1 // texcoord.hxx -- routine(s) to handle texture coordinate generation
3 // Written by Curtis Olson, started March 1999.
5 // Copyright (C) 1999 Curtis L. Olson - http://www.flightgear.org/~curt
7 // This library is free software; you can redistribute it and/or
8 // modify it under the terms of the GNU Library General Public
9 // License as published by the Free Software Foundation; either
10 // version 2 of the License, or (at your option) any later version.
12 // This library is distributed in the hope that it will be useful,
13 // but WITHOUT ANY WARRANTY; without even the implied warranty of
14 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 // Library General Public License for more details.
17 // You should have received a copy of the GNU Library General Public
18 // License along with this library; if not, write to the
19 // Free Software Foundation, Inc., 59 Temple Place - Suite 330,
20 // Boston, MA 02111-1307, USA.
25 /* The following is an explanation of our somewhat conveluted and
26 tricky texture scaling/offset scheme:
28 MAX_TEX_COORD is a value I arrived at by trial and error for my
29 voodoo2/3 video card. If you use texture coordinates that are too
30 big, you quickly start getting into round off problems and the texture
31 jumps and moves relative to the polygon.
33 The point of all of this code is that I wanted to be able to define
34 this size in meters of a texture and have it be applied seamlessly to
35 the terrain. I wanted to be able to change the defined size (in
36 meters) of textures at run time. In other words I want to be able to
37 scale the textures at run time and still have them seamlessly tile
40 The problem is that I have to pregenerate all the texture coordinates
41 when I create the scenery, and I didn't want to burn CPU doing this
42 again when I load the scenery at run time.
44 It ended up taking me a lot of thought, a lot of trial and error, and
45 a lot of fiddling around to come up with a scheme that worked.
49 Ok, so think about what needs to be done to have the texture tile
50 across a series of triangles and fans ...
52 Basically you want to use some function of lon/lat mod your max
53 texture coordinate size to calculate the texture coordinate of each
54 vertex. This should result in nice tiling across distinct triangles
57 Pretend our MAX_TEX_COORD = 4.0 and half of this is 2.0
59 Imagine the following two adjacent polygons with the "X" component of
60 the initial texture coordinate based on longitude (Note they are drawn
61 spaced apart, but in reality the two polygons are adjacent):
67 Now, this exceeds our MAX_TEX_COORD of 4.0 so we have to scale these
68 texture coordinates by some integer value. Let's say we always want
69 to minimize the tex coordinates to minimize rounding error so we will
70 offset the first polygon by 7.0 and the second by 8.0:
72 0.0 --- 1.6 and 0.6 --- 1.0
74 Our tiling is maintianed becuase the coordinates are continous (mod
75 1.0) and we still get the double repeat across both polygons.
77 We want to be able to scale these values by an arbitrary constant and
78 still have proper tiling.
80 Let's try doubling the coordinates:
82 0.0 --- 3.2 and 1.2 --- 2.0
84 Everything still tiles nicely (because the coordinates are continuous
85 mod 1.0) and the texture is now repeated 4x across the two polygons.
86 Before it was repeated 2x.
88 Let's try halving the coordinates:
90 0.0 --- 0.8 and 0.3 --- 0.5
92 Ooop! We lost continuity in texture coordinate space ... no we will
93 have a visual discontinuity in the texture tiling!
95 Ok, so we need some other scheme to keep our texture coordinates
96 smaller than MAX_TEX_COORD that preserves continuity in texture
97 space. <Deep breath> let's try the scheme that I have coded up that
98 you are asking about ... <fingers crossed> :-)
100 Going way back to the top before we shifted the texture coordinates.
101 tmin for the first polygon is 7.0, this is then adjusted to:
103 (int)(tmin.x() / HALF_MAX_TEX_COORD) ) * HALF_MAX_TEX_COORD
105 = (int)(7.0/2.0) * 2.0 = 3.0 * 2.0 = 6.0
107 The two texture coordinates are offset by 6.0 which yields 1.0 -- 2.6
109 tmin for the second polygon is 8.6 which is adjusted to:
111 (int)(tmin.x() / HALF_MAX_TEX_COORD) ) * HALF_MAX_TEX_COORD
112 = (int)( 8.6 / 2.0 ) * 2.0 = 4.0 * 2.0 = 8.0
114 The offset for the second polygon is 8.0 which yields 0.6 --- 1.0
118 1.0 --- 2.6 and 0.6 --- 1.0
120 This still tiles nicely and strethes our texture across completely, so
121 far we haven't done any damage.
