偽三維

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伪三维(又称为2.5D假3D),是一种用于在二维画面中所应用的技术,使得图像或场景有着三维的视觉效果。對於電子遊戲來說,多邊形技術未成熟或是遊戲主機(電腦电子游戏机)機能不足以支持的時代,產生了許多过渡產物。此类游戏雖然有著看似立體的外观,可是卻並非真正意義上的三维遊戲,因此一般都統稱為伪三维遊戲。在部分三维图形游戏中,仅使用平面坐标或使用固定相机位置的游戏同样被称作伪三维游戏。作为对比,没有以上限制的三维图形技术游戏就被称为真三维游戏。

计算机图形[编辑]

等轴测投影和斜向投影[编辑]

Lincity将二维等轴测投影图像组合在一起来形成伪三维效果
等轴测投影的一个示例。左边为2D图形数据,右边为3D模型数据

等轴测投影斜向投影这两种投影的形式中,视角被轻微转动来使得物体的其他面可见,产生一种三维环境的感觉。可以认为一个物体被“倾斜放置“在屏幕前[1]。图像可以被看作该物体在游戏平面上以特定角度的投影[1]且不同边的长度根据投影位置关系被分别调整。

这种效果常用于二维游戏的相机透视效果。在第四世代游戏机掌上游戏机中较为常见。较新的战略游戏角色扮演游戏也经常用到。它结合了俯视视角游戏的角色的操控性和横版游戏的角色辨识度。这样玩家就能够获得游戏世界的一个俯视视角,同时能够看到角色的身体侧面细节,而不是像俯视游戏一样只能够看到角色头顶和肩部。

等轴测投影有三大类别:等距投影二轴测投影(对称与不对称)和三角投影。等距投影在工程绘图较为常见。这种投影方法旋转物体使得三条坐标轴投影的夹角均为120度。这样的好处是物体在三个方向上的缩放长度均相等。在游戏中,为方便在方形像素显示器上进行抗锯齿处理,大部分投影处理选择2:1的像素长宽比例。

Ultima VII: The Black GatePaperboy 均使用等轴测投影,而SimCity 2000、角色扮演游戏DiabloBaldur's Gate 均使用斜向投影。

看板[编辑]

在三维场景中,看板技术常常指视野中的一个二维图像元素。因其效果类似于看板的外观而得名。这项技术常常被用于1990年早期的游戏中,因当时设备机能限制,无法很好的渲染全三维的物体。这项技术也同样被应用于背景中。当一个物体距离相机过远时,可以将其用二维的精灵图代替,同时可以使游戏获得显著的性能提升。在游戏中最为常见的看板技术莫过于粒子(雨滴,烟,火花等)和分辨率要求不高的的植被。侏罗纪公园:侵入者 就是经典案例之一。这项技术曾经被大量游戏应用,如Rome: Total War 将其用于表示战场上成千上万的独立士兵。第一人称射击游戏Duke Nukem 3D 和赛车游戏马力欧卡丁车 均有用到此项技术。

天空盒与穹顶[编辑]

已标记好各个面的天空盒贴图

天空盒或穹顶被用于创造一个看起来比实际视野更加广阔的环境。如果使用的是方形的天空盒,远到无法触及的物体如高山和云等将会以方形贴图技术表现在四周,创造出一种三维空间遥远物体的感觉。穹顶则使用球体或半球体。

天空盒或穹顶一般随着视野位置一起移动,来让观察者感到其贴图上的景物十分遥远。因为在真实世界中,随着观察者移动,远处的景物看起来也并不会有太大改变。因此,天空盒或穹顶中的景物看起来与观察者始终保持着无限远的距离。在背景贴图中不应包含过多分离的物体或较近的物体,否则将会导致观察者移动时发现物体仍然静止,从而产生违和感。

近大远小[编辑]

在某些游戏里,部分物体的大小会随着与玩家的远近不同而改变。世嘉于1986年发布的游戏Out Run 就是一个典型例子。

Out Run 中,玩家需驾驶一辆法拉利深入游戏场景。街道两侧的棕榈树虽使用同一个位图,但被缩放到不同的大小以营造远近感。大量消耗内存的看板被Out Run 用于渲染即使在最高能见度模式下也看不到边际的水波和玉米地。

Drakkhen 作为第一个宣称使用三维场景的角色扮演游戏值得一提。然而,游戏并不包含三维引擎,而是使用缩放玩家的算法来营造类似的效果。玩家的队伍在向量控制的场景内活动,而所有的物体都根据距离进行缩放。 Drakkhen 还带有动态昼夜交替和开放世界游戏,均在同时代游戏中难得一见。此类引擎后来被用于Eternam 游戏。

