# 3DFAQ.TXT

From pat@nick.csh.rit.edu (Pat Fleckenstein (HoseNode)) Newsgroups: rec.games.programmer Subject: FAQ: 3-D Information for the Programmer Date: Thu, 3 Mar 1994 18:26:18 GMT ______________________________________ | |\ | @@@@@ @@@@ @@@@ @@ @@@ |*| | @ @@ @ @@ @@ @ @@ @ |*| | @@ @@ @@ @ @@@@ @@@@ @@ @ |*| | @@ @ @@ @ @@ @@ @ @@ @@ |*| | @@@ @@@@ @@ @@ @ @@@@ |*| |____________________________________|*| \***********************************\*| \*better*graphics*later*I*promise***\| ------------------------------------ Contents: 1) General references for 3-d graphics questions. 2) How do I define an object? 3) How do I define space? 4) How do I define position? 5) How do I define orientation? 6) How do I define a velocity? 7) Drawing three-dimensional objects on a two-dimensional screen. 8) Vector Math - Dot Product and Cross-Product. 9) Matrix Math 10) Collisions. 11) Perspective. 12) Z-Buffering & the Painters Algorithm. 13) Shading. 14) 3-space clipping. 15) 3-d scanning. 16) Publically available source-code. 17) Books on the topics. 18) Other forums. Last update: 21Dec93 What's new? This whole ball of wax. 1) General references for 3-d graphics questions. Well, this FAQ is just getting off the ground. Hopefully it will touch on most of the bases you need to get started for now, and hope- fully it will expand at least as fast as you need it too. But... regardless, things you'll want to locate for more help are Matrix Alge- bra books, Physics books talking about Eulerian motion, and some books on the Graphics Hardware you want to program for. The code examples included in this FAQ will most likely be in C with pseudo-code in com- ments. But, you'll also want to defintely check out the FAQ for comp.graphics. That FAQ touches mainly on 2-D needs, but some 3-D aspects are reviewed there, too. 2) How do I define an object? There are lots of ways to define objects. One of the most commonly used is the OFF (Object File Format). The OFF toolkit and a library of objects are available via anonymous ftp from gatekeeper.dec.com -- XXX ???. The format provides easy methods for extensions and a base set of things you can expect for each object. The toolkit is a bit bulky, but the file format (in ascii) is easy enough to parse by hand. The OFF.aoff file contains information about the object. The most important one there is the location of the surface specification file (usually object name.geom). This file also contains other attributes - and file names relevant to this object. The OFF surface specification begins with the number of points, the number of polygons and the number of segments. npts nplys nsegs This line is followed by the floating point coordinates for the points that make up the object. x1 y1 z1 x2 y2 z2 x3 y3 z3 . x(npts) y(npts) z(npts) Then, it gets a bit more complicated. The following lines begin with a number to indicate the number of vertices in this polygon. That number is followed by that many numbers, one for each vertice. These are given in an order specified in the .aoff (usually conter-clockwise). So, for example, a triangle and a pentagon which share a side are shown below. 3 1 3 4 5 2 4 3 6 7 Here is some quick and dirty sample code to read in the .geom file: struct polygon { int nvert; /* Number of vertices in this polygon */ int *verts; /* Vertices in this polygon */ }; struct object { int npts; /* The number of points */ int npolys; /* The number of polygons */ int nsegs; /* The number of segments */ double *point x,*point y,*point z; - - - struct polygon *polys; }; int read geom file( char *geom file, struct object *obj ) - - - { FILE *fp; int i,j; if (!(fp = fopen(geom file,"r"))) /* Open the .geom file */ - return -1; /* Get header information */ fscanf(fp,"%d %d %d",&obj.npts,&obj.npolys,&obj.nsegs); /* ** Allocate room for the points. */ obj.point x = (double *)malloc(obj.npts*sizeof(double)); - obj.point y = (double *)malloc(obj.npts*sizeof(double)); - obj.point z = (double *)malloc(obj.npts*sizeof(double)); - for (i=0;itriplets, it will require a fair bit of work on reading them in to turn them into spherical coordinates. If you're looking to this FAQ for information on how to define the space your objects will be in, I'd strongly suggest using rectangular coordinates and some derivative of the OFF-format. For starters, let me just throw in that while our universe may be infinite in all directions, that doesn't make