Hidden line removal algorithm
- 2. HLR - Introduction • Hidden line removal (HLR) is an extension of wireframe model rendering where lines (or segments of lines) covered by surfaces are not drawn. • Hidden line removal (HLR) is the method of computing which edges are not hidden by the faces of parts for a specified view and the display of parts in the projection of a model into a 2D plane. • It is considered that information openly exists to define a 2D wireframe model as well as the 3D topological information.
- 5. Visibility Techniques Algorithms might be slow in calculation and storing data. These techniques will rectify the problem. • Minimax Test • Containment Test • Surface Test • Computing Silhouettes • Edge Intersection • Segment Comparison • Homogeneity Test
- 6. Minimax Test/ Bounding Box Test • To check whether a point is in a given bounded surface or volume and checks if two polygons overlap. Conditions
- 7. Containment Test • Determines the given point is located in or out of the polygon and to check the vertices of one polygon for containment in the other. • Types of Method – Intersection Method – Angle Method
- 8. Containment Test Containment proceeds: (a) If the sum of the angles is equal to zero, point P is outside the polygon. (b) If the sum is equal to 360°, point P is inside the polygon.
- 9. Surface Test/ Back Face/Depth Test • The process used to determine which surfaces and parts of surfaces are not visible from a certain viewpoint. Condition : – Faces whose surface normal are positive in Z direction for visible surfaces – Faces whose surface normal are negative in Z direction for non visible surfaces
- 10. Computing Silhouettes • A set of edges that separates visible faces from invisible faces of an object with respect to a given viewing direction is called silhouette edges (or silhouettes). • An edge that is part of the silhouette is characterized as the intersection of one visible face and one invisible face.
- 12. Edge Intersection • To determine the visibility of partially invisible edges. • Calculate line intersections to determine edge visibility. y = y1 + m (xboundary - x1) x = x1 + (yboundary - y1) / m m = (y2 - y1) / (x2 - x1)
- 13. Segment Comparison (Scan Line) • Utilizes the image’s raster scan. • Segment comparison are performed in x & z plane. • Visibility techniques are further performed for each span by comparing depth and edge segments.
- 14. Homogeneity Test • Utilizes neighbourhood points to test its visibility. • A Point can be projected onto neighbourhood of projection of points, then the neighbourhood of point P is decided to be visible or invisible.
- 15. Hidden Line Algorithms • Priority Algorithm • Area Oriented Algorithm • Overlay Algorithm
- 16. Priority algorithm • This algorithm is also known as depth or Z algorithm. • Imagines that objects are modelled with lines and lines are generated where surfaces join. If only the visible surfaces are created then the invisible lines are automatically removed by this algorithm.
- 17. Priority algorithm Face Priority ABCD 1 ADFG 1 DCEF 1 ABHG 2 EFGH 2 BCEH 2
- 18. Priority algorithm • ABCD, ADFG, DCEF are given higher priority-1. Hence, all lines in this faces are visible, that is, AB, BC, CD, DA, AD, DF, FG, AG, DC, CE, EF and DF are visible. • AGHB, EFGH, BCEH are given lower priority-2. Hence, all lines in this faces other than priority-1 are invisible, that is BH, EH and GH. These lines must be eliminated.
- 19. • Identify Silhouette Polygons • Quantitative Hiding (QH) • Visibility of Silhouette segment • Intersect the internal edges • Display the edges Area Oriented algorithm
- 21. • Applicable for curved surfaces by approximating them as planar surfaces. • u-v grid is used to make grid surface by making it as straight edges. Overlay algorithm • STEPS: – To Calculate uv grid using surface equation. – Grid Surface to Linear Edge creation. – Using proper visibility techniques for required output.
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