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Trigons, Normals, and Quadragons

Vector-based Solutions to the Classic

Problems of Computer Graphics

09 July 2013 update

Notes / Options:

Slideshow animated (F5)

See also slide footnotes

J. Philip Barnes

Display a 3D-math-modeled shape on a 2D screen

- Presentation Contents
- “Flexible-cylinder” geometry grid
- Geometry math modeling tools
- Vectors ~ history & key operations
- Fast world-to-pyramid transformation
- Review & Renew: Rotation matrices
- Review & Renew: Z-buffer & Ray-casting
- Vector solution : “Point-in-polygon”
- Vector solution: Inter-vertex interpolation
- Vector solution: Constant-curvature Tesselation
- “Pyranometer” ambient lighting model
- Apply ~ "Render Raptor" & “TrigonoSoar”

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

“Flexible Cylinder” Geometry Grid

y

q

l

q

z

l

x

Vertex

Normal

Vertex

Quadragon

Computer graphics classic problem 1:

Generate and manage 3D geometry

Arrays and Subscripts

Object (n), Row (i), Col (j),

Vertex (k), Quadragon (m)

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Geometry Math-model Building Blocks

1.0

1.0

Pseudo-Cosine

0.8

0.8

0.6

0.6

e = sin(hm p)

0.4

0.4

Pseudo-Sine

0.2

0.2

h

0.0

0.0

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

1.0

1.0

Exponential

Varabola

0.8

0.8

e =e-5hm

e = hm

0.6

0.6

0.4

0.4

0.2

0.2

h

0.0

0.0

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

m = 2.0 1.0 0.5

e y / ymax

= cos m (hp/2)

h x / xmax

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Review, Renew, Theory, & History* of the Casteljau-Bézier Curve

- P. De Casteljau first conceived (1963) today’s “Bézier Curve”
- His notes & algorithm remained proprietary at Citroën for years
- Set (Np) Control points for control polygon near, not on, curve
- (Np-1, Np-2,...) interpolation series on polygon-sides for x(t), y(t)
- De Casteljau’s Algorithm is represented by the Bernstein Basis
- as discovered by A.R. Forrest ~ Geometric construct → math
- S. Bernstein’s polynomial (B) derivative (dB/dt) is introduced herein
- P. Bézier independently conceived (1966) the control polygon
- Weight factors not widely used (differ from Casteljau/Bernstein)
- Bézier’s work at Rénault is the foundation of UNISURF & CATIA

p2

1/3

y

p3

1/3

x,y

1/3

p1

pNp

t = 1/3

x

Animated graphic (shift F5)

* R.T. Farouki, The Bernstein polynomial basis: a centennial retrospective, UC Davis, 03 Mar 2012

* G. Farin, A History of Curves and Surfaces in CAGD, Arizona State University, 2007

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Demo: Casteljau-Bézier 2D Curve Example

x = S Bi Xpi

4

3

N=Vxk

V

y = S Bi Ypi

t

5

6

1

2

V = (dx/dt) i + (dy/dt) j

dx/dt = S (dB/dt)iXpi

dy/dt = S (dB/dt)iYpi

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

“TrigonoSoar” Math-modeled 3D Surface

Aero axes

z

y

x

Chord, thickness, twist, etc.

parametric along the spar

from centerline to winglet tip

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Algebratross

Wandering Albatross

RegenoSoar

Regenerative-soarer

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

“RenderRaptor” ~ Wireframe & Flat-Shade

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Vectors ~ Origin and Key Operations

Unit vectors

a A / |A|

b B / |B|

c C / |C|

cosg = a b

sin g = |axb|

y

C

B

j

g

Oliver Heaviside

J. Willard Gibbs

A

c

k

i

z

x

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Vector Cross Product ~ Elegant Visual Derivation

y

C=AxB

B

j

A

k

i

z

x

y, j

Ay

By

Az

Ax

Bz

Bx

z, k

x,i

Add cross products, each axis

Important ~ Retain order A:B

Cx = AyBz - Az By

Cy= AzBx - AxBz

Cz= Homework !

