# comp.graphics.algorithms FAQ

Last update: 11Jan96

Thanks to all the contributors. Corrections and contributions always welcome.

Changed items are emphasized.
New items are in Bold.
Items needing input are in boring old monospaced and have no hotlinks.

This FAQ is available in HTML or plain text form.

### Preface

1. Charter of comp.graphics.algorithms
2. Are the postings to comp.graphics.algorithms archived?
3. What are some must have books on graphics algorithms?
4. Are there any online references?
5. Where is all the source?

### Algorithms: 2D Geometry

6. How do I rotate a 2D point?
7. How do I find the distance from a point to a line?
8. How do I find the distance from a point to an ellipse?
9. How do I find intersections of 2 2D line segments?
10. How do I generate a circle through three points?
11. How do I generate a bezier curve that is parallel to another bezier?
12. How do I split a bezier at a specific value for t?
13. How do I find a t value at a specific point on a bezier?
14. How do I fit a bezier curve to a circle?
15. How do I generate a spline to approximate (insert curve here)?
16. How do I find the area/orientation of a polygon?
17. How do I find if a point lies within a polygon?
18. How do I find the intersection of two convex polygons?
19. How do I clip a polygon against a rectangle?
20. How do I clip a polygon against another polygon?
21. Where can I get source for Weiler/Atherton clipping?
22. Where can I find graph layout algorithms?
23. Where can I find algorithms for 2D collision detection?

### Algorithms: 2D Imaging

24. Where can I get a floodfill algorithm?
25. How do I rotate a bitmap?
26. How do I display a 24 bit image in 8 bits?
27. How do I fill the area an arbitrary shape?
28. How do I find the 'edges' in a bitmap?
29. How do I enlarge/sharpen/fuzz a bitmap?
30. How do I detect a 'corner' in a collection of points?
31. Where do I get source to display (raster font format)?
32. Are there Bresenham-like algorithms for circles and ellipses?
33. How do I draw an anti-aliased line/polygon/ellipse?
34. How do I quickly draw a filled triangle?
35. What is morphing/how is it done?

### Algorithms: 3D

36. How do I perform basic viewing in 3d?
37. How do I rotate a 3D point?
38. What are these ``quaternions'' anyway?
39. How do I map a texture on to a shape?
40. What is ARCBALL?
41. Where can I find ARCBALL source?
42. How do I find the intersection of a line and a plane?
43. How do I determine the intersection between a ray and a polygon?
44. How do I determine the intersection between a ray and a sphere?
45. How do I determine the intersection between a ray and a quadric?
46. How do I determine the intersection between a ray and a bezier surface?
47. How do I ray trace caustics?
48. Where can I get source for Voronoi/Delaunay triangulation?
49. Where do I get source for convex hull?
50. What is the marching cubes algorithm?
51. What is the status of the patent on the "marching cubes" algorithm?
52. How do I do a hidden surface test (backface culling) with 3d points?
53. How do I do a hidden surface test (backface culling) with 2d points?
54. Where can I find algorithms for 3D collision detection?
55. 3D Noise functions and turbulence in Solid texturing.

### Useful Mathematical Tidbits

56. How do I compute a fast square root
57. How do I generate a vector perpendicular to a given vector

### The FAQ

58. How can you contribute to this FAQ?
59. Who made this all possible?

1. Charter of comp.graphics.algorithms
Comp.graphics.algorithms is an unmoderated newsgroup intended as a forum for the discussion of the algorithms used in the process of generating computer graphics. These algorithms may be recently proposed in published journals or papers, old or previously known algorithms, or hacks used incidental to the process of computer graphics. The scope of these algorithms may range from an efficient way to multiply matrices, all the way to a global illumination method incorporating ray tracing, radiosity, infinite spectrum modeling, and perhaps even mirrored balls and lime jello.

It is hoped that this group will serve as a forum for programmers and researchers to exchange ideas and ask questions on recent papers or current research related to computer graphics.

comp.graphics.algorithms is not:

• for requests for gifs, or other pictures
• for requests for image translator software (i.e. gif <--> jpg)
contents
2. Are the postings to comp.graphics.algorithms archived?
Yes. The "official" archive is stored in http://www.cis.ohio-state/hypertext/faq/usenet/graphics/algorithms-faq/faq.html and is also available on href=ftp://rtfm.mit.edu/pub/usenet-by-group/news.answers/graphics/algorithms-faq ftp://wuarchive.wustl.edu/graphics/graphics/mail-lists/comp.graphics.algorithms

It is archived in the same manner that all other newsgroups are being archived there, namely there is an Index file with all the subjects, and all the articles are being kept in a hierarchy based on the year and month they are posted.

FYI, all usenet FAQ's are available on href=http://www.cis.ohio-state/hypertext/faq/usenet/top.html

contents
3. What are some must have books on graphics algorithms?
The keywords in brackets are used to refer to the books in later questions. They generally refer to the first author except where it is necessary to resolve ambiguity or in the case of the Gems.

### Basic computer graphics, rendering algorithms

[Foley] Computer Graphics: Principles and Practice (2nd Ed.), J.D. Foley, A. van Dam, S.K. Feiner, J.F. Hughes, Addison-Wesley 1990, ISBN 0-201-12110-7

[Rogers:Procedural] Procedural Elements for Computer Graphics, David F. Rogers, McGraw Hill 1985, ISBN 0-07-053534-5

[Rogers:Mathematical] Mathematical Elements for Computer Graphics 2nd Ed., David F. Rogers and J. Alan Adams, McGraw Hill 1990, ISBN 0-07-053530-2

[Watt:3D] _3D Computer Graphics, 2nd Edition_, Alan Watt, Addison-Wesley 1993, ISBN 0-201-63186-5

[Glassner:RayTracing] An Introduction to Ray Tracing, Andrew Glassner (ed.), Academic Press 1989, ISBN 0-12-286160-4