123 Now let's double the coordinates:
125 2.0 --- 5.2 and 1.2 --- 2.0
127 The texture is repeated 4x as it should be and is still continuous.
129 How about halfing the coordinates. This is where the first scheme
130 broke down. Halving the coordinates yields
132 0.5 --- 1.3 and 0.3 --- 0.5
134 Woohoo, we still have texture space continuity (mod 1.0) and the
135 texture is repeated 1x.
137 Note, it took me almost as long to re-figure this out and write this
138 explanation as it did to figure out the scheme originally. I better
139 enter this in the official comments in case I forget again. :-)
143 #include <simgear/compiler.h>
145 // #include STL_IOSTREAM
147 #include "texcoord.hxx"
149 // SG_USING_STD(cout);
150 // SG_USING_STD(endl);
153 #define FG_STANDARD_TEXTURE_DIMENSION 1000.0 // meters
154 #define MAX_TEX_COORD 8.0
155 #define HALF_MAX_TEX_COORD ( MAX_TEX_COORD * 0.5 )
158 // return the basic unshifted/unmoded texture coordinate for a lat/lon
159 static inline Point3D basic_tex_coord( const Point3D& p,
161 double degree_height,
164 return Point3D( p.x() * ( degree_width * scale /
165 FG_STANDARD_TEXTURE_DIMENSION ),
166 p.y() * ( degree_height * scale /
167 FG_STANDARD_TEXTURE_DIMENSION ),
172 // traverse the specified fan/strip/list of vertices and attempt to
173 // calculate "none stretching" texture coordinates
174 point_list sgCalcTexCoords( const SGBucket& b, const point_list& geod_nodes,
175 const int_list& fan, double scale )
177 // cout << "calculating texture coordinates for a specific fan of size = "
178 // << fan.size() << endl;
180 // calculate perimeter based on center of this degree (not center
182 double clat = (int)b.get_center_lat();
184 clat = (int)clat + 0.5;
186 clat = (int)clat - 0.5;
189 double clat_rad = clat * SGD_DEGREES_TO_RADIANS;
190 double cos_lat = cos( clat_rad );
191 double local_radius = cos_lat * SG_EQUATORIAL_RADIUS_M;
192 double local_perimeter = local_radius * SGD_2PI;
193 double degree_width = local_perimeter / 360.0;
195 // cout << "clat = " << clat << endl;
196 // cout << "clat (radians) = " << clat_rad << endl;
197 // cout << "cos(lat) = " << cos_lat << endl;
198 // cout << "local_radius = " << local_radius << endl;
199 // cout << "local_perimeter = " << local_perimeter << endl;
200 // cout << "degree_width = " << degree_width << endl;
202 double perimeter = SG_EQUATORIAL_RADIUS_M * SGD_2PI;
203 double degree_height = perimeter / 360.0;
204 // cout << "degree_height = " << degree_height << endl;
206 // find min/max of fan
207 Point3D tmin, tmax, p, t;
212 for ( i = 0; i < (int)fan.size(); ++i ) {
213 p = geod_nodes[ fan[i] ];
214 // cout << "point p = " << p << endl;
216 t = basic_tex_coord( p, degree_width, degree_height, scale );
217 // cout << "basic_tex_coord = " << t << endl;
223 if ( t.x() < tmin.x() ) {
226 if ( t.y() < tmin.y() ) {
229 if ( t.x() > tmax.x() ) {
232 if ( t.y() > tmax.y() ) {
238 double dx = fabs( tmax.x() - tmin.x() );
239 double dy = fabs( tmax.y() - tmin.y() );
240 // cout << "dx = " << dx << " dy = " << dy << endl;
242 // Point3D mod_shift;
243 if ( (dx > HALF_MAX_TEX_COORD) || (dy > HALF_MAX_TEX_COORD) ) {
244 // structure is too big, we'll just have to shift it so that
245 // tmin = (0,0). This messes up subsequent texture scaling,
246 // but is the best we can do.
