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| 在]领域，'''可见表面确定''' ('''VSD''', 也称为'''隐藏表面移除''' ('''HSR''')，'''occlusion culling'''('''OC'''))是确定从某些视点不可见的面的过程。隐藏表面确定的]是一个可见问题的解，这是一个3D计算机图形领域的问题。 隐藏表面确定的过程有时被称为 '''hiding'''，这样的算法有时被称为一个 '''hider'''。模拟进行的线渲染是隐藏线去除。隐藏表面确定对正确渲染图像是必要的，以使模型本身后面的图形不可见，只允许自然能看到的图形可见。 | |||
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| In ], shown '''surface determination''' (also known as '''hidden surface removal''' ('''HSR'''), '''occlusion culling''' ('''OC''') or '''visible surface determination''' ('''VSD''')) is the process used to determine which surfaces and parts of surfaces are not visible from a certain viewpoint. A hidden surface determination ] is a solution to the ], 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'''. The analogue for line rendering is ]. Hidden surface determination is necessary to render an image correctly, so that one may not view features hidden behind the model itself, allowing only the naturally viewable portion of the graphic to be visible. | |||
| == Background == | |||
| Hidden surface determination is a process by which surfaces which should not be visible to the user (for example, because they lie behind opaque objects such as walls) are prevented from being rendered. Despite advances in hardware capability there is still a need for advanced ]s. The responsibility of a rendering engine is to allow for large world spaces and as the world’s size approaches infinity the engine should not slow down but remain at constant speed. Optimising this process relies on being able to ensure the deployment of as few resources as possible towards the rendering of surfaces that will not end up being rendered to the user. | |||
| There are many techniques for hidden surface determination. They are fundamentally an exercise in ], and usually vary in the order in which the sort is performed and how the problem is subdivided. Sorting large quantities of graphics primitives is usually done by ]. | |||
| == Algorithms == | |||
| Considering the ], the ], the ], and the ] steps are handled differently by the following algorithms: | |||
| * ] During rasterization the depth/Z value of each pixel (or ''sample'' in the case of anti-aliasing, but without loss of generality the term ''pixel'' is used) is checked against an existing depth value. If the current pixel is behind the pixel in the Z-buffer, the pixel is rejected, otherwise it is shaded and its depth value replaces the one in the Z-buffer. Z-buffering supports dynamic scenes easily, and is currently implemented efficiently in graphics hardware. This is the current standard. The cost of using Z-buffering is that it uses up to 4 bytes per pixel, and that the rasterization algorithm needs to check each rasterized sample against the z-buffer. The z-buffer can also suffer from artifacts due to precision errors (also known as ]). | |||
| * Coverage buffers (]) and Surface buffer (]): faster than z-buffers and commonly used in games in the ] era. Instead of storing the Z value per pixel, they store a list of already displayed segments per line of the screen. New polygons are then cut against already displayed segments that would hide them. An S-Buffer can display unsorted polygons, while a C-Buffer requires polygons to be displayed from the nearest to the furthest. Because the C-buffer technique does not require a pixel to be drawn more than once, the process is slightly faster. This was commonly used with ] (BSP) trees, which would provide sorting for the polygons. | |||
| * Sorted Active Edge List: Used in Quake 1, this was storing a list of the edges of already displayed polygons (see ]). Polygons are displayed from the nearest to the furthest. New polygons are clipped against already displayed polygons' edges, creating new polygons to display then storing the additional edges. It's much harder to implement than S/C/Z buffers, but it scales much better with increases in resolution. | |||
| * ] sorts polygons by their ] and draws them back to front. This produces few artifacts when applied to scenes with polygons of similar size forming smooth meshes and ] turned on. The cost here is the sorting step and the fact that visual artifacts can occur. This algorithm is broken by design for general scenes, as it cannot handle polygons in various common configurations. | |||
| * ] (BSP) divides a scene along planes corresponding to polygon boundaries. The subdivision is constructed in such a way as to provide an unambiguous depth ordering from any point in the scene when the BSP tree is traversed. The disadvantage here is that the BSP tree is created with an expensive pre-process. This means that it is less suitable for scenes consisting of dynamic geometry. The advantage is that the data is pre-sorted and error free, ready for the previously mentioned algorithms. Note that the BSP is not a solution to HSR, only an aid. | |||