[Gems1] Graphics Gems, Andrew Glassner (ed.), Academic Press 1990, ISBN 0-12-286165-5

[Gems2] Graphics Gems II, James Arvo (ed.), Academic Press 1991, ISBN 0-12-64480-0

[Gems3] Graphics Gems III, David Kirk (ed.), Academic Press 1992, ISBN 0-12-409670-0 (with IBM disk) or 0-12-409671-9 (with Mac disk)

[Gems4] Graphics Gems IV, Paul S. Heckbert (ed.), Academic Press 1994, ISBN 0-12-336155-9 (with IBM disk) or 0-12-336156-7 (with Mac disk)

[Watt:Animation] Advanced Animation and Rendering Techniques, Alan Watt, Mark Watt, Addison-Wesley 1992, ISBN 0-201-54412-1

[Bartels] An Introduction to Splines for Use in Computer Graphics and Geometric Modeling, Richard H. Bartels, John C. Beatty, Brian A. Barsky, 1987, ISBN 0-934613-27-3

[Farin] Curves and Surfaces for Computer Aided Geometric Design: A Practical Guide, 3rd Edition, Gerald E. Farin, Academic Press 1993. ISBN 0-12-249052-5

[Prusinkiewicz] The Algorithmic Beauty of Plants, Przemyslaw W. Prusinkiewicz, Aristid Lindenmayer, Springer-Verlag, 1990, ISBN 0-387-97297-8, ISBN 3-540-97297-8

[Oliver] Tricks of the Graphics Gurus, Dick Oliver, et al. (2) 3.5 PC disks included, \$39.95 SAMS Publishing

[Hearn] Introduction to computer graphics, Hearn and Baker

### Image processing

[Barnsley] Fractal Image Compression, Michael F. Barnsley and Lyman P. Hurd, AK Peters, Ltd, 1993 ISBN 1-56881-000-8

[Jain] Fundamentals of Image Processing, Anil K. Jain, Prentice-Hall 1989, ISBN 0-13-336165-9

[Castleman] Digital Image Processing, Kenneth R. Castleman, Prentice-Hall 1979, ISBN 0-13-212365-7

[Pratt] Digital Image Processing, Second Edition, William K. Pratt, Wiley-Interscience 1991, ISBN 0-471-85766-1

[Gonzalez] Digital Image Processing (2nd Ed.), Rafael C. Gonzalez, Paul Wintz, Addison-Wesley 1987, ISBN 0-201-11026-1

[Russ] The Image Processing Handbook, John C. Russ, CRC Press 1992, ISBN 0-8493-4233-3

[Wolberg] Digital Image Warping, George Wolberg, IEEE Computer Society Press Monograph 1990, ISBN 0-8186-8944-7

### Computational geometry

[Bowyer] A Programmer's Geometry, Adrian Bowyer, John Woodwark, Butterworths 1983, ISBN 0-408-01242-0 Pbk

[O'Rourke] Computational Geometry in C, Joseph O'Rourke, Cambridge University Press 1994, ISBN 0-521-44592-2 Pbk, ISBN 0-521-44034-3 Hdbk

[Mortenson] Geometric Modeling, Michael E. Mortenson, Wiley 1985, ISBN 0-471-88279-8

[Preparata] Computational Geometry: An Introduction, Franco P. Preparata, Michael Ian Shamos, Springer-Verlag 1985, ISBN 0-387-96131-3

contents
4. Are there any online references?
The computational geometry community maintains its own bibliography of publications in or closely related to that subject. Every four months, additions and corrections are solicited from users, after which the database is updated and released anew. As of September 1993, it contained 5356 bib-tex entries. It can be retrieved from

ftp://cs.usask.ca/pub/geometry/o-cgc19.ps.Z - overview published in '93 in SIGACT News and the Internat. J. Comput. Geom. Appl.

Announcing the ACM SIGGRAPH Online Bibliography Project, by Stephen Spencer (biblio@siggraph.org)

The database is available for anonymous FTP from the ftp://siggraph.org/publications/bibliography directory. Please download and examine the file READ_ME in that directory for more specific information concerning the database.

'netlib' is a useful source for algorithms, member inquiries for SIAM, and bibliographic searches. For information, send mail to netlib@ornl.gov, with "send index" in the body of the mail message.

You can also find free sources for numerical computation in C via ftp://usc.edu/pub/C-numanal In particular, grab numcomp-free-c.gz in that directory.

Check out Nick Fotis's computer graphics resources FAQ -- it's packed with pointers to all sorts of great computer graphics stuff. This FAQ is posted biweekly to comp.graphics.

contents
5. Where is all the source?
Graphics Gems source code. ftp://princeton.edu/pub/Graphics/GraphicsGems

General 'stuff' ftp://wuarchive.wustl.edu/graphics/graphics

contents

# Algorithms: 2D Geometry

6. How do I rotate a 2D point?
In 2-D, the 2x2 matrix is very simple. If you want to rotate a column vector v by t degrees using matrix M, use

M = {{cos t, -sin t}, {sin t, cos t}} in M*v.

If you have a row vector, use the transpose of M (turn rows into columns and vice versa). If you want to combine rotations, in 2-D you can just add their angles, but in higher dimensions you must multiply their matrices.

contents
7. How do I find the distance from a point to a line?

Let the point be C (XC,YC) and the line be AB (XA,YA) to (XB,YB). The length of the line segment AB is L:

```              ___________________
|        2         2
L = \| (XB-XA) + (YB-YA)
```
and
```            (YA-YC)(YA-YB)-(XA-XC)(XB-XA)
r = -----------------------------
L**2

(YA-YC)(XB-XA)-(XA-XC)(YB-YA)
s = -----------------------------
L**2
```
Let I be the point of perpendicular projection of C onto AB, the
```        XI=XA+r(XB-XA)
YI=YA+r(YB-YA)
```
Distance from A to I = r*L
Distance from C to I = s*L

If r < 0 I is on backward extension of AB
If r>1 I is on ahead extension of AB
If 0<=r<=1 I is on AB

If s < 0 C is left of AB (you can just check the numerator)
If s>0 C is right of AB
If s=0 C is on AB

contents
8. How do I find the distance from a point to an ellipse?

