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This document discusses various techniques for achieving visual realism in 3D computer graphics, including hidden surface removal algorithms, shading methods, and color models. It describes algorithms for hidden line elimination, hidden surface removal, and hidden solid removal. It also covers shading techniques like constant intensity shading, Gouraud shading, and Phong shading. Finally, it discusses color models such as RGB, CMY, HSV, and HSL that can be used for computer graphics applications.
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- 1. UNIT – III VISUALREALISM
- 2. SYLLABUS Hidden LineRemoval algorithm Hidden Surface Removal algorithm Hidden Solid Removal algorithm Shading Coloring Computer animation
- 3. INTRODUCTION Benefits ofCAD systems is their ability to provide their users with visual displays of the models. Visualization has always been most effective way to communicate new ideas among engineers and designers. CAD systems utilize visualization both in geometric modelling and in scientific computing.
- 4. APPLICATIONS IN GEOMETRIC MODELING An effective and less expensive way of reviewing various design alternatives For determining relationships in design applications For design of complex surfaces, like as those in automobile bodies and aircraft frames.
- 5. Cont…, IN SCIENTIFIC COMPUTING For robot simulations and trajectory planning. For displaying results of finite element analysis, heat transfer analysis, computational fluid dynamics, structural dynamics and vibration. In medical field for hip replacement operations For verification of NC tool paths.
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- 12. STRUCTURAL DYNAMICS &VIBRATIONS
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- 15. Model cleanup Thecleanup process begins by adding the proper orthographic views to a drawing. Visual realism is added to each view separately, in the drawing mode, by handling hidden lines and adding center lines, dimensions, notes and labels. A major advantage of manual model cleanup is the control it gives the user over which entities should be removed and which should be dashed. The automatic elimination of hidden lines may result in a loss of depth information. A hybrid (a mix of manual and automatic activities) approach to model cleanup is the best and fastest way to prepare views in a engineering drawing.
- 17. REMOVALALGORITHMS Edges andsurfaces of given model may be visible or invisible depending on viewing directions. Removal algorithm are usually built into the local hardware processor of a graphics display to speed up their execution. Removal algorithms have been classified as Object space method Image space method Raster algorithms (pattern of lines and columns) Vector algorithms Hybrid method
- 18. HIDDEN LINE ELIMINATIONPROCESS The appearance of the object is greatly complicated by the visibility of hidden details. Therefore, it is necessary to remove hidden details such as edges and surfaces. The edges and surfaces which need to be removed depending on its view port, viewing direction and viewing window. One of the most challenging problems considered in computer graphics is the determination of hidden edges and surfaces.
- 19. HIDDEN LINE ELIMINATIONPROCESS
- 20. VISIBILITY TECHNIQUES Thevisibility technique normally checks the overlapping of pairs of polygon in the viewing plane. There are different types of Visibility techniques and algorithms exist to differentiate 2D elements. The simplest approach involves in evaluating the depth elements from the view point along the viewing direction. Boundingbox or MinimaxTest Containment test Surface test Computing silhouettes Edgeintersections Segment comparisons Homogeneitytest
- 21. MINIMAX TEST Inthis test, a complex object is converted into a simpler outline of the object. This test checks for the intersection for any two boxes in both X and Y directions. This method is also called as overlap test as it provides a quick method to check if two polygons overlap.
- 23. CONTAINMENT TEST Containmenttest checks whether a given point lies inside the given polygon. There are different methods to compute containment. Convex polygon Non convex polygon
- 24. Convexpolygon -A polygon that has all interior angles less than 180° Non convex polygon - A polygon that has any of its interior angles more than 180°
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- 28. Dot product ofnormal vector 𝑁. 𝑆 = 𝑁 𝑆 cos 𝜃
- 29. COMPUTING SILHOUETTES Aset of edges which separates visible faces from invisible faces of an object with respect to a given viewing direction is called silhouette edges (or silhouettes).
- 32. EDGE INTERSECTIONS Fullyvisible edge AB CD marks the disappearance of partially hidden edge AB CD marks the appearance of partially hidden edge AB
- 34. SEGMENT COMPARISONS Thesegment comparison technique is used for identifying visible surfaces in the raster image space.