for good programming. We have to limit ourselves to small enough numbers that we can multiply them together without overflowing them, we can divide them without crashing our systems, and we can add them without accidentally flipping a sign bit. Now, the fun begins. The simplest form of defining the Universe is to flat out say that the Universe stretches over these coordinates, say in the bounding box of <-65536, -65536, -65536> to <65536, 65536, 65536>. This is often referred to as a Universal Coordinate system or an Absolute Coordinate system. Then, each object in the Universe will be centered about some coordinate in that range. This includes your viewpoint. Several strategies are available for dealing with the edge of the Universe. One can make the Universe wrap around so that an object leaving the cube at < X, Y, 65536> will re-appear in the Universe at < X, Y, -65536>. Or, one can make objects bounce or stop at the edge of the Universe. And, given any approach, one can have the edge of the Universe be transparent or opaque. In an Absolute Coordinate system, all objects must be shown from the position of your viewpoint. This involves lots of interesting math that we'll get into later. But, in general, an objects position with respect to you is it's absolute position - your absolute position (with all kinds of hell breaking loose if you can see past the edge of the Universe). Then, after this position is calculated, it must be rotated based on your orientation in the Universe. Another possibility for defining space is a Relative Coordinate system or a View-Centered Coordinate system. In this sort of system, the Viewpoint is always at coordinates <0,0,0> and everything else in the Universe is based relatively to this home position. This causes funky math to come into play when dealing with velocities of objects, but... it does wonders for not having to deal with the 'edge of the Universe'. This is the Schroedinger's cat method of the 'edge of the Universe'.... in the truest sense of out of sight is out of mind. Small provisions have to be made if objects aren't to wrap around. But... a Relative Coordinate system can be used to give the illusion of infinite space on a finite machine. (Yes, even your 486/66DX is finite). I'll leave spherical coordinates to a later version if people think they'll be of use... 4) How do I define position? Position in an Absolute Coordinate system is easy. Each object has three coordinates. These are often stored in a data-type called a vec- tor to abstract further the notion that these numbers belong together. typedef struct { long x; long y; long z; } VECT; Usually, each object in the Universe is defined about its center with each coordinate on its surface being centered at its own <0,0,0>. This helps tremendously in rotating the object, and I would highly recommend this. Then, the object as a whole is given a position in space. When it comes time to draw this object, its points' coordinates get added on to its position. In a Relative Coordinate system, position is also fairly straight forward. The view-point always has position VECT={ 0, 0, 0 };. Other objects follow the same sort of system that they would in Absolute Coor- dinate systems. 5) How do I define orientation? Orientation can be quite tricky. I interchange some of the terms here quite often. In 3-space, orientation must be defined be two-and- a-half angles. "Two and a half?" you say. Well, almost everyone uses three because two just isn't enough, but if you want to be technical, one of those angles only has to range from 0 - 180 degrees (0 - PI/2 radians). But, taking that for granted now.... you have to pick an orienta- tion for your view. I personally prefer to have the X-axis run from left to right across the center of my screen. I also like to have the Y-axis run from the bottom of my screen; and I also like to have the Z- axis running from me straight into my screen. With some tweaking of plus and minus signs and a bit of re-ordering, all of the math here-in can be modified to reflect any orientation of the coordinate system. Some people prefer to have the Y-axis heading into the screen with the Z-axis going vertically. It's all a matter of how you want to define stuff. Given that you've agreed with me that Z can go into the screen, what 3-angles do you need? (Here's where I stand the biggest chance of mucking up the terms.) You need roll, pitch, and