Well, there are two ways, the right way and the fast way. Representing the ellipse in implicit form:

```   2              2
A x  + B x y + C y  + D x + E y + F = 0
```
The fast way is to calculate
```         2              2
A x  + B x y + C y  + D x + E y + F
-------------------------------------------------
sqrt((2 A x + B y + D)^2 + (B x + 2 C y + E)^2)
```
The right way is to rotate and translate the coordinate system so that the ellipse is in canconical form:
```   2      2
a x  + b y  = 1
```
and solve for lambda in
```                2      2
a x  + b y  = 0
x + lambda (2 a x) = x0
y + lambda (2 b y) = y0
```
This gives a quadric equation in lambda:
```    2                2      2                2                     2                2
a x0 (1 + 2 b lambda) + b y0 (1 + 2 a lambda)   =  (1 + 2 a lambda) (1 + 2 b lambda)
```
contents
9. How do I find intersections of 2 2D line segments?
This problem can be extremely easy or extremely difficult depends on your applications. If all you want is the intersection point, the following should work:

Let A,B,C,D be 2-space position vectors. Then the directed line segments AB and CD are given by:

```        AB=A+r(B-A), r in [0,1]
CD=C+s(D-C), s in [0,1]
```
If AB and CD intersect, then
```        A+r(B-A)=C+s(D-C)
```
or
```        XA+r(XB-XA)=XC+s(XD-XC)
YA+r(YB-YA)=YC+s(YD-YC)  for some r,s in [0,1]
```
Solving the above for r and s yields
```            (YA-YC)(XD-XC)-(XA-XC)(YD-YC)
r = -----------------------------  (eqn 1)
(XB-XA)(YD-YC)-(YB-YA)(XD-XC)

(YA-YC)(XB-XA)-(XA-XC)(YB-YA)
s = -----------------------------  (eqn 2)
(XB-XA)(YD-YC)-(YB-YA)(XD-XC)
```
Let I be the position vector of the intersection point, then
```        I=A+r(B-A)
```
or
```        XI=XA+r(XB-XA)
YI=YA+r(YB-YA)
```
By examining the values of r and s, you can also determine some other limiting conditions:

If 0 <= r <= 1 and 0 <= s <= 1, intersection exists
r < 0 or r>1 or s < 0 or s>1 line segments do not intersect

If the denominator in eqn 1 is zero, AB and CD are parallel
If the numerator in eqn 1 is also zero, AB and CD are coincident

If the intersection point of the 2 lines are needed (lines in this context mean infinite lines) regardless whether the two line segments intersect, then

If r > 1, I is located on extension of AB
If r < 0, I is located on extension of BA
If s > 1, I is located on extension of CD
If s < 0, I is located on extension of DC

Also note that the denominators of equations 1 and 2 are identical.

References:

[O'Rourke] pp. 249-51
[Gems3] pp. 199-202 "Faster Line Segment Intersection,"

contents
10. How do I generate a circle through three points?
Let the three given points be a, b, c. Use _0 and _1 to represent x and y coordinates. The coordinates of the center p=(p_0,p_1) of the circle determined by a, b, and c are:
```            p_0 =
( b_1 a_0^2
- c_1 a_0^2
- b_1^2 a_1
+ c_1^2 a_1
+ b_0^2 c_1
+ a_1^2 b_1
+ c_0^2 a_1
- c_1^2 b_1
- c_0^2 b_1
- b_0^2 a_1
+ b_1^2 c_1
- a_1^2 c_1 )
/ D

p_1 =
( a_0^2 c_0
+ a_1^2 c_0
+ b_0^2 a_0
- b_0^2 c_0
+ b_1^2 a_0
- b_1^2 c_0
- a_0^2 b_0
- a_1^2 b_0
- c_0^2 a_0
+ c_0^2 b_0
- c_1^2 a_0
+ c_1^2 b_0)
/ D
```
where
```      D = 2( a_1 c_0 + b_1 a_0 - b_1 c_0 -a_1 b_0 -c_1 a_0 + c_1 b_0 )
```
The radius of the circle is then:
```      r^2 = (a_0 - p_0)^2 + (a_1 - p_1)^2
```
Reference:

[O'Rourke] p. 201

contents
11. How do I generate a bezier curve that is parallel to another bezier?
You can't. The only case where this is possible is when the bezier can be represented by a straight line. And then the parallel 'bezier' can also be represented by a straight line.

contents
12. How do I split a bezier at a specific value for t?
A Bezier curve is a parametric polynomial function from the interval [0..1] to a space, usually 2-D or 3-D. Common Bezier curves use cubic polynomials, so have the form

