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Curves and Surfaces CSE3AGR - Paul Taylor 2009
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Polynomials of Degree n Degree is equal to the highest exponent of a term. Higher exponents result in more and more possible curves A degree n Polynomial can represent all curves that can be represented by ANY <n polynomials.
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Linear x https://harmon-middle- school.wikispaces.com/file/view/Graph.psd.jpg
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Quadratic x 2 http://www.nipissingu.ca/calculus/tutorials/quadratics.html
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Cubic x 3 http://en.wikipedia.org/wiki/File:Polynomialdeg3.svg
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Quartic x 4 http://en.wikipedia.org/wiki/Quartic_equation
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Quintic x 5 http://en.wikipedia.org/wiki/File:Polynomialdeg5.png
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Representin’ Explicit Implicit Parametric
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Explicit Solve the new state of the system based on its previous state
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Explicit Just like in High-School One variable is dependant, the other Independant Y = f(x) – Eg: y = x 2 Sometimes the function is invertertable X = g(y) – Eg: x = y (1/2)
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Problems Explicit Functions can fail on specific orientations!!! A Simple Line y = mx + c A Vertical Line all values of Y will only exist for 1 value of x
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A Circle A circle is a curve with a constant rate of bending The closest we can get using an explicit rep: Y = (r 2 + x 2 ) Y = - (r 2 - x 2 ) Given 0 ≤ |x| ≤ r
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3D Explicit Functions We need two functions to represent a curve in 3D. Given X we have 2 dependant variables: – Y = f(x) – Z = f(x)
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Explicit Surfaces A Surface requires two independent variables Z = f( x, y)
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Pros & Cons of Explicits Mathematically Perfect – But Mathematicians never had to render an infinite value!!! – A Good use of explicits is intersection testing Did the ray Intersect the Circle? r = (y 2 + x 2 ) – A sphere r = - (z 2 + y 2 + x 2 )
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Implicit Solve the new state of the system based on its previous state and the current state
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Implicits F( x, y) = 0 In 2 Dimensions: A Line: Ax + by + c = 0 A Circle: x 2 + y 2 – r 2 = 0
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Pros & Cons of Implicits Also very useful for membership testing. Eg: Does (3,7) belong to the curve? Curves must be described as the intersection of two curved surfaces. :-S Quadrics are a quadratic polynomial type of surface, which can be represented implicitly. -Quadrics are also simple to intersect with lines
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Parametric Equations These guys are pretty much the staple diet of Cg curves! Basically they are created using three related explicit functions
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Pros & Cons of Parametrics Always solvable! Great for cheating with pre-calculated Basis Matrices
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Joining Cubics Continuity is what we need!! – We have two levels of continuity Geometric and Parametric Geometric is simply ‘it looks pretty good!’ Parametric is where the continuity is perfectly defined. – Parametric Continuity infers geometric Continuity » This is not transient – We can create Geometric Continuity by slacking off on Parametric Equations
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3 Levels of Cohesion Level 0 – Both curves share a point in space Level 1 – Both curves share the point in space and have tangents (Gradients) that point in the same direction. (matching Velocity in time based visualisation) Level 2 – Both curves share the point in space and have tangents (Gradients) that point in the same direction with the same magnitude (matching velocity and acceleration in time based visualisation)
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Geometric Continuity Levels G 0 – Curves are Joined G 1 – First Derivatives are Proportional G 2 – First & Second Derivatives are Proportional
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Polynomial Continuity Levels C 0 – Curves are Joined C 1 – First Derivatives are Equal C 2 – First & Second Derivatives are Equal
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Hermite Form Curves Specified by: – The 2 Endpoints – The Tangents at the endpoints Both Direction and Velocity
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Bezier Form Curves
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Natural Cubic Spline This type of spline can contain n control points Generating the coefficients requires an n+1 square Matrix!!!
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Spline A special mathematical function defined piecewise by polynomials – In Human Readable Format: Joining Polynomials Together
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B-Spline is short for basis spline
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Non-Uniform Rational Basis Splines (NURBS)
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Lerping It’s a bad name, but we need to do it! Thus we can convert our curves to line segments, and surfaces to polygonsm – Remember that Quad patches may look easy, but may not turn out flat!!!
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Considerations How large is the spline being represented? How close is it to the screen? How much time do you want to spend rendering? Do you want to generate micro-polygons for a awesomely shaded image? (RenderMan style)
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Generating Normals whilst Lerping Perpendicular line gradients have a product of -1 Thus the normal x gradient = -1 -1 / gradient = normal * In 1 axis only, you will need to accumulate and normalise the Vector!! (or is it already normalised??)
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References http://spec.winprog.org/curves/ http://graphics.cs.msu.ru/courses/cg99/notes/le ct13/curves.htm
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