or scene that calculates illumination for each pixel on a rendered image. This is in contrast to other popular methods of lighting such as vertex lighting, which calculates illumination at each vertex of a 3D model and then interpolates the resulting values over the model's faces to calculate the final per-pixel color values.

Computer graphics lighting is the collection of techniques used to simulate light in computer graphics scenes. While lighting techniques offer flexibility in the level of detail and functionality available, they also operate at different levels of computational demand and complexity. Graphics artists can choose from a variety of light sources, models, shading techniques, and effects to suit the needs of each application.

PICA200 is a graphics processing unit (GPU) designed by Digital Media Professionals Inc. (DMP), a Japanese GPU design startup company, for use in embedded devices such as vehicle systems, mobile phones, cameras, and game consoles. The PICA200 is an IP Core which can be licensed to other companies to incorporate into their SOCs. It was most notably licensed for use in the Nintendo 3DS.

This is a glossary of terms relating to computer graphics.

References

  1. ↑ Lance Williams. "Casting curved shadows on curved surfaces" (PDF). Retrieved 2020-12-22.{{cite journal}}: Cite journal requires |journal= (help)
  2. ↑ Akenine-Mo ̈ller, Tomas; Haines, Eric; Hoffman, Naty (2018-08-06). Real-Time Rendering, Fourth Edition. CRC Press. ISBN   978-1-351-81615-1.
  3. ↑ "Cascaded shadow maps" (PDF). NVidia . Retrieved 2008-02-14.{{cite journal}}: Cite journal requires |journal= (help)
  4. ↑ Tobias Martin; Tiow-Seng Tan. "Anti-aliasing and Continuity with Trapezoidal Shadow Maps" . Retrieved 2008-02-14.{{cite journal}}: Cite journal requires |journal= (help)
  5. ↑ Michael Wimmer; Daniel Scherzer; Werner Purgathofer. "Light Space Perspective Shadow Maps" . Retrieved 2008-02-14.{{cite journal}}: Cite journal requires |journal= (help)
  6. ↑ Fan Zhang; Hanqiu Sun; Oskari Nyman. "Parallel-Split Shadow Maps on Programmable GPUs". GPU Gems 3. Archived from the original on January 17, 2010. Retrieved 2008-02-14.
  7. ↑ "Shadow Map Antialiasing". NVidia . Retrieved 2008-02-14.
  8. ↑ Eric Chan, Fredo Durand, Marco Corbetta. "Rendering Fake Soft Shadows with Smoothies" . Retrieved 2008-02-14.{{cite journal}}: Cite journal requires |journal= (help)CS1 maint: multiple names: authors list (link)
  9. ↑ William Donnelly; Andrew Lauritzen. "Variance Shadow Maps" . Retrieved 2008-02-14.
  10. ↑ "Common Techniques to Improve Shadow Depth Maps". Msdn.microsoft.com. Retrieved November 7, 2021.
  11. ↑ "Cascaded Shadow Maps". Msdn.microsoft.com. Retrieved November 7, 2021.
  12. ↑ Donnelly, William; Lauritzen, Andrew (14 March 2006). "Variance shadow maps". Proceedings of the 2006 symposium on Interactive 3D graphics and games - SI3D '06. Association for Computing Machinery. pp. 161–165. doi:10.1145/1111411.1111440. ISBN   159593295X. S2CID   538139 . Retrieved 7 November 2021– via ACM Digital Library.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.