某些移动端游戏,比如Asphalt: Urban GT 2Asphalt 3: Street Rules 的移动版,使用这种方法渲染景物和楼房。

平行滚动[编辑]

平行滚动的一个简易示范

平行滚动指一组二维精灵图或图片层独立于背景以不同的速度移动来营造景深的效果。[2]:103 上世纪40年代,传统动画所使用的多平面相机衍生出了这项技术。[3] 这种技术最初被用于1982年的街机游戏Moon Patrol。.[4] Rise of the Triad 的天空,Rygar刺猬索尼克Street Fighter IIShadow of the BeastDracula X Chronicles超级马力欧世界 中靠近靠近幻象森林出口的一关和未使用的一关。

Mode 7[编辑]

Mode 7是电子游戏机超级任天堂的一种图形方式,可让逐扫描线式的背景层支持旋转与缩放以创造大量不同的三维效果。

射线渲染[编辑]

简易的射线渲染示范

使用射线渲染技术的游戏在相机位置从不同角度射出多条“射线”,当它们碰到墙或物体时,将会在屏幕对应位置渲染图像。[5] 早期的第一人称射击游戏使用射线渲染来创造一个类似于三维的效果。即使看上去是三维世界,玩家既不能上下转动视角,也不能跳跃或蹲下,因为地图仍是二维的。

凹凸贴图,法线贴图和视差贴图[编辑]

凹凸贴图法线贴图视差贴图是在3D渲染(例如视频游戏)中处理纹理的技术,以在不需要渲染额外多边形的情况下表现出物体表面上的凹凸纹路。经过此类技术处理后,诸如石墙等表面的纹理外观将具有更明显的深度,看起来更为真实,对渲染的负载影响也较小。

凹凸贴图技术根据一张叫做“高度图”的灰度图片,在光照计算期间扰动物体的表面法线来实现物体表面凹凸不平的效果。尽管物体的表面形状实际上没有改变,视觉上该物体的表面已经变得凹凸不平。凹凸贴图技术由Blinn于1978年引入。[6]

左图为未处理的球体。中图为即将使用的“模板”灰度图片。右图为处理后的球体

法线贴图将阴影点到光源方向的单位矢量与垂直于该表面的单位矢量相乘,得到的数量积即为该表面上的亮度。你无法用多边形构造一个完美的球体,但通过在你的“球体”上应用一个3通道位图贴图,可以加入更精致的多边形效果。该位图中的每个通道对应一个空间维度(XYZ),这些空间维度关联于某个恒定坐标系来计算基于该对象的物体空间凹凸贴图,或某个缓慢变化的坐标系(由贴图材质坐标的位置决定)来计算基于对象切面的切空间凹凸贴图。与先进的照明技术相结合,法线贴图可以有效地为你的“球体”表面增加更多细节。

Parallax mapping (also called offset mapping or virtual displacement mapping) is an enhancement of the bump mapping and normal mapping techniques implemented by displacing the texture coordinates at a point on the rendered polygon by a function of the view angle in tangent space (the angle relative to the surface normal) and the value of the height map at that point. At steeper view-angles, the texture coordinates are displaced more, giving the illusion of depth due to parallax effects as the view changes.

Film and animation techniques[编辑]

The term is also used to describe an animation effect commonly used in music videos and, more frequently, title sequences. Brought to wide attention by the motion picture The Kid Stays in the Picture based on the book by film producer Robert Evans, it involves the layering and animating of two-dimensional pictures in three dimensional space. Earlier examples of this technique include Liz Phair's music video Down directed by Rodney Ascher and "A Special Tree" directed by musician Giorgio Moroder.

Graphic design[编辑]

Faux shading and reflections make these desktop icons appear three-dimensional.

The term also refers to an often-used effect in the design of icons and graphical user interfaces (GUIs), where a slight 3D illusion is created by the presence of a virtual light source to the left (or in some cases right) side, and above a person's computer monitor. The light source itself is always invisible, but its effects are seen in the lighter colors for the top and left side, simulating reflection, and the darker colours to the right and below of such objects, simulating shadow.

An advanced version of this technique can be found in some specialised graphic design software, such as Pixologic's ZBrush. The idea is that the program's canvas represents a normal 2D painting surface, but that the data structure that holds the pixel information is also able to store information with respect to a z-index, as well material settings, specularity, etc. Again, with this data it is thus possible to simulate lighting, shadows, and so forth.

