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Rendering Engines usually come with a number of features packed within them. The look of the rendered output usually is dependent on the eff ecti ve use of those features. All the producti on renderers available in market comprise of several common features. These common features are essenti al for any renderer to produce high-quality images. Research and development usually focus on developing more features and efficient simulati on of light eff ects.
Some of the common features of rendering engines are briefly explained below :Shading
In 3D computer graphics, the process in which the computer simulates how the faces of a polygon will look when illuminated by a virtual light source is known as Shading.
- Flat shading - A technique that shades each polygon of an object based on the polygon's normal and the positi on and intensity of a light source.
- Gouraud shading - Invented by Henri Gouraud in 1971, a fast and resource-conscious technique used to simulate smoothly shaded surfaces by interpolati ng vertex colors across a polygon's surface.
- Phong shading - Invented by Bui Tuong Phong, a smooth shading technique that approximates curved-surface lighti ng by interpolati ng the vertex normals of a polygon across the surface
Texture mapping is a process which is usually used to add detail, surface texture, or color to CG object. Its was invented/developed by Dr Edwin Catmull in his Ph.D. thesis of 1974.
A texture map is applied (mapped) to the surface of a shape or polygon. This process is similar to wrapping a texture like a cloth around an object. Before the process of mapping, the 3d Objects surface has to be projected onto a 2d Plane (explained in chapter 3). This is known as UV mapping.
Multi texturing is the use of more than one texture at a ti me on a polygon. Usually to get the required feel and look to a surface more then one texture is applied to it to control its various att ributes (like bump, reflecti on, transparency,etc). This highly increases the appeal of a complex surface.
Texture filtering controls the calculati on the resulti ng pixels on the screen from texels. Usually the neartest-neighbours of the texels are interpolated to get the final pixel value. This is the most basic and fastest way to calculate. More advanced techniques like bilinear, trilinear and anisotropic are used nowadays to reduce aliasing or jaggedness at the edges. If the mapped texture coordinates exceed beyond the bounday of the texture, then it is usually clamped or wrapped.Bump Mapping
Bump mapping works like Texture Mapping. When Texture Mapping defines the color of the surface, Bump Mapping provides the surface some definiti on(detail) by creati ng bumpiness or roughness. This usually boosts the visual appeal and look of the polygon object. Bump Mapping can add minute detail to an object which would otherwise require a large number of polygons.
Bump mapping is an extension of the Phong Shading technique. In Phong Shading, the surface normal was interpolated over the polygon, and that vector was used to calculate the brightness of that pixel. When you add bump mapping, you are altering the normal vector slightly, based on informati on in the bump map. Adjusti ng the normal vector causes changes in the brightness of the pixels in the polygon.Fogging/Parti cipati ng Medium
Parti cipati ng media eff ects are used to simulate any type of atmospheric eff ect that absorbs and scatt ers light, as well as the eff ect on shadowing. The environment we usually deal with in CG can be thought of as a vacuum; atmospheric parti cles are nonexistent. When such eff ects are needed such as haze, mist, and so on, mental ray uses a PM shader with certain variables to determine how to simulate the existence of an atmosphere. Calculati ng the influence of suspended parti cles in air requires a technique known as ray marching.Shadows
Shadows is one of the main features which decide the look and feel of the image. It is usually caused by obstructi on of light. Usually shadows are generated by two main methods by rendering engines :
- Shadow maps
- Ray Tracing.
In the process of shadow mapping, a pixel is first tested if it is visible from the light source. The resulti ng values are then compared to a z-buff er image. Then based on the output values shadows are created.
Raytraced shadows are shadows produced during raytracing. Raytraced shadows produce very good results in most situati ons; however, you must raytrace your enti re scene to use raytraced shadows, and this is oft en very ti me consuming.
It is adviceable to avoid using raytraced shadows to produce soft -edged shadows. Raytracing high quality soft -edged shadows is very ti me consuming.Transparency and Translucency
Transparency is the physical property of allowing light to pass through a material.
Translucency only allows light to pass through diff usely.
