The 3D model is an objectpolygonRepresented by computer or othersVideo equipmentDisplay.The displayed objects can be real world entities or imaginary objects.Anything that exists in physical nature can use three-dimensionalModel representation。
3D models are often used3D modelingThis special software generation tool[1], but it can also be generated by other methods.As the data of points and other information sets, 3D models can be generated manually or according to certain algorithms.Although it usually exists on the computer or computer in a virtual wayfileHowever, similar models described on paper can also be regarded as 3D models.3D models are widely used3D graphicsThe place.In fact, they were used beforepersonal computerThe popularity of 3D graphics.manyComputer gamesUse pre rendered 3D model images as sprites forReal-time computerRender.
3D models have been used in various fields.Use them to make accurate models of organs in the medical industry;The film industry uses them for the characters, objects and real movies of activities;Video gamesThe industry uses them as resources in computer and video games;They are used as precise models of compounds in the scientific field;The construction industry uses them to display the proposed buildings or landscape performance;The engineering community uses them to design new equipment, vehicles, structures and othersapplication area ；In recent decades,Earth scienceThe field began to build a 3D geological model.
The 3D model itself is invisible, and can be rendered at different levels of detail according to simple wireframes or shaded in different ways.However, many 3D models use texture to cover, and the process of putting texture arrangement on the 3D model is calledtexture mapping 。Texture is an image, but it can make the model more detailed and more realistic.For example, if a 3D model of a person has skin and clothing textures, it will look better than a simple monochrome model orWireframe modelMore realistic.
In addition to texture, other effects can also be used for 3D models to increase realism.For example, you can adjust the surfacenormalTo achieve their lighting effect, some surfaces can use bump textureMapping methodAnd othersvolume rendering Skills.
3D models are often animated. For example, 3D models are widely used in feature films, movies, computers and video games.They can be used in 3D modeling tools or separately.In order to form animation easily, some additional data are usually added to the model, for example, some 3D models of humans or animals have completeSkeletal system, so that the motion looks more realistic, and you can control the motion through joints and bones.
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grid
Mesh is composed of many objectsPoint cloudIt forms a 3D model mesh through point clouds.Point cloud includes 3D coordinates (XYZ), laser reflection intensity (Intensity) and color information（RGB）, and finally drawn into a grid.These meshes are usuallytriangle、quadrilateralOr other simpleConvex polygonComposition, which simplifies the rendering process.However, the mesh can also include ordinarypolygonA constituent object.
texture
Mapping
Texture includes the texture of the object surface in general, even if the object surface is unevenFurrowAt the same time, it also includes the color patterns on the smooth surface of the object, also called texture, which can make the object look more realistic when the texture is mapped to the object surface in a specific way.The technique of texture mapping grid endowing image data;After processing the image obtained by photographing the object, texture mapping on each grid will finally form a 3D model.
three-dimensional model
Construction method
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There are generally three methods for object modeling: the first method uses 3DSoftware modeling；The second way is to model by measuring instruments and equipment;The third method uses images or videos to model.
3D software modeling
Manual modeling
We can see many excellent modeling software in the market, including 3DMAX,
SoftImage, Maya, UG andAutoCADwait.Their common feature is to use some basic geometric elements, such asCube, sphere, etc., through a series of geometric operations, such as translation, rotation, stretchingBoolean operationTo build complex geometric scenes.Building 3D models by modeling mainly includesGeometric modelingGeometric Modeling, KinematicModelingPhysical modelingPhysical Modeling, Object Behavior, Model Segmentation, etc.Among them, the creation and description of geometric modeling areVirtual sceneThe focus of modeling.
Modeling with instruments and equipment
3D scanner(3 Dimensional Scanner) is also called3D digitizer(3 Dimensional Digitizer)。It is one of the important tools for 3D modeling of actual objects currently used.It can quickly and convenientlyreal world 3D color ofInformation conversionIt can be directly processed by computerdigital signalIt provides an effective means for the digitization of objects.It is different from the traditional planeScanner, camera, graphicsAcquisition cardThere is a big difference: first, the scanning object is not a plane pattern, but a three-dimensional object.Secondly, by scanning, we can get everySampling pointColor scanning can also obtain the color of each sampling point.Some scanning devices can even obtain the structure data inside the object.The camera can only shoot one side of the object, and will lose a lot of depth information.Finally, its output is not2D image, but contains the 3D of each sampling point on the object surfaceSpatial coordinatesAnd colorDigital modelFile.This can be used directly in CAD orthree-dimensional animation。color scanner You can also output object surface color texture maps.Early used for3D measurementYescoordinate measuring machine (CMM)。It installs a probe on a servo device with three degrees of freedom (or more), and drives the probe to move in three directions.When the probe contacts the object surface, measure its movement in three directions, and you can know the point of the object surface3D coordinates。The three-dimensional measurement of the whole surface can be completed by controlling the probe to move and touch on the object surface.Its advantage is high measurement accuracy;Its disadvantages are high price, complex control when the object shape is complex, slow speed, and no color information.With the aid of the principle of radar, people have developed the use of laser or ultrasonic media to replace the probe for depth measurement.Distance finderSend a signal to the surface of the object to be measured. According to the reflection time or phase change of the signal, thespatial location , called "flying point method" or "image radar".
Modeling from images or videos
Image based modeling
Image Based Modeling and Rendering,IBMR) Is currentcomputer graphicsIt is an extremely active research field.Compared with traditional geometry based modeling and rendering, IBMR technology has many unique advantages.Image based modeling and rendering technology provides us with the most natural way to obtain photo realism. With IBMR technology, modeling becomes faster and more convenient, and high rendering speed and high realism can be obtained.The latest research progress of IBMR has made many fruitful achievements, and may fundamentally change our understanding and concept of computer graphics.Because the image itself contains rich scene information, it is naturally easy to obtain photo realisticScene model。The main purpose of image-based modeling is toimage restoration 3D of sceneryGeometry。The 3D objects recovered from 2D images originally belong to computer graphics andcomputer vision The content of.Because of its broad application prospects, researchers in computer graphics and computer vision are interested in this field.Compared with traditional modeling software or3D scannerCompared with the method of obtaining stereo model, the method based on image modeling is cheap and realistic,Degree of automationTherefore, it has a wide application prospect.