- 36. HOMOGENEITY TEST Thehomogeneity of a point can be computed using the motion of neighborhood. Computing homogeneity of points is used to determine visibility. Three types of points can be identified Homogeneously visible Homogeneously invisible In-homogeneously visible
- 38. SORTING Sorting isthe process of arranging items in some sequence (or) in different sets according to a selected criterion. The records of the scene database contain geometrical, topological and viewing information about the polygons and faces that make the scene. The sorting time required depends on the following The number of records to be processed The algorithm performing the sort The statical properties of the initial ordering of records.
- 39. COHERENCE Naturally theelements of a scene or its image have some interrelationships, known as coherence. Coherence is a measure of how rapidly a scene or its image chances.
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- 42. Hidden line removalalgorithms Depth or priority algorithm Area-oriented algorithm Overlay algorithm Robert’s algorithm
- 43. PRIORITYALGORITHM The following stepsprovide guidance for implementing the algorithm 1. Utilize the proper orthographic projection to obtain the desired view of the scene 2. Utilize the surface test to remove back faces to improve the efficiency of the priority algorithm 3. Assign priorities to the faces in the face list 4. Recorder the face and priority lists so that the highest priority is on top of the list.
- 48. AREA ORIENTED ALGORITHM 1.Identify silhouette polygons. 2. Assign quantitative hiding (QH) values to edges of silhouette polygons. 3. Determine the visible silhouette segments. 4. Intersect the visible silhouette segments with partially visible faces. 5. Display the interior of the visible or partially visible polygons.
- 51. OVERLAYALGORITHM In thismethod, the u·v grid is used to create grid surface which consists of the regions having straight-edges. The curves in each region of u-v grid are approximated as a line segment. This algorithm is called overlay algorithm In this algorithm, the first step is to calculate the u·v grid using the surface equation. Then the grid surface with linear edges is created.
- 52. ROBERT’s ALGORITHM Steps: Treateach volume separately and eliminate self-hidden (back-faces) planes and self hidden lines. Treat each edge (or line segment) separately eliminates those which are entire hidden by one or more other volumes. It identifies those lines which are entirely visible. For each of the remaining edge, junction lines are constructed. New edges are constructed if there is inter-penetration of two volumes
- 53. HIDDEN SURFACE REMOVAL The elimination of parts of solid objects that are covered by another solid objects called hidden surface removal. Painter’s algorithm Depth buffer algorithm Warnocks’s area subdivision algorithm Watkin’s algorithm
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- 55. The basicfunctions performed by the algorithm are Sort all polygons according to their z coordinates. Resolve any ambiguities that sorting may cause when the z extends overlap, splitting polygons if necessary Scan convert each polygon in ascending order of their z coordinates
- 58. DEPTH BUFFER ALGORITHM This algorithm compares surface depths at each pixel position on the projection plane. Since the object depth is usually measured from the view plane along the z-axis of a viewing system, this algorithm is also caned z-buffer algorithm. Each surface of a scene is processed separately, one point is at a time across the surface. Thus, for each pixel position (x, y) on the view plane. Two buffers are required for each pixel. Depth buffer or z buffer which stores the smallest z value for each pixel Refresh buffer or frame buffer which stores the intensity value for each position.
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- 60. WARNOCKS’S ALGORITHM JohnWarnock proposed an elegant divide-and-conquer hidden surface algorithm. This algorithm relies on the area coherence of polygons to resolve the visibility of many polygons in image space. Depth sorting and performed only in those cases involving the image space overlap. This method is also called area-subdivision method as the process involves the division of viewing window into four equal sub-windows or sub-division
- 63. The scanline algorithm is identical to z-buffer algorithm except that one scan line at a time is processed, hence, a much smaller buffer is needed. It processes the image one scan-line at a time rather than one pixel at a time. In this method, as each scan line is processed, a polygon surface intersecting that lines are examined to determine which are visible. WATKIN’S ALGORITHM
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- 65. HIDDEN SOLID REMOVAL Hidden solid removal is the process of displaying the solid models with hidden lines or surfaces removed. This process is carried out automatically because of the completeness and unambiguities of solid models. One approach utilizes the technique of converting CSG model into B-rep model which can be then rendered with standard hidden surface removal algorithms. Another important approach utilizes the technique of ray- tracing.
- 66. RAY TRACING METHOD Ray-tracing is the process of tracking and plotting the path taken by the rays of light starting at a light source to the centre of projection (viewing position). If we shoot a ray from the view point through the pixel, the first object which hits is the one that is visible at the pixel as shown in Figure. Algorithms adopting this method is called ray-tracing algorithm. This concept is used in computer graphics to eliminates the hidden surfaces and solids.
