Hidden surface determination
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In 3D computer graphics, hidden surface determination is the process used to determine which surfaces and parts of surfaces are not visible from a certain viewpoint. A hidden surface determination algorithm is a solution to the visibility problem, which was one of the first major problems in the field of 3D computer graphics. The process of hidden surface determination is sometimes called hiding, and such an algorithm is sometimes called a hider.
Hidden surface determination is necessary to render an image correctly, as parts of the image that are not visible should not be drawn. It also speeds up rendering since objects that aren't visible can be removed from the graphics pipeline.
There are many techniques for hidden surface determination, and the core differences between most rendering algorithms is how they handle this problem. There are also different stages of hidden surface determination. These stages include:
- Backface culling
- Since meshes are hollow shells, not solid objects, the back side of some faces, or polygons, in the mesh will face the camera. Typically, there is no reason to draw such faces.
- Viewing frustum culling
- The viewing frustum is a geometric representation of the volume visible to the virtual camera. Naturally, objects outside this volume will not be visible in the final image, so they are discarded. Often, objects lie on the boundary of the viewing frustum. These objects are cut into pieces along this boundary in a process called clipping, and the pieces that lie outside the frustum are discarded.
- Occlusion culling
- Occlusion culling is the process of determining which portions of objects are hidden by other objects from a given viewpoint. This is one of the fundamental problems in computer graphics, and many different occlusion culling algorithms have been developed. The simplest is painter's algorithm, in which polygons are sorted, then drawn back to front. The most common in real-time computer graphics is z-buffering, in which the depth value at each pixel is stored as each polygon is rendered. The pixel is only overwritten if the depth value of the current point is less than the depth value stored in the z-buffer. Both of these methods operate on polygon meshes. Ray tracing, which can also operate on parametric geometry, attempts to model the path of light rays into a viewpoint by tracing rays from the viewpoint into the scene. The first object the ray intersects is rendered, as it naturally is the object visible to the camera.
- Contribution culling
- Often, objects are so far away that they do not contribute significantly to the final image. These objects are thrown away if their screen projection is too small.
Though hidden surface determination is most often used to determine what is visible in the final image, it is also has other applications, such as determining which parts of objects are in shadow.de:Sichtbarkeitsproblem