CS 450: COMPUTER GRAPHICS REVIEW: CLIPPING SPRING 2015 DR. MICHAEL J. REALE

Transcription

1 CS 45: COMPUTER GRAPHICS REVIEW: CLIPPING SPRING 215 DR. MICHAEL J. REALE

2 CLIPPING Clipping = removing part or all of the primitives otside of the clipping volme/window Clipping volme in OpenGL nit cbe from (-1,-1,-1) to (1,1,1) Clipping window in OpenGL (-1,-1) to (1,1)

3 VIEWPORT Viewport = deteres where on displa window the clipping window will be displaed Defined with starting point and width/height (in piels) Ma be (and most probabl is) a different aspect ratio than clipping window Can have mltiple viewports

4 POINT AND LINE CLIPPING IN 2D Point clipping keep point if within bondaries; discard otherwise: Line clipping ma ma Keep line if endpoints inside window Discard line if endpoints completel on otside of bondar Otherwise, intersect with bondaries Generall want to avoid nnecessar intersections

5 COHEN-SUTHERLAND LINE CLIPPING Cohen-Stherland line clipping Assmes rectanglar clipping region Assign all endpoints 4-bit region code Bit = 1 OUTSIDE of that window border Bit = INSIDE or ON window border Basic algorithm (given region codes R 1 and R 2 ): If R 1 R 2 == false both mst be completel inside If R 1 & R 2 == tre one bit mst be shared completel otside Otherwise intersect with bondar, change endpoint, reassign region codes, and tr again with remaining part of line

6 LIANG-BARSKY LINE CLIPPING Liang-Barsk Line Clipping Does even MORE testing before intersection calclations faster Uses parametric line eqations Can be sed with non-rectanglar clipping regions Generall more efficient than Cohen-Stherland 1,2,3,4 ) ( k q p k k ma ma ma ma ma ma q p q p q p q p Testing conditions:

7 LIANG-BARSKY LINE CLIPPING: FULL ALGORITHM Start with 1 and 2 (starting and ending parameter vales of line) 1 =, 2 = 1 For each clipping bondar (can implement as nested if statements): Compte p k and q k vales If (p k!= ) Compte r k If (p k < ) OUTSIDE to INSIDE 1 = r k ONLY if line will be shorter If (p k > ) INSIDE to OUTSIDE 2 = r k ONLY if line will be shorter If at an point ( 1 > 2 ) REJECT LINE Otherwise (p k == ) q rk p If (q k < ) line is parallel to AND otside bondar REJECT LINE If after all bondar checks line is NOT rejected se vales of 1 and 2 to compte line endpoints k k

8 POLYGON CLIPPING Polgon clipping Cannot jst clip lines (end p with disconnected lines) Trivial cases: All vertices inside KEEP polgon All vertices otside one bondar DISCARD polgon Otherwise, check intersections: Conve ONE otpt verte list Concave MULTIPLE otpt verte lists Conve Concave

9 SUTHERLAND-HODGMAN POLYGON CLIPPING Stherland-Hodgman Polgon Clipping B defalt, onl handles CONVEX polgons one otpt verte list Creates etra lines if appl to concave Sends pairs of endpoints throgh a series of clippers: left, right, bottom, top For a given pair of endpoints v1 and v2, 4 possibilities: in in Otpt: v2 in ot Otpt: v2 ot ot Otpt: none ot in Otpt: v1, v2

10 WEILER-ATHERTON POLYGON CLIPPING Weiler-Atherton Polgon Clipping Works with both conve and concave polgons Also works with an shape clipping region Basic algorithm: Trace arond perimeter of polgon Hit IN-OUT intersection follow clipping bondar (keep track of that point P) When hit alread processed verte otpt crrent verte list start over at point P Can get mltiple fill regions separate, nconnected polgons

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12 3D CLIPPING Use PLANE bondaries with 3D clipping Planes ma be arbitrar or aligned with aes (see right) z z ma ma ma Man 2D clipping algorithms etended to 3D: Trivial cases ver similar Cohen-Stherland region codes Liang-Barsk sing parametric lines for intersection with bondaries P P ( P P1) D objects sall clip srface triangles individall Ma check individal vertices or bondar bo/sphere first for trivial cases