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- 68. RAY TRACING METHOD It represents the first surface seen at the given pixel, the pixel is set to the colour of the surface at the point where the light ray strikes it shown in figure. If the resolution of screen is x x y, there are 'xy' pixels and so ‘xy’ light rays are traced. After calculating all ray surface intersection, the visible surface can be identified as the one whose intersection point is the closest to the pixel.
- 71. SHADING Shading isthe process of altering the colour of an object surface polygon in the 3D scene based or its angle on light and its distance from lights to create a photorealistic effect.
- 73. A numberof different lighting techniques are used to make the rendering of a scene look more realistic. Point lighting Ambient lighting
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- 76. SHADING ALGORITHMS Constant-intensityor Lambert shading Gourand or first-derivative shading Phong or second-derivative shading Half-tone shading
- 77. CONSTANT-INTENSITY SHADING Thefast and simple method for shading polygon is constant intensity shading which is also known as Lambert shading or faceted shading or flat shading. In this method, the surface of the object is approximated by polygons (flat surface) even though it is smooth. It gives a rise to the appearance of the smooth solid as a faceted solid. The surface normal for each polygon is computed and the illumination model is applied to get the intensity of the surface. The polygon is then filled with this intensity. The surface can become ,smooth by increase the number of polygons. However for complex objects the number of polygon may be too large to get a smooth appearance.
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- 79. GOURAND SHADING Gourandshading is invented by Henri Gourand in 1971 and hence the name. It is a method to display each surface polygon with an intensity that varies smoothly. Therefore, it is also called as smooth shading. The smoothness of polygon surface is obtained by linearly interpolating a colour or shading intensity values across the surface. Here, the intensity values for each polygon are matched with the values of adjacent polygons among the common edges. It eliminates the intensity discontinuities that can occur in constant- intensity shading.
- 80. PHONG SHADING Phongshading uses similar principle as Gourand shading but instead of interpolating the shade intensities, it interpolates normal vectors N at the surface vertices. Hence, this algorithm is also called normal-vector interpolation shading. Interpolation occurs across a polygon span on a scan line between starting and ending normal for the span. The interpolated normal are then used to evaluate a shade at each pixel.
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- 82. HALF-TONE SHADING Whenwe view a small area consisting of several pixel positions, Our eyes tend to integrate or average the fine detail into an overall intensity. This phenomenon of apparent increases the number of available intensities by considering combined intensity of multiple pixels known as half-toning. This technique is commonly used in printing black-and-white photographs for publication in newspapers, magazines and books. The pictures reproduced using half-toning processes are called halftones.
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- 84. SHADING ENHANCEMENTS The visualrealism of images can be enhanced by including shading effects such as Shadows Transparency Surface details Texture
- 85. SHADOWS A shadowedobject is one which is hidden from the light source. Shadows help to depict relationships between objects which are near each other and relative size. They also help to resolve visual ambiguities where the shaded images alone cannot be done the same. When the observer's position is coincident with the light source, no shadows are seen. As the positions of the observer and the light source separate, shadows will appear.
- 86. TRANSPARENCY When thelight passes from one medium to another, e.g air to water or water will cause specular transmittance. This is called as Transparency
- 87. SURFACE DETAILS Someof the surfaces may contain surface details such as logo, textures pattern etc., which need to be modeled to add the realism to the surface image. These surface details can be modeled using a surface-detail polygon. Surface-detail polygons are generally overlaid on a larger surface polygon and they are processed with the parent surface.
- 88. TEXTURE Texture detailsgive the appearance of a roughened surface on the model. The method of adding surface detail is to map the texture patterns onto the surfaces of objects is referred as texture mapping or pattern mapping.
- 89. COLOURING The useof colors in CAD/CAM has two main objectives. Colours can be used in geometric construction and engineering application. There are three colour parameters Hue Saturation brightness
- 90. Colour models arebroadly divided into two categories, namely Additive colour models Subtractive colour models
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- 95. There arelarge number of colour models. Some of the popular models are RGB CMY HSV HSL
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- 100. COMPUTER ANIMATION Theprocess of moving something which can’t move by itself is called animation. A typical animation sequence for animated cartoons as follows Storyboard layout Object definitions Key frame specifications Generation of in-between frames Recording the animation sequence
- 102. Types of animation Frame buffer animation Real time playback Real time animation
- 103. Computer animation techniques Keyframing Simulation Motion capture Combinations (motion capture + simulation)
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