359 lines
11 KiB
C++
359 lines
11 KiB
C++
// texcoord.hxx -- routine(s) to handle texture coordinate generation
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//
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// Written by Curtis Olson, started March 1999.
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//
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// Copyright (C) 1999 Curtis L. Olson - http://www.flightgear.org/~curt
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//
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// This library is free software; you can redistribute it and/or
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// modify it under the terms of the GNU Library General Public
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// License as published by the Free Software Foundation; either
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// version 2 of the License, or (at your option) any later version.
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//
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// This library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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// Library General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program; if not, write to the Free Software
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// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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//
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// $Id$
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/* The following is an explanation of our somewhat conveluted and
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tricky texture scaling/offset scheme:
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MAX_TEX_COORD is a value I arrived at by trial and error for my
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voodoo2/3 video card. If you use texture coordinates that are too
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big, you quickly start getting into round off problems and the texture
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jumps and moves relative to the polygon.
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The point of all of this code is that I wanted to be able to define
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this size in meters of a texture and have it be applied seamlessly to
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the terrain. I wanted to be able to change the defined size (in
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meters) of textures at run time. In other words I want to be able to
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scale the textures at run time and still have them seamlessly tile
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together across fans.
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The problem is that I have to pregenerate all the texture coordinates
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when I create the scenery, and I didn't want to burn CPU doing this
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again when I load the scenery at run time.
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It ended up taking me a lot of thought, a lot of trial and error, and
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a lot of fiddling around to come up with a scheme that worked.
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----------
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Ok, so think about what needs to be done to have the texture tile
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across a series of triangles and fans ...
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Basically you want to use some function of lon/lat mod your max
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texture coordinate size to calculate the texture coordinate of each
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vertex. This should result in nice tiling across distinct triangles
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and fans.
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Pretend our MAX_TEX_COORD = 4.0 and half of this is 2.0
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Imagine the following two adjacent polygons with the "X" component of
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the initial texture coordinate based on longitude (Note they are drawn
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spaced apart, but in reality the two polygons are adjacent):
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7.0 8.6 8.6 9.0
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*-----* *------*
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| | | |
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Now, this exceeds our MAX_TEX_COORD of 4.0 so we have to scale these
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texture coordinates by some integer value. Let's say we always want
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to minimize the tex coordinates to minimize rounding error so we will
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offset the first polygon by 7.0 and the second by 8.0:
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0.0 --- 1.6 and 0.6 --- 1.0
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Our tiling is maintianed becuase the coordinates are continous (mod
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1.0) and we still get the double repeat across both polygons.
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We want to be able to scale these values by an arbitrary constant and
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still have proper tiling.
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Let's try doubling the coordinates:
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0.0 --- 3.2 and 1.2 --- 2.0
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Everything still tiles nicely (because the coordinates are continuous
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mod 1.0) and the texture is now repeated 4x across the two polygons.
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Before it was repeated 2x.
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Let's try halving the coordinates:
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0.0 --- 0.8 and 0.3 --- 0.5
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Ooop! We lost continuity in texture coordinate space ... no we will
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have a visual discontinuity in the texture tiling!
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Ok, so we need some other scheme to keep our texture coordinates
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smaller than MAX_TEX_COORD that preserves continuity in texture
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space. <Deep breath> let's try the scheme that I have coded up that
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you are asking about ... <fingers crossed> :-)
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Going way back to the top before we shifted the texture coordinates.
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tmin for the first polygon is 7.0, this is then adjusted to:
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(int)(tmin.x() / HALF_MAX_TEX_COORD) ) * HALF_MAX_TEX_COORD
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= (int)(7.0/2.0) * 2.0 = 3.0 * 2.0 = 6.0
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The two texture coordinates are offset by 6.0 which yields 1.0 -- 2.6
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tmin for the second polygon is 8.6 which is adjusted to:
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(int)(tmin.x() / HALF_MAX_TEX_COORD) ) * HALF_MAX_TEX_COORD
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= (int)( 8.6 / 2.0 ) * 2.0 = 4.0 * 2.0 = 8.0
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The offset for the second polygon is 8.0 which yields 0.6 --- 1.0
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So now we have:
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1.0 --- 2.6 and 0.6 --- 1.0
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This still tiles nicely and strethes our texture across completely, so
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far we haven't done any damage.