2.5D platform games[编辑]

Trine 2 features 3D graphics, yet its gameplay is restricted to a 2D plane.

The term "2.5D" is also applied to 3D games that use polygonal graphics to render the world and/or characters, but whose gameplay is restricted to a 2D plane. Examples include Pandemonium!, Einhänder, Klonoa: Door to Phantomile, Kirby 64: The Crystal Shards (although the scene moves towards and away from the camera), LittleBigPlanet (although it features a playing field three layers thick), Nights into Dreams... and New Super Mario Bros. The Crash Bandicoot series is sometimes referred to as 2.5D because the characters and scenery are rendered in 3D, yet most of its levels are not as free-roaming as "true" 3D platformers.

Some fighting games such as the Super Smash Bros. series, Marvel vs. Capcom 3, Street Fighter IV, Mortal Kombat, BlazBlue, Guilty Gear Xrd and The King of Fighters XIV also utilize 2.5D to showcase 3D backdrops and/or characters while limiting the action to a 2D plane. 2D scrolling shooters are another genre in which 3D graphics are often used in a 2D game, such as in Ikaruga.

In some games, such as Goemon's Great Adventure and Pandemonium!, the area of gameplay can be described as a two-dimensional surface twisting and bending in a three-dimensional space. Inside this surface, the character and physics behave like in a traditional sidescrolling platformer. There are, however, a number of twists that aren't possible with normal sidescroller platformers: it is common in such games to let the two-dimensional plane cross itself or other planes on certain points, thus creating "track switches" in the course. Players can explore different areas of the 3D world that way or can be brought back to previous points seamlessly. Interactions with the "background" (non-accessible points in the 3D landscape) are also used extensively. Other 2.5D games allow the character to interact with objects out of the primare 2D plane; for example in the platformer-shooter Shadow Complex, the player may need to target and fire at enemies that are in the background of the scene.[7]

History[编辑]

The first video games that used pseudo-3D were primarily arcade games, the earliest known examples dating back to the mid-1970s, when they began using microprocessors. In 1975, Taito released Interceptor,[8] an early first-person shooter and combat flight simulator that involved piloting a jet fighter, using an eight-way joystick to aim with a crosshair and shoot at enemy aircraft that move in formations of two and increase/decrease in size depending on their distance to the player.[9] In 1976, Sega released Moto-Cross, an early black-and-white motorbike racing video game, based on the motocross competition, that was most notable for introducing an early three-dimensional third-person perspective.[10] Later that year, Sega-Gremlin re-branded the game as Fonz, as a tie-in for the popular sitcom, Happy Days.[11] Both versions of the game displayed a constantly changing forward-scrolling road and the player's bike in a third-person perspective where objects nearer to the player are larger than those nearer to the horizon, and the aim was to steer the vehicle across the road, racing against the clock, while avoiding any on-coming motorcycles or driving off the road.[10][11] That same year also saw the release of two arcade games that extended the car driving subgenre into three dimensions with a first-person perspective: Sega's Road Race, which displayed a constantly changing forward-scrolling S-shaped road with two obstacle race cars moving along the road that the player must avoid crashing while racing against the clock,[12] and Atari's Night Driver, which presented a series of posts by the edge of the road though there was no view of the road or the player's car. Games using vector graphics had an advantage in creating pseudo-3D effects. 1979's Speed Freak recreated the perspective of Night Driver in greater detail.

In 1979, Nintendo debuted Radar Scope, a shoot 'em up that introduced a three-dimensional third-person perspective to the genre, imitated years later by shooters such as Konami's Juno First and Activision's Beamrider.[13] In 1980, Atari's Battlezone was a breakthrough for pseudo-3D gaming, recreating a 3D perspective with unprecedented realism, though the gameplay was still planar. It was followed up that same year by Red Baron, which used scaling vector images to create a forward scrolling rail shooter.