Transparent materials are clear, while translucent ones cannot be seen through clearly. This eff ect of transparency is also a common feature of rendering engines and can be achieved by using any method of rendering (both scan-line and ray tracing). In ray tracing, a light ray is shot. As it hits the transparent/translucent surface, based on the amount of transparency/translucency, the ray travels further interacti ng with more objects in the scene ti ll it hits an opaque surface. The resultant value of the end pixel is then calculated.Reflecti on
Renderers can also simulate reflecti ons like in mirrors and shiny surfaces. Reflecti on is usually emulated by a ray trace renderer. There a ray is cast from the eye to the mirror and then calculati ng where it bounces from, and conti nuing the process unti l no surface is found, or a non-reflecti ve surface is found. Reflecti on on shiny surfaces improves and provides photorealism to a render.
The reflecti ons are usually of four kinds :
- Polished - A Polished Reflecti on is an undisturbed reflecti on, like a mirror or chrome.
- Blurry - A Blurry Reflecti on means that ti ny random bumps on the surface of the material cause the reflecti on to be blurry.
- Metallic - A reflecti on is Metallic if the highlights and reflecti ons retain the color of the reflecti ve object.
- Glossy - Blurry Reflecti ons are known as glossy reflecti ons. This is because, usually when the glossiness of a surface is low, reflecti ons appear blurred.
Refracti on is caused dude to the bending of light when it travels from one medium to another. Common examples include rainbows, mirages and Fata Morgana.
In Computer graphics, refracti on is calculated by ray tracing. There a ray is cast from the eye to the surface and then calculati ng where it bounces from, and conti nuing the process unti l no surface is found, or a non-refracti ve surface is foundGlobal Illuminati on / Indirect Illuminati on
Global Illuminati on (GI) is a term for a group of techniques used in CG, which provide more realisti c lighti ng. These algorithms simulate the not only the light rays coming from the source, but also the light rays that are reflected by the surfaces they hit (bouncing of light). These surfaces can be both reflecti ve and non-reflecti ve (indirect illuminati on). Reflecti ons, refracti ons, and shadows are all eff ected by global illuminati on, because the GI simulates the eff ect of one object on the other one. In practi ce, however, only the simulati on of diff use inter-reflecti on or causti cs is called global illuminati on.
Images are more photorealisti c when Global Illuminati on is used. But on the downside, calculati on of Global Illuminati on is very slow and require more computati on power. One usual procedure followed is storing of Global Illuminati on data within the geomerty, i.e. Radiosity. We can generate walkthroughs of a scene without having to calculate the Illuminati on every frame by using the stored date from the geometry.
Radiosity, ray tracing, beam tracing, cone tracing, path tracing, Metropolis light transport, ambient occlusion, photon mapping, and image based lighti ng are examples of algorithms used in Global Illuminati on.Causti cs
The eff ects of the refracti on of light through a transparent medium are called causti cs. A causti c is a patt ern of light that is focused on a surface aft er having had the original path of light rays bent by an intermediate surface.
This eff ect of causti cs is achieved by using photon mapping. Rendering Engines shoot photons with energy into the scene which interact with the surfaces based on their material att ributes and returns values. Those values are used to simulate causti cs.Depth Of Field
Depth of field is the eff ect in which objects within some range of distances in a scene appear in focus, and objects nearer or farther than this range appear out of focus. Depth of field is frequently used in photography and cinematography to direct the viewer's att enti on within the scene, and to give a bett er sense of depth within a scene.
This is usually achieved by using several methods :
- Distributi ng traced rays across the surface of a lens
- Rendering from multi ple cameras (accumulati on-buff er technique)
- Rendering multi ple layers
- Forward-mapped z-buff er techniques
- Reverse-mapped z-buff er techniques
Moti on blur can be used to add a great deal of realism to mental ray renders. Adding moti on blur not only provides an extra touch of photographic realism, it also provides for smoother animati on. Objects that move fast with no blur simply appear "wrong" and "rigid," and so moti on blur helps smooth the visual appearance of object translati on over several frames.
Moti on blur in rendering engines realisti cally simulates camera moti on blur, handling both standard features, such as correctly blurring surface texture colors, and advanced features, such as indirect illuminati on eff ects (i.e., global illuminati on, causti c light, and final gather). In film, there are two primary causes for moti on blur :
- the moti on of objects in front of the camera during the shutt er interval
- movement of the camera so that anything within the camera's view appears blurred
If both the camera and an object are in moti on, then as with real cameras, the environment would blur but not the object, because its visual cues remain stati c in the "eyes" of the camera. Accommodati ng all these features provides yet another powerful tool for simulati ng realism.