| * ] attempts to model the path of light rays to a viewpoint by tracing rays from the viewpoint into the scene. Although not a hidden surface removal algorithm as such, it implicitly solves the hidden surface removal problem by finding the nearest surface along each view-ray. Effectively this is equivalent to sorting all the geometry on a per pixel basis. | |||
| * The ] divides the screen into smaller areas and sorts triangles within these. If there is ambiguity (i.e., polygons overlap in depth extent within these areas), then further subdivision occurs. At the limit, subdivision may occur down to the pixel level. | |||
| == Culling and visible surface determination == | |||
| A related area to visible surface determination (VSD) is ''culling'', which usually happens before VSD in a rendering pipeline. Primitives or batches of primitives can be rejected in their entirety, which ''usually'' reduces the load on a well-designed system. | |||
| The advantage of culling early on the pipeline is that entire objects that are invisible do not have to be fetched, transformed, rasterized or shaded. Here are some types of culling algorithms: | |||
| === Viewing frustum culling === | |||
| The ] is a geometric representation of the volume visible to the ]. 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 ], and the pieces that lie outside the frustum are discarded as there is no place to draw them. | |||
| === Backface culling === | |||
| {{main|Back-face culling}} | |||
| With 3D objects, only half of the surfaces are facing the camera, and the rest are facing away from the camera, i.e. are on the back side of the object, hindered by the front side. If the object is completely opaque, those surfaces never need to be drawn. They are determined by the vertex winding order: if the triangle drawn has its vertices in clockwise order on the projection plane when facing the camera, they switch into counter-clockwise order when the surface turns away from the camera. | |||
| Incidentally, this also makes the objects completely transparent when the viewpoint camera is located inside them, because then all the surfaces of the object are facing away from the camera and are culled by the renderer. To prevent this the object must be set as double-sided (i.e. no backface culling is done) or have separate inside surfaces. | |||
| === 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 ] is too small. See ]. | |||
| === Occlusion culling === | |||
| {{See also|Z-buffering#Z-culling}} | |||
| Objects that are entirely behind other opaque objects may be culled. This is a very popular mechanism to speed up the rendering of large scenes that have a moderate to high ]. There are several types of occlusion culling approaches: | |||
| * ] or ''PVS'' rendering, divides a scene into regions and pre-computes visibility for them. These visibility sets are then indexed at run-time to obtain high quality visibility sets (accounting for complex occluder interactions) quickly. | |||
| * ] divides a scene into cells/sectors (rooms) and portals (doors), and computes which sectors are visible by clipping them against portals. | |||
| Hansong Zhang's dissertation "Effective Occlusion Culling for the Interactive Display of Arbitrary Models"<ref>{{cite web|url=http://www.cs.unc.edu/~zhangh/hom.html|title=Occlusion Culling with Hierarchical Occlusion Maps|website=www.cs.unc.edu}}</ref> describes an occlusion culling approach. | |||
| == Divide and conquer == | |||
| A popular theme in the VSD literature is ]. The ] pioneered dividing the screen. ] is a ray-tracing approach which divides the visible volumes into beams. Various screen-space subdivision approaches reducing the number of primitives considered per region, e.g. tiling, or screen-space BSP clipping. Tiling may be used as a preprocess to other techniques. ZBuffer hardware may typically include a coarse 'hi-Z' against which primitives can be rejected early without rasterization, this is a form of occlusion culling. | |||
| ] (BVHs) are often used to subdivide the scene's space (examples are the ], the ] and the ]). This allows visibility determination to be performed hierarchically: effectively, if a node in the tree is considered to be ''invisible'' then all of its child nodes are also invisible, and no further processing is necessary (they can all be rejected by the renderer). If a node is considered ''visible'', then each of its children need to be evaluated. This traversal is effectively a tree walk where invisibility/occlusion or reaching a leaf node determines whether to stop or whether to recurse respectively. | |||
| ==See also== | |||
| *] | |||
| *] | |||
| *] | |||
| ==Sources== | |||
| {{reflist}} | |||
| *http://www.cs.washington.edu/education/courses/cse557/07wi/lectures/hidden-surfaces.pdf | |||
| *http://design.osu.edu/carlson/history/PDFs/ten-hidden-surface.pdf | |||
| {{Mixed reality}} | |||
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Revision as of 12:29, 15 November 2017
在3D计算机图形领域，可见表面确定 (VSD, 也称为隐藏表面移除 (HSR)，occlusion culling(OC))是确定从某些视点不可见的面的过程。隐藏表面确定的算法是一个可见问题的解，这是一个3D计算机图形领域的问题。 隐藏表面确定的过程有时被称为 hiding，这样的算法有时被称为一个 hider。模拟进行的线渲染是隐藏线去除。隐藏表面确定对正确渲染图像是必要的，以使模型本身后面的图形不可见，只允许自然能看到的图形可见。
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