247 // cout << "SHIFTING" << endl;
248 if ( tmin.x() < 0 ) {
249 tmin.setx( (double)( (int)tmin.x() - 1 ) );
251 tmin.setx( (int)tmin.x() );
253 if ( tmin.y() < 0 ) {
254 tmin.sety( (double)( (int)tmin.y() - 1 ) );
256 tmin.sety( (int)tmin.y() );
258 // cout << "found tmin = " << tmin << endl;
260 if ( tmin.x() < 0 ) {
261 tmin.setx( ( (int)(tmin.x() / HALF_MAX_TEX_COORD) - 1 )
262 * HALF_MAX_TEX_COORD );
264 tmin.setx( ( (int)(tmin.x() / HALF_MAX_TEX_COORD) )
265 * HALF_MAX_TEX_COORD );
267 if ( tmin.y() < 0 ) {
268 tmin.sety( ( (int)(tmin.y() / HALF_MAX_TEX_COORD) - 1 )
269 * HALF_MAX_TEX_COORD );
271 tmin.sety( ( (int)(tmin.y() / HALF_MAX_TEX_COORD) )
272 * HALF_MAX_TEX_COORD );
275 // structure is small enough ... we can mod it so we can
276 // properly scale the texture coordinates later.
277 // cout << "MODDING" << endl;
278 double x1 = fmod(tmin.x(), MAX_TEX_COORD);
279 while ( x1 < 0 ) { x1 += MAX_TEX_COORD; }
281 double y1 = fmod(tmin.y(), MAX_TEX_COORD);
282 while ( y1 < 0 ) { y1 += MAX_TEX_COORD; }
284 double x2 = fmod(tmax.x(), MAX_TEX_COORD);
285 while ( x2 < 0 ) { x2 += MAX_TEX_COORD; }
287 double y2 = fmod(tmax.y(), MAX_TEX_COORD);
288 while ( y2 < 0 ) { y2 += MAX_TEX_COORD; }
290 // At this point we know that the object is < 16 wide in
291 // texture coordinate space. If the modulo of the tmin is >
292 // the mod of the tmax at this point, then we know that the
293 // starting tex coordinate for the tmax > 16 so we can shift
294 // everything down by 16 and get it within the 0-32 range.
297 mod_shift.setx( HALF_MAX_TEX_COORD );
299 mod_shift.setx( 0.0 );
303 mod_shift.sety( HALF_MAX_TEX_COORD );
305 mod_shift.sety( 0.0 );
308 // cout << "mod_shift = " << mod_shift << endl;
315 for ( i = 0; i < (int)fan.size(); ++i ) {
316 p = geod_nodes[ fan[i] ];
317 t = basic_tex_coord( p, degree_width, degree_height, scale );
318 // cout << "second t = " << t << endl;
320 adjusted_t = t - tmin;
323 adjusted_t.setx( fmod(t.x() + mod_shift.x(), MAX_TEX_COORD) );
324 while ( adjusted_t.x() < 0 ) {
325 adjusted_t.setx( adjusted_t.x() + MAX_TEX_COORD );
327 adjusted_t.sety( fmod(t.y() + mod_shift.y(), MAX_TEX_COORD) );
328 while ( adjusted_t.y() < 0 ) {
329 adjusted_t.sety( adjusted_t.y() + MAX_TEX_COORD );
331 // cout << "adjusted_t " << adjusted_t << endl;
334 if ( adjusted_t.x() < SG_EPSILON ) {
335 adjusted_t.setx( 0.0 );
337 if ( adjusted_t.y() < SG_EPSILON ) {
338 adjusted_t.sety( 0.0 );
340 adjusted_t.setz( 0.0 );
341 // cout << "adjusted_t = " << adjusted_t << endl;
343 tex.push_back( adjusted_t );