7-Surface Detection Methods.ppt

7-Surface Detection Methods.ppt

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  CG 3/1/2023 LECTURE No. (7) Dr.Majid D. Y. [email protected] Surface Detection Methods
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  2 of 20 Why? We must determinewhat is visible within a scene from a chosen viewing position For 3D worlds this is known as visible surface detection or hidden surface elimination
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  3 of 20 Two Main Approaches Visiblesurface detection algorithms are broadly classified as: – Object Space Methods: Compares objects and parts of objects to each other within the scene definition to determine which surfaces are visible – Image Space Methods: Visibility is decided point-by-point at each pixel position on the projection plane Image space methods are by far the more common
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  4 of 20 Back-Face Detection The simplestthing we can do is find the faces on the backs of polyhedra and discard them
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  5 of 20 Back-Face Detection Method •A fast and simple object-space method for identifying the back faces of a polyhedron is based on the "inside- outside" tests. A point (x, y, z) is "inside" a polygon surface with plane parameters A, B, C, and D if • When an inside point is along the line of sight to the surface, the polygon must be a back face (we are inside that face and cannot see the front of it from our viewing position). 0     D Cz By Ax D Cz By Ax S    
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  6 of 20 Back-Face Detection Method •We can simplify this test by considering the normal vector N to a polygon surface, which has Cartesian components (A, B, C). In general, if V is a vector in the viewing direction from the eye (or "camera") position, as shown in Fig, then this polygon is a back face if V.N>0
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  7 of 20 N calculation P1(X1,Y1,Z1) P2(X2,Y2,Z2) P3(X3,Y3,Z3) ) P - (P ) P - (P N 2 3 2 1                   2 3 2 3 2 3 2 1 2 1 2 1 N Z Z Y Y X X Z Z Y Y X X k j i k Y Y X X Y Y X X j Z Z X X Z Z X X i Z Z Y Y Z Z Y Y                                  2 3 2 3 2 1 2 1 2 3 2 3 2 1 2 1 2 3 2 3 2 1 2 1 N         2 3 2 1 2 3 2 1 A * N Y Y Z Z Z Z Y Y                2 3 2 1 2 3 2 1 B * N X X Z Z Z Z X X                2 3 2 1 2 3 2 1 C * N X X Y Y Y Y X X       
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  8 of 20 Back-Face Detection Method •Ensure we have a right handed system with the viewing direction along the negative z-axis Now we can simply say that if the z component (C) of the polygon’s normal is less than zero the surface cannot be seen V = (0, 0, Vz)  V.N=Vz.C so that we only need to consider the sign of C, the ; component of the normal vector N .
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  9 of 20 Back-Face Detection Method •In a right-handed viewing system with viewing direction along the negative z, axis, the polygon is a back face if C <= 0. • In a Left-handed viewing system with viewing direction along the positive z, axis, the polygon is a back face if C > 0.
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  10 of 20 Back-Face Detection (cont…) Ingeneral back-face detection can be expected to eliminate about half of the polygon surfaces in a scene from further visibility tests More complicated surfaces though scupper us! We need better techniques to handle these kind of situations
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  11 of 20 Depth-Buffer Method Compares surfacedepth values throughout a scene for each pixel position on the projection plane Usually applied to scenes only containing polygons As depth values can be computed easily, this tends to be very fast Also often called the z-buffer method
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  12 of 20 Depth-Buffer Method (cont…) Images taken from Hearn & Baker, “Computer Graphics with OpenGL” (2004)
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  13 of 20 Depth-Buffer Algorithm 1. Initialisethe depth buffer and frame buffer so that for all buffer positions (x, y) depthBuff(x, y) = 1.0 frameBuff(x, y) = Color
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  14 of 20 Depth-Buffer Algorithm (cont…) 2.Process each polygon in a scene, one at a time – For each projected (x, y) pixel position of a polygon, calculate the depth z (if not already known) – If z < depthBuff(x, y), compute the surface colour at that position and set depthBuff(x, y) = z frameBuff(x, y) = surfColour(x, y) After all surfaces are processed depthBuff and frameBuff will store correct values
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  15 of 20 A-Buffer Method The A-buffermethod is an extension of the depth-buffer method The A-buffer method is visibility detection method developed at Lucasfilm Studios for the rendering system REYES (Renders Everything You Ever Saw)
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  16 of 20 A-Buffer Method (cont…) TheA-buffer expands on the depth buffer method to allow transparencies The key data structure in the A-buffer is the accumulation buffer
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  17 of 20 A-Buffer Method (cont…) Ifdepth is >= 0, then the surface data field stores the depth of that pixel position as before If depth < 0 then the data filed stores a pointer to a linked list of surface data
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  18 of 20 A-Buffer Method (cont…) Surfaceinformation in the A-buffer includes: – RGB intensity components – Opacity parameter – Depth – Percent of area coverage – Surface identifier – Other surface rendering parameters The algorithm proceeds just like the depth buffer algorithm The depth and opacity values are used to determine the final colour of a pixel
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  19 of 20 Scan-Line Method An imagespace method for identifying visible surfaces Computes and compares depth values along the various scan-lines for a scene
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  20 of 20 Depth-Sorting Method A visiblesurface detection method that uses both image-space and object-space operations Basically, the following two operations are performed – Surfaces are sorted in order of decreasing depth – Surfaces are scan-converted in order, starting with the surface of greatest depth The depth-sorting method is often also known as the painter’s method