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Now let's double the coordinates:
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2.0 --- 5.2 and 1.2 --- 2.0
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The texture is repeated 4x as it should be and is still continuous.
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How about halfing the coordinates. This is where the first scheme
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broke down. Halving the coordinates yields
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0.5 --- 1.3 and 0.3 --- 0.5
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Woohoo, we still have texture space continuity (mod 1.0) and the
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texture is repeated 1x.
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Note, it took me almost as long to re-figure this out and write this
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explanation as it did to figure out the scheme originally. I better
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enter this in the official comments in case I forget again. :-)
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*/
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#ifdef HAVE_CONFIG_H
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# include <simgear_config.h>
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#endif
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#include <simgear/compiler.h>
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// #include <iostream>
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#include "texcoord.hxx"
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// using std::cout;
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// using std::endl;
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#define FG_STANDARD_TEXTURE_DIMENSION 1000.0 // meters
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#define MAX_TEX_COORD 8.0
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#define HALF_MAX_TEX_COORD ( MAX_TEX_COORD * 0.5 )
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// return the basic unshifted/unmoded texture coordinate for a lat/lon
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static inline SGVec2f basic_tex_coord( const SGGeod& p,
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double degree_width,
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double degree_height,
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double scale )
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{
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return SGVec2f( p.getLongitudeDeg() * ( degree_width * scale /
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FG_STANDARD_TEXTURE_DIMENSION ),
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p.getLatitudeDeg() * ( degree_height * scale /
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FG_STANDARD_TEXTURE_DIMENSION )
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);
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}
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// traverse the specified fan/strip/list of vertices and attempt to
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// calculate "none stretching" texture coordinates
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std::vector<SGVec2f> sgCalcTexCoords( const SGBucket& b, const std::vector<SGGeod>& geod_nodes,
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const int_list& fan, double scale )
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{
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return sgCalcTexCoords(b.get_center_lat(), geod_nodes, fan, scale);
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}
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std::vector<SGVec2f> sgCalcTexCoords( double centerLat, const std::vector<SGGeod>& geod_nodes,
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const int_list& fan, double scale )
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{
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// cout << "calculating texture coordinates for a specific fan of size = "
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// << fan.size() << endl;
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// calculate perimeter based on center of this degree (not center
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// of bucket)
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double clat = (int)centerLat;
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if ( clat > 0 ) {
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clat = (int)clat + 0.5;
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} else {
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clat = (int)clat - 0.5;
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}
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double clat_rad = clat * SGD_DEGREES_TO_RADIANS;
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double cos_lat = cos( clat_rad );
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double local_radius = cos_lat * SG_EQUATORIAL_RADIUS_M;
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double local_perimeter = local_radius * SGD_2PI;
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double degree_width = local_perimeter / 360.0;
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// cout << "clat = " << clat << endl;
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// cout << "clat (radians) = " << clat_rad << endl;
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// cout << "cos(lat) = " << cos_lat << endl;
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// cout << "local_radius = " << local_radius << endl;
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// cout << "local_perimeter = " << local_perimeter << endl;
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// cout << "degree_width = " << degree_width << endl;
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double perimeter = SG_EQUATORIAL_RADIUS_M * SGD_2PI;
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double degree_height = perimeter / 360.0;
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// cout << "degree_height = " << degree_height << endl;
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// find min/max of fan
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SGVec2f tmin(0.0, 0.0);
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SGVec2f tmax(0.0, 0.0);
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bool first = true;
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int i;
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for ( i = 0; i < (int)fan.size(); ++i ) {
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SGGeod p = geod_nodes[ fan[i] ];
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// cout << "point p = " << p << endl;
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SGVec2f t = basic_tex_coord( p, degree_width, degree_height, scale );
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// cout << "basic_tex_coord = " << t << endl;
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if ( first ) {
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tmin = tmax = t;
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first = false;
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} else {
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if ( t.x() < tmin.x() ) {
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tmin.x() = t.x();
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}
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if ( t.y() < tmin.y() ) {
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tmin.y() = t.y();
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}
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if ( t.x() > tmax.x() ) {
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tmax.x() = t.x();
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}
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if ( t.y() > tmax.y() ) {
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tmax.y() = t.y();
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}
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}
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}
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double dx = fabs( tmax.x() - tmin.x() );
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double dy = fabs( tmax.y() - tmin.y() );
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// cout << "dx = " << dx << " dy = " << dy << endl;
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// Point3D mod_shift;
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if ( (dx > HALF_MAX_TEX_COORD) || (dy > HALF_MAX_TEX_COORD) ) {
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// structure is too big, we'll just have to shift it so that
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// tmin = (0,0). This messes up subsequent texture scaling,
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// but is the best we can do.