Sega's arcade shooter Space Tactics, released in 1980, allowed players to take aim using crosshairs and shoot lasers into the screen at enemies coming towards them, creating an early 3D effect.[14] It was followed by other arcade shooters with a first-person perspective during the early 1980s, including Taito's 1981 release Space Seeker,[15] and Sega's Star Trek in 1982.[16] Sega's SubRoc-3D in 1982 also featured a first-person perspective and introduced the use of stereoscopic 3-D through a special eyepiece.[17] Sega's Astron Belt in 1983 was the first laserdisc video game, using full-motion video to display the graphics from a first-person perspective.[18] Third-person rail shooters were also released in arcades at the time, including Sega's Tac/Scan in 1982,[19] Nippon's Ambush in 1983,[20] Nichibutsu's Tube Panic in 1983,[21] and Sega's 1982 release Buck Rogers: Planet of Zoom,[22] notable for its fast pseudo-3D scaling and detailed sprites.[23]

In 1981, Sega's Turbo was the first racing game to feature a third-person perspective, rear view format.[24] It was also the first racing game to use sprite scaling with full-colour graphics.[23] Pole Position by Namco is one of the first racing games to use the trailing camera effect that is now so familiar. In this particular example, the effect was produced by linescroll—the practice of scrolling each line independently in order to warp an image. In this case, the warping would simulate curves and steering. To make the road appear to move towards the player, per-line color changes were used, though many console versions opted for palette animation instead.

Zaxxon, a shooter introduced by Sega in 1982, was the first game to use isometric axonometric projection, from which its name is derived. Though Zaxxon's playing field is semantically 3D, the game has many constraints which classify it as 2.5D: a fixed point of view, scene composition from sprites, and movements such as bullet shots restricted to straight lines along the axes. It was also one of the first video games to display shadows.[25] The following year, Sega released the first pseudo-3D isometric platformer, Congo Bongo.[26] Another early pseudo-3D platform game released that year was Konami's Antarctic Adventure, where the player controls a penguin in a forward-scrolling third-person perspective while having to jump over pits and obstacles.[27][28][29] It was one of the earliest pseudo-3D games available on a computer, released for the MSX in 1983.[29] That same year, Irem's Moon Patrol was a side-scrolling run & gun platform-shooter that introduced the use of layered parallax scrolling to give a pseudo-3D effect.[30] In 1985, Space Harrier introduced Sega's "Super Scaler" technology that allowed pseudo-3D sprite-scaling at high frame rates,[31] with the ability to scale 32,000 sprites and fill a moving landscape with them.[32]

The first original home console game to use pseudo-3D, and also the first to use multiple camera angles mirrored on television sports broadcasts, was Intellivision World Series Baseball (1983) by Don Daglow and Eddie Dombrower, published by Mattel. Its television sports style of display was later adopted by 3D sports games and is now used by virtually all major team sports titles. In 1984, Sega ported several pseudo-3D arcade games to the Sega SG-1000 console, including a smooth conversion of the third-person pseudo-3D rail shooter Buck Rogers: Planet of Zoom.[31]

With the advent of consoles and computer systems that were able to handle several thousands of polygons (the most basic element of 3D computer graphics) per second and the usage of 3D specialized graphics processing unit, pseudo 3D became obsolete. But even today, there are computer systems in production, such as cellphones, which are often not powerful enough to display true 3D graphics, and therefore use pseudo-3D for that purpose. Interestingly, many games from the 1980s' pseudo-3D arcade era and 16-bit console era are ported to these systems, giving the manufactures the possibility to earn revenues from games that are now nearly twenty years old.

By 1989, 2.5D representations were surfaces drawn with depth cues and a part of graphic libraries like GINO.[33] 2.5D was also used in terrain modeling with software packages such as ISM from Dynamic Graphics, GEOPAK from Uniras and the Intergraph DTM system.[33] 2.5D surface techniques gained popularity within the geography community because of its ability to visualize the normal thickness to area ratio used in many geographic models; this ratio was very small and reflected the thinness of the object in relation to its width, which made it the object realistic in a specific plane.[33] These representations were axiomatic in that the entire subsurface domain was not used or the entire domain could not be reconstructed; therefore, it used only a surface and a surface is one aspect not the full 3D identity.[33]

Fly through the Trenta Valley

The resurgence of 2.5D or visual analysis, in natural and earth science, has increased the role of computer systems in the creation of spatial information in mapping.[34] GVIS has made real the search for unknowns, real-time interaction with spatial data, and control over map display and has paid particular attention to three-dimensional representations.[34] Efforts in GVIS have attempted to expand higher dimensions and make them more visible; most efforts have focused on "tricking" vision into seeing three dimensions in a 2D plane.[34] Much like 2.5D displays where the surface of a three dimensional object is represented but locations within the solid are distorted or not accessible.[34]

Technical aspects and generalizations[编辑]

The reason for using pseudo-3D instead of "real" 3D computer graphics is that the system that has to simulate a three dimensional-looking graphic is not powerful enough to handle the calculation-intensive routines of 3D computer graphics, yet is capable of using tricks of modifying 2D graphics like bitmaps. One of these tricks is to stretch a bitmap more and more, therefore making it larger with each step, as to give the effect of an object coming closer and closer towards the player.