yaw. (I often mix up roll and yaw and such... so if you can follow along without getting locked into my terminology, future FAQ's will correct it.) Look at your monitor as you're reading this. Now tilt your head so that your right ear is on your right shoulder. This change in orienta- tion is roll (or yaw... but I call it roll). Ok, now sit up straight again. Now bring your chin down to meet your chest. (Hmmm... LOOK BACK NOW!!!, whew... glad you heard me.) That motion was pitch. Ok, now look over your right shoulder keeping your head vertical to see who's behind you. (LOOK BACK AGAIN!!.) Ok... that was yaw (or roll, but I call it yaw). That's the basics. Now, what do I do with them? Well, here's where a nice book on Matrix Arithmetic will help you out. You have to use these three angles to make a Transformation matrix. [See the sec- tion on Matrix Math]. Here is a typical method of doing these transfor- mations: [Note, if you don't have Z going into your screen you'll have to munge these considerably]. typedef double matrix[4][4]; double sr,sp,sy,cr,cp,cy; matrix mr, mp, my; /* individual transformations */ matrix s; /* final matrix */ sr = sin( roll ); cr = cos( roll ); sp = sin( pitch ); cp = cos( pitch ); sy = sin( yaw ); cy = cos( yaw ); /* clear all matrixes ** [See the section on Matrix Math] */ identity( &mr ); identity( &mp ); identity( &my ); /* prepare roll matrix */ mr[0][0] = mr[1][1] = cr; mr[1][0] = - (mr[0][1] = sr); /* prepare pitch matrix */ mp[1][1] = mp[2][2] = cp; mp[1][2] = - (mp[2][1] = sp); /* prepare yaw matrix */ my[0][0] = my[2][2] = cy; my[0][2] = - (my[2][0] = sy); multiply( &mr, &my, &s ); multiply( &s, &mp, &s ); 6) How do I define a velocity? Sticky question. I'll get to it in the next rev. 7) Drawing three-dimensional objects on a two-dimensional screen. From comp.graphics FAQ: "There are many ways to do this. Some approaches map the viewing rectangle onto the scene, by shooting rays through each pixel center and assigning color according to the object hit by the ray. Other approaches map the scene onto the view- ing rectangle, by drawing each object into the region, keeping track of which object is in front of which. The mapping mentioned above is also referred to as a 'projec- tion', and the two most popular projections are perspective projection and parallel projection. For example, to do a parallel projection of a scene onto a viewing rectangle, you can just discard the Z coordinate (divide by depth), and 'clip' the objects to the viewing rectangle (discard portions that lie outside the region). For details on 3D rendering, the Foley, van Dam, Feiner and Hughes book, reading. Chapter 6 is 'Viewing in 3D', and chapter 15 is 'Visible-Surface Determination'. For more information go to chapter 16 for shading, chapter 19 for clip- ping, and branch out from there." 8) Vector Math - Dot Product and Cross-Product. Adding and subtracting vectors is as easy as subtracting their respective parts: + = - = Scaling vectors is as simple as multiplying each part by a con- stant: S * = ~~The Dot-Product of two vectors is simply the sum of the products of their respective parts: .~~= A*D + B*E + C*F Note that this value is not a vector. The Cross-Product of two vectors is a bit more complex (it is the determinant of the matrix with the direction vector as the first row, the first vector as the second row, and the second vector as the third row): X = Note that: X= -1 * ( X ) -and- ( X ) . = ( X ) . = 0 More later. 9) Matrix Math The identity matrix is the one with all elements {i,j} given by: { 1.0 if i == j m[i][j] = { { 0.0 otherwise Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 10) Collisions. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 11) Perspective. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 12) Z-Buffering & the Painters Algorithm. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 13) Shading. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 14) 3-space clipping. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 15) 3-d scanning. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 16) Publically available source-code. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 17) Books on the topics. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. 18) Other forums. Sorry... wanted to get at least a partial FAQ out soon, so this section is mostly blank for now. Will do more later. -- mu �