f(t) = a3 t^3 + a2 t^2 + a1 t + a0,

where the coefficients are points in 3-D. Blossoming converts this polynomial to a more helpful form. Let s = 1-t, and think of tri-linear interpolation:

```        F([s0,t0],[s1,t1],[s2,t2]) =
s0(s1(s2 c30 + t2 c21) + t1(s2 c21 + t2 c12)) +
t0(s1(s2 c21 + t2 c12) + t1(s2 c12 + t2 c03))
=
c30(s0 s1 s2) +
c21(s0 s1 t2 + s0 t1 s2 + t0 s1 s2) +
c12(s0 t1 t2 + t0 s1 t2 + t0 t1 s2) +
c03(t0 t1 t2).
```
The data points c30, c21, c12, and c03 have been used in such a way as to make the resulting function give the same value if any two arguments, say [s0,t0] and [s2,t2], are swapped. When [1-t,t] is used for all three arguments, the result is the cubic Bezier curve with those data points controlling it:
```          f(t) = F([1-t,t],[1-t,t],[1-t,t])
=  (1-t)^3 c30 +
3(1-t)^2 t c21 +
3(1-t) t^2 c12 +
t^3 c03.
```
Notice that
```           F([1,0],[1,0],[1,0]) = c30,
F([1,0],[1,0],[0,1]) = c21,
F([1,0],[0,1],[0,1]) = c12, _
F([0,1],[0,1],[0,1]) = c03.
```
In other words, cij is obtained by giving F argument t's i of which are 0 and j of which are 1. Since F is a linear polynomial in each argument, we can find f(t) using a series of simple steps. Begin with
```        f000 = c30, f001 = c21, f011 = c12, f111 = c03.
```
Then compute in succession:
```        f00t = s f000 + t f001, f01t = s f001 + t f011,
f11t = s f011 + t f111,
f0tt = s f00t + t f01t, f1tt = s f01t + t f11t,
fttt = s f0tt + t f1tt.
```
This is the de Casteljau algorithm for computing f(t) = fttt.

It also has split the curve for the intervals [0..t] and [t..1]. The control points for the first interval are f000, f00t, f0tt, fttt, while those for the second interval are fttt, f1tt, f11t, f111.

If you evaluate 3 F([1-t,t],[1-t,t],[-1,1]) you will get the derivate of f at t. Similarly, 2*3 F([1-t,t],[-1,1],[-1,1]) gives the second derivative of f at t, and finally using 1*2*3 F([-1,1],[-1,1],[-1,1]) gives the third derivative.

This procedure is easily generalized to different degrees, triangular patches, and B-spline curves.

contents
13. How do I find a t value at a specific point on a bezier?
In general, you'll need to find t closest to your search point. There are a number of ways you can do this. Look at [Gems1, p607], there's a chapter on finding the nearest point on the bezier curve. In my experience, digitizing the bezier curve is an acceptable method. You can also try recursively subdividing the curve, see if you point is in the convex hull of the control points, and checking is the control points are close enough to a linear line segment and find the nearest point on the line segment, using linear interpolation and keeping track of the subdivision level, you'll be able to find t.

contents
14. How do I fit a bezier curve to a circle?
Interestingly enough, bezier curves can approximate a circle but not perfectly fit a circle.

Michael Goldapp, "Approximation of circular arcs by cubic polynomials" Computer Aided Geometric Design (#8 1991 pp.227-238)

Tor Dokken and Morten Daehlen, "Good Approximations of circles by curvature-continuous Bezier curves" Computer Aided Geometric Design (#7 1990 pp. 33-41).

contents
15. How do I generate a spline to approximate (insert curve here)?
contents
16. How do I find the area/orientation of a polygon?
Compute the signed area. The orientation is counter-clockwise if this area is positive. There's a Gem on computing signed areas.

A slightly faster method is based on the observation that it isn't necessary to compute the area. One can find the lowest, rightmost point of the polygon, and then take the cross product of the edges fore and aft of it. Both methods are O(n) for n vertices, but it does seem a waste to add up the total area when a single cross product (of just the right edges) suffices.

The reason that the lowest, rightmost point works is that the internal angle at this vertex is necessarily convex, strictly less than pi (even if there are several equally-lowest points).

The key formula is this:

If the coordinates of vertex v_i are x_i and y_i, twice the area of a polygon is given by

```             n - 1
----
\
2 A(P) =  >   (x  y      - x      y )
/      i  i + 1    i + 1  i
----
i = 0
```
Reference:

[O'Rourke] pp. 18-27.

contents
17. How do I find if a point lies within a polygon?
A quick comment - the code in the Sedgewick book Algorithms is wrong.

The short answer, for the FAQ, could be:

```    int pnpoly(int npol, float *xp, float *yp, float x, float y)
{
int i, j, c = 0;
for (i = 0, j = npol-1; i < npol; j = i++) {
if ((((yp[i] <= y) && (y < yp[j])) ||
((yp[j] <= y) && (y < yp[i]))) &&
(x < (xp[j] - xp[i]) * (y - yp[i]) /
(yp[j] - yp[i]) + xp[i]))
c = !c;
}
return c;
}
```
This code is from Wm. Randolph Franklin, wrf@ecse.rpi.edu, a professor at RPI, with some minor modifications for speed by me. A good reference for this problem will be:

References:

[O'Rourke] pp. 233-238
[Gems4] pp. 23-45
[Glassner:RayTracing]

contents
18. How do I find the intersection of two convex polygons?
[O'Rourke] pp. 242-252

contents
19. How do I clip a polygon against a rectangle?
This is the Sutherland-Hodgman algorithm, an old favorite that should be covered in any textbook. Any of the references listed in the FAQ should have it. According to Vatti (q.v.) "This [algorithm] produces degenerate edges in certain concave / self intersecting polygons that need to be removed as a special extension to the main algorithm" (though this is not a problem if all you are doing with the results is scan converting them.)

contents
20. How do I clip a polygon against another polygon?
People interested in some alpha state code, written in C++ with templates (for example: g++) can contact Klamer Schutte, klamer@mi.el.utwente.nl. Or with Mosaic, href=http://ph.tn.tudelft.nl:2000/People/klamer/Klamer.html#clippoly Also from ftp://galaxy.ph.tn.tudelft.nl/pub/klamer/clippoly.tar.gz

contents
21. Where can I get source for Weiler/Atherton clipping?
contents
22. Where can I find graph layout algorithms?
See the paper by Eades and Tamassia, "Algorithms for Drawing Graphs: An Annotated Bibliography," Tech Rep CS-89-09, Dept. of CS, Brown Univ, Feb. 1989.