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// cout << "SHIFTING" << endl;
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if ( tmin.x() < 0 ) {
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tmin.x() = (double)( (int)tmin.x() - 1 ) ;
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} else {
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tmin.x() = (int)tmin.x();
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}
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if ( tmin.y() < 0 ) {
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tmin.y() = (double)( (int)tmin.y() - 1 );
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} else {
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tmin.y() = (int)tmin.y();
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}
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// cout << "found tmin = " << tmin << endl;
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} else {
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if ( tmin.x() < 0 ) {
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tmin.x() = ( (int)(tmin.x() / HALF_MAX_TEX_COORD) - 1 )
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* HALF_MAX_TEX_COORD ;
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} else {
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tmin.x() = ( (int)(tmin.x() / HALF_MAX_TEX_COORD) )
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* HALF_MAX_TEX_COORD ;
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}
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if ( tmin.y() < 0 ) {
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tmin.y() = ( (int)(tmin.y() / HALF_MAX_TEX_COORD) - 1 )
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* HALF_MAX_TEX_COORD ;
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} else {
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tmin.y() = ( (int)(tmin.y() / HALF_MAX_TEX_COORD) )
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* HALF_MAX_TEX_COORD ;
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}
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#if 0
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// structure is small enough ... we can mod it so we can
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// properly scale the texture coordinates later.
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// cout << "MODDING" << endl;
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double x1 = fmod(tmin.x(), MAX_TEX_COORD);
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while ( x1 < 0 ) { x1 += MAX_TEX_COORD; }
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double y1 = fmod(tmin.y(), MAX_TEX_COORD);
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while ( y1 < 0 ) { y1 += MAX_TEX_COORD; }
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double x2 = fmod(tmax.x(), MAX_TEX_COORD);
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while ( x2 < 0 ) { x2 += MAX_TEX_COORD; }
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double y2 = fmod(tmax.y(), MAX_TEX_COORD);
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while ( y2 < 0 ) { y2 += MAX_TEX_COORD; }
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// At this point we know that the object is < 16 wide in
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// texture coordinate space. If the modulo of the tmin is >
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// the mod of the tmax at this point, then we know that the
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// starting tex coordinate for the tmax > 16 so we can shift
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// everything down by 16 and get it within the 0-32 range.
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if ( x1 > x2 ) {
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mod_shift.setx( HALF_MAX_TEX_COORD );
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} else {
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mod_shift.setx( 0.0 );
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}
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if ( y1 > y2 ) {
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mod_shift.sety( HALF_MAX_TEX_COORD );
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} else {
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mod_shift.sety( 0.0 );
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}
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#endif
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// cout << "mod_shift = " << mod_shift << endl;
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}
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// generate tex_list
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SGVec2f adjusted_t;
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std::vector<SGVec2f> tex;
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tex.clear();
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for ( i = 0; i < (int)fan.size(); ++i ) {
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SGGeod p = geod_nodes[ fan[i] ];
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SGVec2f t = basic_tex_coord( p, degree_width, degree_height, scale );
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// cout << "second t = " << t << endl;
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adjusted_t = t - tmin;
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#if 0
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} else {
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adjusted_t.setx( fmod(t.x() + mod_shift.x(), MAX_TEX_COORD) );
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while ( adjusted_t.x() < 0 ) {
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adjusted_t.setx( adjusted_t.x() + MAX_TEX_COORD );
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}
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adjusted_t.sety( fmod(t.y() + mod_shift.y(), MAX_TEX_COORD) );
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while ( adjusted_t.y() < 0 ) {
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adjusted_t.sety( adjusted_t.y() + MAX_TEX_COORD );
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}
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// cout << "adjusted_t " << adjusted_t << endl;
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}
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#endif
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if ( adjusted_t.x() < SG_EPSILON ) {
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adjusted_t.x() = 0.0;
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}
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if ( adjusted_t.y() < SG_EPSILON ) {
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adjusted_t.y() = 0.0;
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}
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// cout << "adjusted_t = " << adjusted_t << endl;
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tex.push_back( adjusted_t );
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}
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return tex;
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}
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