Even simple shading and size of an image could be considered pseudo-3D, as shading makes it look more realistic. If the light in a 2D game were 2D, it would only be visible on the outline, and because outlines are often dark, they would not be very clearly visible. However, any visible shading would indicate the usage of pseudo-3D lighting and that the image uses pseudo-3D graphics. Changing the size of an image can cause the image to appear to be moving closer or further away, which could be considered simulating a third dimension.

Dimensions are the variables of the data and can be mapped to specific locations in space; 2D data can be given 3D volume by adding a value to the x, y, or z plane. "Assigning height to 2D regions of a topographic map" associating every 2D location with a height/elevation value creates a 2.5D projection; this is not considered a "true 3D representation", however is used like 3D visual representation to "simplify visual processing of imagery and the resulting spatial cognition".

See also[编辑]

References[编辑]

  1. ^ 1.0 1.1 Axonometric Projection, Merriam-Webster
  2. ^ Pile Jr, John. 2D Graphics Programming for Games. New York, NY: CRC Press. May 2013. ISBN 1466501898. 
  3. ^ Paul, Wyatt. The Art of Parallax Scrolling (PDF). August 2007 [2009-07-06]. (原始内容 (PDF)存档于2009-10-07). 
  4. ^ Stahl, Ted. Chronology of the History of Video Games: Golden Age. 2006-07-26 [2009-11-21]. (原始内容存档于27 November 2009). 
  5. ^ http://lodev.org/cgtutor/raycasting.html
  6. ^ Blinn, James F. "Simulation of Wrinkled Surfaces", Computer Graphics, Vol. 12 (3), pp. 286–292 SIGGRAPH-ACM (August 1978)
  7. ^ Bakalar, Jeff; Stein, Scott. Shadow Complex: Classic gaming bliss in two-and-a-half dimensions. CNet. August 21, 2009 [August 26, 2016]. 
  8. ^ Tomohiro Nishikado's biography at his company's web site. Dreams, Inc. [2011-03-27]. (原始内容存档于2009-04-01). 
  9. ^ Killer List of Videogames上的《偽三維
  10. ^ 10.0 10.1 Killer List of Videogames上的《偽三維
  11. ^ 11.0 11.1 Killer List of Videogames上的《Fonz
  12. ^ Killer List of Videogames上的《偽三維
  13. ^ Where Were They Then: The First Games of Nintendo, Konami, and More (Nintendo), 1UP
  14. ^ Killer List of Videogames上的《Space Tactics
  15. ^ Killer List of Videogames上的《Space Seeker
  16. ^ Killer List of Videogames上的《偽三維
  17. ^ Killer List of Videogames上的《偽三維
  18. ^ Allgame上的偽三維
  19. ^ Killer List of Videogames上的《Tac/Scan
  20. ^ Killer List of Videogames上的《偽三維
  21. ^ Allgame上的Tube Panic
  22. ^ Killer List of Videogames上的《偽三維
  23. ^ 23.0 23.1 IGN Presents the History of SEGA, IGN
  24. ^ Killer List of Videogames上的《Turbo
  25. ^ Bernard Perron & Mark J. P. Wolf (2008), Video game theory reader two, p. 158, Taylor & Francis, ISBN 0-415-96282-X
  26. ^ Killer List of Videogames上的《Congo Bongo
  27. ^ Killer List of Videogames上的《偽三維
  28. ^ Allgame上的Antarctic Adventure
  29. ^ 29.0 29.1 MobyGames上的《Antarctic Adventure(英文)
  30. ^ Gaming's most important evolutions. GamesRadar. (原始内容存档于2011-06-15). 
  31. ^ 31.0 31.1 IGN Presents the History of SEGA: World War, IGN
  32. ^ Bernard Perron & Mark J. P. Wolf (2008), Video game theory reader two, p. 157, Taylor & Francis, ISBN 0-415-96282-X
  33. ^ 33.0 33.1 33.2 33.3 Raper, Jonathan. “The 3-dimensional geoscientific mapping and modeling system: a conceptual design.” In Three dimensional applications in Geographic Information Systems, edited by Jonathan F. Raper, 11–19. Philadelphia: Taylor and Francis Inc., 19.
  34. ^ 34.0 34.1 34.2 34.3 MacEachren, Alan. "GVIS Facilitating Visual Thinking." In How Maps Work: Representation, Visualization, and Design, 355–458. New York: The Guilford Press, 1995.