A revised version of the annotated bibliography on graph drawing algorithms by Giuseppe Di Battista, Peter Eades, and Roberto Tamassia is now available from

contents
23. Where can I find algorithms for 2D collision detection?
contents

# Algorithms: 2D Imaging

24. Where can I get a floodfill algorithm?
contents
25. How do I rotate a bitmap?
The easiest way, according to the comp.graphics faq, is to take the rotation transformation and invert it. Then you just iterate over the destination image, apply this inverse transformation and find which source pixel to copy there.

A much nicer way comes from the observation that the rotation matrix:

R(T) = { { cos(T), -sin(T) }, { sin(T), cos(T) } }

is formed my multiplying three matrices, namely:

R(T) = M1(T) * M2(T) * M3(T)

or

```       [ 1, -tan(T/2) ] [ 1,      0 ] [ 1, -tan(T/2) ]
[ 0, 1         ] [ sin(T), 1 ] [ 0,         1 ]
```
Each transformation can be performed in a separate pass, and because these transformations are either row-preserving or column-preserving, anti-aliasing is quite easy.

Reference:

Paeth, A. W., "A Fast Algorithm for General Raster Rotation", Proceedings Graphics Interface '89, Canadian Information Processing Society, 1986, 77-81 [Note - e-mail copies of this paper are no longer available]

contents
26. How do I display a 24 bit image in 8 bits?
[Gems1] pp. 287-293, "A Simple Method for Color Quantization: Octree Quantization"

B. Kurz. Optimal Color Quantization for Color Displays. Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition, 1983, pp. 217-224.

[Gems2] pp. 116-125, "Efficient Inverse Color Map Computation"

This describes an efficient technique to map actual colors to a reduced color map, selected by some other technique described in the other papers.

[Gems2] pp. 126-133, "Efficient Statistical Computations for Optimal Color Quantization"

Xiaolin Wu. Color Quantization by Dynamic Programming and Principal Analysis. ACM Transactions on Graphics, Vol. 11, No. 4, October 1992, pp 348-372.

contents
27. How do I fill the area of an arbitrary shape?

"A Fast Algorithm for the Restoration of Images Based on Chain Codes Description and Its Applications", L.W. Chang and K.L. Leu, Computer Vision, Graphics, and Image Processing, vol.50, pp296-307 (1990)

"An Introductory Course in Computer Graphics" by Richard Kingslake, (2nd edition) published by Chartwell-Bratt ISBN 0-86238-284-X

contents
28. How do I find the 'edges' in a bitmap?
A simple method is to put the bitmap through the filter:
```        -1    -1    -1
-1     8    -1
-1    -1    -1
```
This will highlight changes in contrast. Then any part of the picture where the absolute filtered value is higher than some threshold is an "edge".

contents
29. How do I enlarge/sharpen/fuzz a bitmap?
contents
30. How do I detect a 'corner' in a collection of points?
For the general solution to the problem get Ata Etemadi's software package and associated papers from one of the addresses below. It's very fast and detects polygons too.

contents
31. Where do I get source to display (raster font format)?
contents

32. Are there Bresenham-like algorithms for circles and ellipses?
Yes, there is a general conic drawer on ftp://ftp.dai.ed.ac.uk./pub/vision/src/conic.cc .

This version completes (and corrects a bug in) the version printed in [Foley].

contents
33. How do I draw an anti-aliased line/polygon/ellipse?
contents
34. How do I quickly draw a filled triangle?
The easiest way is to render each line separately into an edge buffer. This buffer is a structure which looks like this (in C):
```        struct { int xmin, xmax; } edgebuffer[YDIM];
```
There is one entry for each scan line on the screen, and each entry is to be interpreted as a horizontal line to be drawn from xmin to xmax.

Since most people who ask this question are trying to write fast games on the PC, I'll tell you where to find code. Look at:

```
ftp://ftp.uwp.edu/pub/msdos/demos/programming/source
```
contents
35. What is morphing/how is it done?
contents

# Algorithms: 3D

36. How do I perform basic viewing in 3d?
We describe the shape and position of objects using numbers, usually (x,y,z) coordinates. For example, the corners of a cube might be {(0,0,0), (1,0,0), (1,1,0), (0,1,0), (0,0,1), (1,0,1), (1,1,1), (0,1,1)}. A deep understanding of what we are saying with these numbers requires mathematical study, but I will try to keep this simple. At the least, we must understand that we have designated some point in space as the origin--coordinates (0,0,0)--and marked off lines in 3 mutually perpendicular directions using equally spaced units to give us (x,y,z) values. It might be helpful to know if we are using 1 to mean 1 foot, 1 meter, or 1 parsec; the numbers alone do not tell us.

A picture on a screen is two steps removed from the 3D world it depicts. First, it is a 2D projection; and second, it is a finite resolution approximation to the infinitely precise projection. I will ignore the approximation (sampling) for now. To know what the projection looks like, we need to know where our viewpoint is, and where the plane of the projection is, both in the 3D world. Think of it as looking out a window into a scene. As artists discovered some 500 years ago, each point in the world appears to be at a point on the window. If you move your head or look out a different window, everything changes. When the mathematicians understood what the artists were doing, they invented perspective geometry.

If your viewpoint is at the origin--(0,0,0)--and the window sits parallel to the x-y plane but at z=1, projection is no more than (x,y,z) in the world appears at (x/z,y/z,1) on the plane. Distant points will have large z values, causing them to shrink in the picture. That's perspective.

The trick is to take an arbitrary viewpoint and plane, and transform the world so we have the simple viewing situation. There are two steps: move the viewpoint to the origin, then move the viewplane to the z=1 plane. If the viewpoint is at (vx,vy,vz), transform every point by the translation (x,y,z) --> (x-vx,y-vy,z-vz). This includes the viewpoint and the viewplane. Now we need to rotate so that the z axis points straight at the viewplane, then scale so it is 1 unit away.