To obtain the cross product AxB above, simply add the orthogonal cross product components as shown at upper right

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

xp

e

k

xs, zs

z-ze

Fast World-to-Pyramid Transform & 90oExclusionCriteria

Graphics classic problem 2:

Efficiently project any vertex (v) in 3D onto 2D screen, given: eye point (e) and focus point (f)

Approach:

Twice rotate the world about axes through (e), operating on coordinate differences (x-xe etc.)

First test a new "90-deg exclusion" criterion. Skip transformations for any quadragon which, after transformation, would have all vertices (v) at least 90o from the line of sight, based on the angle included by the vectors ev& ef using only the dot-product sign. This adds 3 multiplications (xv-xe)(xf -xe) + (yv-ye)(yf -ye) + (zv-ze)(zf -ze) for all vertex coordinates, but avoids at least 9 to transform excluded vertices & unit-normals.

After

world

rotations

y-ye

ef

e

View Pyramid

n

Focus

point

Eye

point

ef

f

e

v

ev

ef cosn

Before

world

rotations

n

1. cosn = |zf - ze| / ef ; sinn= -(yf - ye) / ef

2. sine = (xf - xe) / (ef cosn) ; cose = (zf - ze) / (ef cosn)

3. Rotate (e) about y-ye axis through point (e)

4. Rotate (n) about x-xe axis through point (e)

5. Optionally rotate (k) for “cockpit-roll” effect

6. Project to screen (perspective transformation)

x-xe

Animated graphic (shift F5)

Chart origin: JPB, 2007

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Rotation Matrix Concatenation ~ Review and Renew

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Demo: object-maneuver and world-pyramid rotations

Stationary object, moving observer

Fixed observer, maneuvering object

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Polygon Pipeline Overview (incl. inner surfaces when applicable)

Polygon Pipeline Filter J. Philip Barnes, 2013

Initialize: No q\'gons-in-pyramid ("QiP"=0) ~ partly/fully

Init: “raybox” corners to screen center (if ray-casting)

For each quadragon of each polyhedron:

Get/set vertex model coords., normals, & in/out colors

Model-to-World transform q\'gon vertex coords.only

Check: all q\'gon vertices pass the 90o exclusion test

OK, World-pyramid & perspective x\'form vertex coords.

Test: q\'gon at least partially overlaps the screen ; OK,

Model-world and world-pyramid x\'form vertex normals

Get 4 corners of q\'gon bounding box and 2 of pixel-box

Update raybox top-left, low-right corners (if ray-casting)

Get q\'gon centroid (zc) ; assess in(out)-facing

Add q’gon as member of “quadragons in pyramid” (QiP)

Screen Plane

Raybox

y

Screen

z

pixel box

x

quadragon

bounding

box

pixel (p)

Z-buffer(E. Catmull / W. Straber, 1974)

Suggested Implementation J. Philip Barnes, 2013

Sort QiPs by in(out)-facing and (zc)

Initialize: For each pixel (i,j), zref = -

For each in(out)-facing QiP (q\'gon-in-pyramid)

Get pixel box upper-left/lower-right pixel indices

For each pixel (i,j) of the pixel box

Get pixel screen coordinates (xp,yp)

Test: (p) inside q\'gon projection ; OK, interp. (zp)

Test at pixel (i,j): zp > zref ; OK, zref = zp

Interp. normals at (p) ; shade pixel if it is visible

Ray-casting(Arthur Appel, 1968)

Projection-Ray / Z-bufferHybrid, J. Philip Barnes, 2013

Sort QiPs by (zc) ; Initialize: “hit_index” = -1

For each “screen-raybox” pixel (p); zref = -

Get pixel sceen x-y ; Cast eye-to-pixel ray

For each QiP, test: 2D overlap box includes (p)

OK, test: point (p) is inside QiP projection

OK, interpolate (zp) on QiP ; Test: zp > zref

OK, hit_index = QiP_index ; zref = zp

At hit, interp. normals ; shade pixel if it is visible

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Vector determination of "point-in-polygon"

y

3

E2

E3

P

2

E1

P1

P

1

x

Computer graphics classic problem 3:

Given 2D coordinates of vertices and point (P), determine if (P) resides inside

Define:

EiEdge vector, vertex(i)-to-next(i+1)

PiPoint vector, vertex(i)-to-point (P)

zUnit vector out of the page

Observe:

(P) inside: E1 x P1 points out of the page

(P) outside: E1 x P1 points into the page

Therefore:

Point (P) is outside if, for any vertex (i),

( Ei x Pi ) ● k < 0

Postulate: Applies to any convex polygon

Chart origin: JPB, 2007

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Inter-vertex Interpolation ~ Vector Solution

4

Computer graphics classic problem 4:

Given coords. & properties at vertices 1-2-3, interpolate the properties at point (p)

y

3

Classic method: “scan line” (comp. intensive)

Contemporary method: Barycentric Coords.

