diff --git a/simgear/misc/texcoord.cxx b/simgear/misc/texcoord.cxx index 804a36b8..52cd16ee 100644 --- a/simgear/misc/texcoord.cxx +++ b/simgear/misc/texcoord.cxx @@ -22,12 +22,130 @@ // $Id$ +/* The following is an explanation of our somewhat conveluted and + tricky texture scaling/offset scheme: + +MAX_TEX_COORD is a value I arrived at by trial and error for my +voodoo2/3 video card. If you use texture coordinates that are too +big, you quickly start getting into round off problems and the texture +jumps and moves relative to the polygon. + +The point of all of this code is that I wanted to be able to define +this size in meters of a texture and have it be applied seamlessly to +the terrain. I wanted to be able to change the defined size (in +meters) of textures at run time. In other words I want to be able to +scale the textures at run time and still have them seamlessly tile +together across fans. + +The problem is that I have to pregenerate all the texture coordinates +when I create the scenery, and I didn't want to burn CPU doing this +again when I load the scenery at run time. + +It ended up taking me a lot of thought, a lot of trial and error, and +a lot of fiddling around to come up with a scheme that worked. + +---------- + +Ok, so think about what needs to be done to have the texture tile +across a series of triangles and fans ... + +Basically you want to use some function of lon/lat mod your max +texture coordinate size to calculate the texture coordinate of each +vertex. This should result in nice tiling across distinct triangles +and fans. + +Pretend our MAX_TEX_COORD = 4.0 and half of this is 2.0 + +Imagine the following two adjacent polygons with the "X" component of +the initial texture coordinate based on longitude (Note they are drawn +spaced apart, but in reality the two polygons are adjacent): + + 7.0 8.6 8.6 9.0 + *-----* *------* + | | | | + +Now, this exceeds our MAX_TEX_COORD of 4.0 so we have to scale these +texture coordinates by some integer value. Let's say we always want +to minimize the tex coordinates to minimize rounding error so we will +offset the first polygon by 7.0 and the second by 8.0: + + 0.0 --- 1.6 and 0.6 --- 1.0 + +Our tiling is maintianed becuase the coordinates are continous (mod +1.0) and we still get the double repeat across both polygons. + +We want to be able to scale these values by an arbitrary constant and +still have proper tiling. + +Let's try doubling the coordinates: + + 0.0 --- 3.2 and 1.2 --- 2.0 + +Everything still tiles nicely (because the coordinates are continuous +mod 1.0) and the texture is now repeated 4x across the two polygons. +Before it was repeated 2x. + +Let's try halving the coordinates: + + 0.0 --- 0.8 and 0.3 --- 0.5 + +Ooop! We lost continuity in texture coordinate space ... no we will +have a visual discontinuity in the texture tiling! + +Ok, so we need some other scheme to keep our texture coordinates +smaller than MAX_TEX_COORD that preserves continuity in texture +space. let's try the scheme that I have coded up that +you are asking about ... :-) + +Going way back to the top before we shifted the texture coordinates. +tmin for the first polygon is 7.0, this is then adjusted to: + + (int)(tmin.x() / HALF_MAX_TEX_COORD) ) * HALF_MAX_TEX_COORD + + = (int)(7.0/2.0) * 2.0 = 3.0 * 2.0 = 6.0 + +The two texture coordinates are offset by 6.0 which yields 1.0 -- 2.6 + +tmin for the second polygon is 8.6 which is adjusted to: + + (int)(tmin.x() / HALF_MAX_TEX_COORD) ) * HALF_MAX_TEX_COORD + = (int)( 8.6 / 2.0 ) * 2.0 = 4.0 * 2.0 = 8.0 + +The offset for the second polygon is 8.0 which yields 0.6 --- 1.0 + +So now we have: + + 1.0 --- 2.6 and 0.6 --- 1.0 + +This still tiles nicely and strethes our texture across completely, so +far we haven't done any damage. + +Now let's double the coordinates: + + 2.0 --- 5.2 and 1.2 --- 2.0 + +The texture is repeated 4x as it should be and is still continuous. + +How about halfing the coordinates. This is where the first scheme +broke down. Halving the coordinates yields + + 0.5 --- 1.3 and 0.3 --- 0.5 + +Woohoo, we still have texture space continuity (mod 1.0) and the +texture is repeated 1x. + +Note, it took me almost as long to re-figure this out and write this +explanation as it did to figure out the scheme originally. I better +enter this in the official comments in case I forget again. :-) + +*/ + #include "texcoord.hxx" #define FG_STANDARD_TEXTURE_DIMENSION 1000.0 // meters #define MAX_TEX_COORD 8.0 -#define HALF_MAX_TEX_COORD ( MAX_TEX_COORD / 2.0 ) +#define HALF_MAX_TEX_COORD ( MAX_TEX_COORD * 0.5 ) // return the basic unshifted/unmoded texture coordinate for a lat/lon @@ -113,14 +231,12 @@ point_list calc_tex_coords( const FGBucket& b, const point_list& geod_nodes, double dy = fabs( tmax.y() - tmin.y() ); // cout << "dx = " << dx << " dy = " << dy << endl; - bool do_shift = false; // Point3D mod_shift; if ( (dx > HALF_MAX_TEX_COORD) || (dy > HALF_MAX_TEX_COORD) ) { // structure is too big, we'll just have to shift it so that // tmin = (0,0). This messes up subsequent texture scaling, // but is the best we can do. // cout << "SHIFTING" << endl; - do_shift = true; if ( tmin.x() < 0 ) { tmin.setx( (double)( (int)tmin.x() - 1 ) ); } else { @@ -193,7 +309,6 @@ point_list calc_tex_coords( const FGBucket& b, const point_list& geod_nodes, t = basic_tex_coord( p, degree_width, degree_height, scale ); // cout << "second t = " << t << endl; - // if ( do_shift ) { adjusted_t = t - tmin; #if 0 } else {