After all this, we may find ourselves looking out upside- down. It is traditional to specify some direction in the world or viewplane as "up", and rotate so the positive y axis points that way (as nearly as possible if it's a world vector). Finally, we have acted so far as if the window was the entire plane instead of a limited portal. A final shift and scale transforms coordinates in the plane to coordinates on the screen, so that a rectangular region of interest (our "window") in the plane fills a rectangular region of the screen (our "canvas" if you like).

I have left out details of how you define and perform the rotation of the viewplane, but I'm sure someone else will be happy to supply those if there is demand. It requires knowing how to describe a plane, and how to rotate vectors to line up. Neither is difficult, but this is already using a lot of net space. One further practical difficulty is the need to clip away parts of the world behind us, so -(x,y,z) doesn't pop up at (x/z,y/z,1). (Notice the mathematics of projection alone would allow that!) But all the viewing transformations can be done using translation, rotation, scale, and a final perspective divide. If a 4x4 homogeneous matrix is used, it can represent everything needed, which saves a lot of work.

contents
37. How do I rotate a 3D point?
Assuming you want to rotate vectors around the origin of your coordinate system. (If you want to rotate around some other point, subtract its coordinates from the point you are rotating, do the rotation, and then add back what you subtracted.) In 3-D, you need not only an angle, but also an axis. (In higher dimensions it gets much worse, very quickly.) Actually, you need 3 independent numbers, and these come in a variety of flavors. The flavor I recommend is unit quaternions.

contents
38. What are these ``quaternions'' anyway?
Unit quaternions are a way of representing 3D rotations. Their advantages are that they are easy to normalize and quick to compose. Their main disadvantage is that they are not widely known.

OK, so what *actually* are they? 4 numbers that square and add up to +1. You can write these as [(x,y,z),w], with 4 real numbers, or [v,w], with v, a 3-D vector pointing along the axis. The concept of an axis is unique to 3-D. It is a line through the origin containing all the points which do not move during the rotation. So we know if we are turning forwards or back, we use a vector pointing out along the line. Suppose you want to use unit vector u as your axis, and rotate by 2t degrees. (Yes, that's twice t.) Make a quaternion [u sin t, cos t]. You can use the quaternion -- call it q -- directly on a vector v with quaternion multiplication, q v q^-1, or just convert the quaternion to a 3x3 matrix M. If the components of q are {(x,y,z),w], then you want the matrix

```        M = {{1-2(yy+zz),  2(xy-wz),  2(xz+wy)},
{  2(xy+wz),1-2(xx+zz),  2(yz-wx)},
{  2(xz-wy),  2(yz+wx),1-2(xx+yy)}}.
```
Rotations, translations, and much more are explained in all basic computer graphics texts. Quaternions are covered briefly in
[Foley], and more extensively in several Graphics Gems, and the SIGGRAPH 85 proceedings.

contents
39. How do I map a texture on to a shape?
Paul S. Heckbert, "Survey of Texture Mapping", IEEE Computer Graphics and Applications V6, #11, Nov. 1986, pp 56-67 revised from Graphics Interface '86 version

Eric A. Bier and Kenneth R. Sloan, Jr., "Two-Part Texture Mappings", IEEE Computer Graphics and Applications V6 #9, Sept. 1986, pp 40-53 (projection parameterizations)

contents
40. What is ARCBALL?
Arcball is a general purpose 3-D rotation controller described by Ken Shoemake in the Graphics Interface '92 Proceedings. It features good behavior, easy implementation, cheap execution, and optional axis constraints. A Macintosh demo and electronic version of the original paper (Microsoft Word format) may be obtained from ftp::/ftp.cis.upenn.edu/pub/graphics .

contents
41. Where can I find ARCBALL source?
Complete source code appears in Graphics Gems IV pp. 175-192. A fairly complete sketch of the code appeared in the original article, in Graphics Interface 92 Proceedings, available from Morgan Kaufmann.

contents
42. How do I find the intersection of a line and a plane?
If the plane is defined as:

a*x + b*y + c*z + d = 0

and the line is defined as:

x = x1 + (x2 - x1)*t = x1 + i*t
y = y1 + (y2 - y1)*t = y1 + j*t
z = z1 + (z2 - z1)*t = z1 + k*t

Then just substitute these into the plane equation. You end up with:

t = - (a*x1 + b*y1 + c*z1 + d)/(a*i + b*j + c*k)

If the denominator is zero, then the vector (a,b,c) and the vector (i,j,k) are perpendicular. Note that (a,b,c) is the normal to the plane and (i,j,k) is the direction of the line. It follows that the line is either parallel to the plane or contained in the plane. In either case there is no unique intersection point.

contents
43. How do I determine the intersection between a ray and a polygon
First find the intersection between the ray and the plane in which the polygon is situated. Then see if the point of intersection is in the polygon.

Reference:

contents
44. How do I determine the intersection between a ray and a sphere
Given a ray defined as:
```        x = x1 + (x2 - x1)*t = x1 + i*t
y = y1 + (y2 - y1)*t = y1 + j*t
z = z1 + (z2 - z1)*t = z1 + k*t
```
and a sphere defined as:
```               2          2          2    2
(x - l)  + (y - m)  + (z - n)  = r
```
Substituting in gives the quadratic equation:
```           2
a*t  + b*t + c = 0
```
where:
```             2    2    2
a = i  + j  + k

b = 2*i*(x1 - l) + 2*j*(y1 - m) + 2*k*(x1 - n)

2    2    2     2     2    2                              2
c = l  + m  + n  + x1  + y1 + z1
- 2*(l*x1 + m*y1 + n*z1) - r
```
If the determinant of this equation is less than 0, the line does not intersect the sphere. If it is zero, the line is tangential to the sphere and if it is greater than zero it intersects at two points. Solving the equation and substituting the values of t into the ray equation will give you the points.