Complementary Alternative : Vectors

2

p

Apply vector tip-to-tail addition & scaling

Solve for & store two “position parameters”

Ro4 = Ro1 + R14 = Ro1 + aR1p

= Ro2 + R24 = Ro2 + bR23

1

x

o

Equate Ro4 (Dx) & (Dy) to get a, b

Finally, interpolate at (p):

zp = z1 + [z2 + b (z3-z2) -z1] / a

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Constant-curvature Tesselation

Computer graphics classic problem 5:

Tesselation of curved polygon Cartesian coordinates

Apply: Breakup curved polygon to avoid edge-rendering artifacts

Whenever at least one vertex is not visible

Task: Get Cartesian coordinates of curved-polygon edge midpoint

Problem: Barycentric coordinates assume a planar trigon surface

But the trigon plane typically models a sphere, bowl, cylinder, or saddle surface

Solution: Assume each trigon edge has constant curvature

Method is exact for spherical surfaces, and perhaps so for cylindrical surfaces

Unit-normal dot product (ua●ub) gets cos (q) ; solve for local curve radius (r)

Vector cross product then gets from (m) to (e), [convex / concave automatic]

c

e

b

f

Tesselate

abc toefg

m

a

Side view of trigon edge ab, midpoint (m),

true surface point (e),

& unit normals

g

b

r

e

a

q

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

“Pyranometer Model” of Ambient Lighting

Sun shade

pyranometer disk sees

only the sky "dome"

Computer graphics classic problem 6:

Model ambient light before illumination

- Classic lighting model: Uniform ambient
- Then add illumination
- Problem:
- “Grey silhouette” until lights are added
- Poor foundation of total reflection model
- Idea:
- Vary ambient reflection with orientation
- ~ good foundation before illumination
- Implementation:
- pyranometer measures "skydome" light
- - Rotated on it side ~ half the intensity
- - Inverted ~ zero intensity (@ albedo=0)

- Define:
- pyranometer tilt angle (t)
- Ground albedo (a)
- Ambient Reflectance
- r = 1 - t/p + a t/p

Chart origin: JPB, 2007

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Pyranometer Vs. Uniform Ambient Model (cloudy day)

Ground

Albedo = 0.0

“Pyranometer”

Ambient

Ground

Albedo = 0.5

“Uniform” ambient

“Pyranometer”

Ambient

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

"Render Raptor" ~ Math-modeled Aircraft

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Summary ~ Trigons, Normals, and Quadragons

- New Solutions: computer graphics classic problems
- Complementary, faster, or better, with added insight
- Graphical derivation of vector cross product
- Fast world-to-pyramid transform ~ just two rotations
- Rotation concatenation ~ Review and Renew
- 90o exclusion criterion for world-to-pyramid transform
- Polygon Pipeline Overview ~ Z-buffer & Ray Casting
- Point-in-polygon ~ vector-based determination
- Interpolation at point (p) ~ vector-based alternative
- Constant-curvature Tesselation
- “Pyranometer” ambient-lighting model foundation

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

Phil Barnes has a Bachelor’s Degree in Mechanical Engineering from the University of Arizona and a Master’s Degree in Aerospace Engineering from Cal Poly Pomona. He has 32-years of experience in performance analysis and computer modeling of aerospace vehicles, engines, and subsystems, primarily at Northrop Grumman. He has authored SAE technical papers on aerodynamics, dynamic soaring, and regenerative soaring. This latest presentation brings together Phil’s knowledge and passions for computer graphics, and geometry math modeling.

Trigons, Normals, and Quadragons J. Philip Barnes 09 July 2013 www.HowFliesTheAlbatross.com

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