Reference:

contents
45. How do I determine the intersection between a ray and a Quadric?
Represent the ray as:
x = a + t b
where a, b and x are 3-vectors

x' A x + x' B + C = 0
where A is a symmetric 3x3 matrix, B a 3x1 vector, and C a scalar. Note that x' A x is the dot product of x and A x.

Substituting in gives the quadratic equation:

```              2
l2 * t  + l1 * t + l0 = 0
```
where:
```	l2 = b' A b

l1 = 2 b' A a + b' B

l0 = a' A a + b' B + C
```
In this notation, If the determinant of this equation is less than 0, the line does not intersect the quadric. Solving the equation and substituting the values of t into the ray equation will give you the points.

contents
46. How do I determine the intersection between a ray and a bezier surface?

Joy I.K. and Bhetanabhotla M.N., "Ray tracing parametric surfaces utilizing numeric techniques and ray coherence", Computer Graphics, 16, 1986, 279-286

Joy and Bhetanabhotla is only one approach of three major method classes: tessellation, direct computation, and a hybrid of the two. Tessellation is relatively easy (you intersect the polygons making up the tessellation) and has no numerical problems, but can be inaccurate; direct is cheap on memory, but very expensive computationally, and can (and usually does) suffer from precision problems within the root solver; hybrids try to blend the two.

Reference:

contents
47. How do I ray trace caustics?
There is a discussion in [Watt:Animation], but I don't know how good it is.

It's hard to get right.

One right (but incredibly expensive) answer is:

```    @InProceedings{mitchell-1992-illumination,
author         = "Don P. Mitchell and Pat Hanrahan",
title          = "Illumination From Curved Reflectors",
year           = "1992",
month          = "July",
volume         = "26",
booktitle      = "Computer Graphics (SIGGRAPH '92 Proceedings)",
pages          = "283--291",
keywords       = "caustics, interval arithmetic, ray tracing",
editor         = "Edwin E. Catmull",
}
```
A cheat:
```    @Article{inakage-1986-caustics,
author         = "Masa Inakage",
title          = "Caustics and Specular Reflection Models for
Spherical Objects and Lenses ",
pages          = "379--383",
journal        = "The Visual Computer",
volume         = "2",
number         = "6",
year           = "1986",
keywords       = "ray tracing effects",
}
```
very specialized:
```    @Article{yuan-1988-gemstone,
author         = "Ying Yuan and Tosiyasu L. Kunii and Naota
Inamato and Lining Sun ",
title          = "Gemstone Fire: Adaptive Dispersive Ray Tracing
of Polyhedrons ",
year           = "1988",
month          = "November",
journal        = "The Visual Computer",
volume         = "4",
number         = "5",
pages          = "259--70",
keywords       = "caustics",
}
```
contents
48. Where can I get source for Voronoi/Delaunay triangulation?
Source code in C for general dimension convex hull and Delaunay triangulation (qhull) is now available from:
```
ftp://geom.umn.edu/pub/software/qhull.tar.Z

ftp://geom.umn.edu/pub/software/qhull.sit.hqx for Macintosh
```
You can view the results in 3-d and 4-d with geomview from:
```
ftp://geom.umn.edu:/pub/software/geomview/geomview-sgi.tar.Z  - Iris

ftp://geom.umn.edu:/pub/software/geomview/geomview-next.tar.Z - Next
```
Convex hull is an easy way to build convex polytopes (just list the vertices). The algorithm also produces approximate convex hulls. For example, we have produced the approximate convex hull of 12,000 points in R^6, randomly distributed in the unit cube.

The distribution includes .ps files for:

Barber, C.B., Dobkin, D.P., and Huhdanpaa, H., "The Quickhull Algorithm for Convex Hull," Technical Report GCG53, The Geometry Center, Univ. of Minnesota, July 1993.

References:

[O'Rourke] pp. 168-204 Three-dimensional convex hull C code in Chapter 4 (p.130-60).

contents
49. Where do I get source for convex hull?
See the previous question on Delaunay triangulation. The two are the same because the Delaunay triangulation of a set of points is the convex hull of the points lifted to a paraboloid.

References: [O'Rourke] pp. 70-112 C code for Graham's algorithm on p.80-96. Three-dimensional convex hull discussed in Chapter 4 (p.113-67). C code for the incremental algorithm on p.130-60.

contents
50. What is the marching cubes algorithm?
The marching cubes algorithm is used in volume rendering to construct an isosurface from a 3D field of values.

The 2D analog would be to take an image, and for each pixel, set it to black if the value is below some threshold, and set it to white if it's above the threshold. Then smooth the jagged black outlines by skinning them with lines.

The marching cubes algorithm tests the corner of each cube (or voxel) in the scalar field as being either above or below a given threshold. This yields a collection of boxes with classified corners. Since there are eight corners with one of two states, there are 256 different possible combinations for each cube. Then, for each cube, you replace the cube with a surface that meets the classification of the cube. For example, the following are some 2D examples showing the cubes and their associated surface.

```    - ----- +       - ----- -       - ----- +       - ----- +
|:::'   |       |:::::::|       |::::   |       |   ':::|
|:'     |       |:::::::|       |::::   |       |.    ':|
|       |       |       |       |::::   |       |::.    |
+ ----- +       + ----- +       - ----- +       + ----- -
```
The result of the marching cubes algorithm is a smooth surface that approximates the isosurface that is constant along a given threshold. This is useful for displaying a volume of oil in a geological volume, for example.

contents
51. How do I generate a spline to approximate (insert curve here)?
contents
52. How do I do a hidden surface test (backface culling) with 3d points?
Just define all points of all polygons in clockwise order.
```            c = (x3*((z1*y2)-(y1*z2))+
(y3*((x1*z2)-(z1*x2))+
(z3*((y1*x2)-(x1*y2))
```
x1,y1,z1, x2,y2,z2, x3,y3,z3 = the first three points of the polygon.

If c is positive the polygon is visible. If c is negative the polygon is invisible (or the other way).

contents
53. How do I do a hidden surface test (backface culling) with 2d points?
c = (x1-x2)*(y3-y2)-(y1-y2)*(x3-x2)

x1,y1, x2,y2, x3,y3 = the first three points of the polygon.

If c is positive the polygon is visible. If c is negative the polygon is invisible (or the other way).

contents
54. Where can I find algorithms for 3D collision detection?
Check out "proxima", from Purdue, which is a C++ library for 3D collision detection for arbitrary polyhedra. It's a nice system; the algorithms are sophisticated, but the code is of modest size.

There's a copy in ftp://shasta.stanford.edu/pub/nagle/proxima ; it may not be the latest version.

contents
55. 3D Noise functions and turbulence in Solid texturing.
See: ftp://gondwana.ecr.mu.oz.au/pub/siggraph92_C23.shar ftp://ftp.cis.ohio-state.edu/siggraph92/siggraph92_C23.shar

In it there are implementations of Perlin's noise and turbulence functions, (By the man himself) as well as Lewis' sparse convolution noise function (by D. Peachey) There is also some of other stuff in there (Musgrave's Earth texture functions, and some stuff on animating gases by Ebert).

contents

# Useful Mathematical Tidbits

56. How do I compute a fast square root?
This routine is `off the net somewhere'
```unsigned short psi_sqrt(unsigned long v)
/*
// Calculates the square root of a 32-bit number.
*/
{
register long t = 1L << 30, r = 0, s;

#define STEP(k) \
s = t + r; \
r >>= 1; \
if (s <= v) { \
v -= s; \
r |= t; \
}

STEP(15); t >>= 2;
STEP(14); t >>= 2;
STEP(13); t >>= 2;
STEP(12); t >>= 2;
STEP(11); t >>= 2;
STEP(10); t >>= 2;
STEP(9); t >>= 2;
STEP(8); t >>= 2;
STEP(7); t >>= 2;
STEP(6); t >>= 2;
STEP(5); t >>= 2;
STEP(4); t >>= 2;
STEP(3); t >>= 2;
STEP(2); t >>= 2;
STEP(1); t >>= 2;
STEP(0);

return (unsigned short) r;
}

```
contents
57. How do I generate a vector perpendicular to a given vector?
Sometimes you have a 3D vector and you want some vector perpendicular to it. Of course there are an infinite number of possible ones, but you just need any old perpendicular. The thing to do is to find some other vector that is not parallel to the given one and to calculate the cross product, which gives your answer. The tricky thing is guaranteeing that the "other" vector is not parallel to the original one and ideally that it is maximally non-parallel to ensure an accurate cross product. What we do is to find which of the principal axes is most perpendicular to the vector and then cross-product with that. Any other way will lead you to grief in the end. The following routine is not the fastest way of doing this but it is clear.
```Vector3 get_perpendicular(const Vector3& A)
// returns a line perpendicular to A
{
// find smallest projection onto a principal axis
// (ie the nearest to 90 degrees)
// cross this with the original and return

Vector3 X(1,0,0);
Vector3 Y(0,1,0);
Vector3 Z(0,0,1);

Vector3& axis = Z;
if (A < A) {
// Not Y
if (A < A)
axis = X;
}
else
// Not X
if (A < A) {
axis = Y;
}

return (A ^ axis).normalize();
}
```
contents

# The FAQ

58. How can you contribute to this FAQ?
Send email to anson@hookup.net with your suggestions, possible topics, corrections, or pointers to information. Remember, I am not an expert on many of these topics. I'm the editor.

Here are some possible topics that may interest many of our readers.

59. Clipping...
60. Splines
61. Nurbs
62. Image Warping/Transformation/Filtering
63. Anti-aliasing
64. Volume Rendering
65. Morphing (synonymous with generalized Warping)
66. MPEG
67. JPEG
68. Z-buffer/A-buffer/etc.
69. interpolation (linear, spline, fft, etc.)
70. Modeling tricks (fractal mountains, trees, seashells)
71. Surfaces
72. Ray Tracing
73. Reflection/Refraction
74. 1) Computing the minimum bounding boxes of various geometric elements such as circular arcs, parabolas, clothoids, splines, etc. What is the most efficient way to do them for the following cases:

i) The boxes are all orthogonal to the XY plane;
ii) The boxes is oriented such that the smallest area rectangular bounding boxes are computed.

2) What is the most efficient way to tell if a polygon crosses itself? i.e. without intersecting every edge with every edge.

contents
75. Who made this all possible?
```    andrewfg@aifh.ed.ac.uk (Andrew Fitzgibbon)
atsao@tkk.win.net (Anson Tsao)
bromage@mundil.cs.mu.OZ.AU (Andrew James Bromage)
cek@Princeton.EDU (Craig Kolb)
fritz@riverside.MR.Net (Fritz Lott)
hollasch@kgc.com (Steve Hollasch)
jdstone@ingr.com (Jon Stone)
jens_alfke@powertalk.apple.com (Jens Alfke)
lhf@visgraf.impa.br (Luiz Henrique de Figueiredo)
orourke@cs.smith.edu (Joseph O'Rourke)
paik@mlo.dec.com (Samuel S. Paik)
sammy@icarus.smds.com (Samuel Murphy)
sanguish@digifix.com (Scott Anguish)
shoemake@graphics.cis.upenn.edu (Ken Shoemake)
slin@esri.com (Sum Lin)
spl@szechuan.ucsd.edu (Steve Lamont)
weilej@rpi.edu (Jason Weiler)
```
contents

```Jon Stone                             Intergraph Corporation
jdstone@ingr.com                      Boulder, CO
```