if (normalisedZ < 0){ *targetU += PI;} if (normalisedZ < 0){ *targetU = PI - *targetU;} 
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[Request for help] Texture mapping a subdivided icosahedron
514
September 27, 2002 10:35 AM
And maybe you should replace :
with:
1,410
September 27, 2002 10:47 AM
vincoof:
I don't think so. The equations to that point should leave the values in the range 0 -> PI/2 */* -PI/2 -> PI/2 */* -PI/2 -> 0 where */* indicates that value change at the functions tangents. By then adding PI to the middle section you get a distribution of 0 -> PI/2 */* PI/2 -> 3PI/2 */* -PI/2 -> 0. Then adding 2PI if the value is < 0 maps the distribution to 0 -> PI/2 */* PI/2 -> 3PI/2 */* 3PI/2 -> 2PI.
Your proposed modification would leave the distribution as 0 -> PI/2 */* 3PI/2 -> PI/2 */* 3PI/2 -> 0, reversing the texture on one side of the planet and adding extra seams either side.
At least I *think* that's what would happen!
Enigma
EDIT: spelling
[edited by - enigma on September 27, 2002 12:13:16 PM]
I don't think so. The equations to that point should leave the values in the range 0 -> PI/2 */* -PI/2 -> PI/2 */* -PI/2 -> 0 where */* indicates that value change at the functions tangents. By then adding PI to the middle section you get a distribution of 0 -> PI/2 */* PI/2 -> 3PI/2 */* -PI/2 -> 0. Then adding 2PI if the value is < 0 maps the distribution to 0 -> PI/2 */* PI/2 -> 3PI/2 */* 3PI/2 -> 2PI.
Your proposed modification would leave the distribution as 0 -> PI/2 */* 3PI/2 -> PI/2 */* 3PI/2 -> 0, reversing the texture on one side of the planet and adding extra seams either side.
At least I *think* that's what would happen!
Enigma
EDIT: spelling
[edited by - enigma on September 27, 2002 12:13:16 PM]
514
September 27, 2002 11:15 AM
Aw sorry I made a mistake. Thanks for poiiting it out. It was too hurried.
I meant :
I meant :
if (normalisedX < 0){ *targetU = PI - *targetU;} September 27, 2002 05:44 PM
Almost!
Now I have (with Enigma''s function):
Vincoof - I couldn''t quite see where you meant to put that line - doing this gives me more bizarreness:
So the only problem that remains is correcting those two seams; would it make it easier if I went for square textures instead? As I realised that that''s going to be what I need to do eventually anyway.... (DOH)
If I followed your explanation correcting I''m suspecting that all I need do to change to square textures is change the last line from
to
I tried to fiddle Enigma''s function so that the seams were corrected, but got nowhere
Thanks again for your continued help guys.
Brgds
I.
Now I have (with Enigma''s function):
Vincoof - I couldn''t quite see where you meant to put that line - doing this gives me more bizarreness:
float normalisedX = 0; float normalisedZ = -1; if (((normal->x * normal->x) + (normal->z * normal->z)) > 0){ normalisedX = sqrt((normal->x * normal->x) / ((normal->x * normal->x) + (normal->z * normal->z))); if (normal->x < 0){ normalisedX = -normalisedX; } normalisedZ = sqrt((normal->z * normal->z) / ((normal->x * normal->x) + (normal->z * normal->z))); if (normal->z < 0){ normalisedZ = -normalisedZ; } } if (normalisedZ == 0){ *targetU = ((normalisedX * ASE_PI) / 2); } else if (normalisedX < 0){ *targetU = ASE_PI - *targetU; } else { *targetU = atan(normalisedX / normalisedZ); if (normalisedZ < 0){ *targetU += ASE_PI; //*targetU = ASE_PI - *targetU; } if (*targetU < 0){ *targetU += 2 * ASE_PI; } } *targetU /= 2 * ASE_PI; *targetV = (-normal->y + 1) / 2; So the only problem that remains is correcting those two seams; would it make it easier if I went for square textures instead? As I realised that that''s going to be what I need to do eventually anyway.... (DOH)
If I followed your explanation correcting I''m suspecting that all I need do to change to square textures is change the last line from
*targetV = (-normal->y + 1) / 2; *targetV = -normal->y / 2; I tried to fiddle Enigma''s function so that the seams were corrected, but got nowhere
Thanks again for your continued help guys.
Brgds
I.
[size="2"]www.coldcity.com code, art, life
1,410
September 27, 2002 06:21 PM
I''m not sure what you mean by ''square textures''. In what way is your current texture not square? (I realize the image is a rectangle, but assumed that you would have stretched it to a square texture).
The first seam (zigzagging line) exists because the texture coordinates around the planet are distributed as follows: The first X coordinate is zero or a small positive fraction. The X coordinates then increase linearly around the planet until we reach the point we started. Unfortunately the point we started has a texture coordinate which is about zero so we get a steep drop in coordinates here. What we wanted was to wrap around from one edge of the image to the other, but OpenGL doesn''t know this and instead draws nearly the entire texture ''backwards''. The solution is to spot the seam and give the initial vertex different texture coords according to which polygon it is part of. For the first polygon you give it the normal X coordinate. For the final polygon you give it the X coordinate + 1.
Without going through your source codes more thoroughly than I''ve had time to thus far, I can''t say how to implement this in you class.
As for the second seam - It shouldn''t be there! something has gone wrong! I''ll have a look at it and hopefully I''ll have an answer sometime tomorrow.
Enigma
The first seam (zigzagging line) exists because the texture coordinates around the planet are distributed as follows: The first X coordinate is zero or a small positive fraction. The X coordinates then increase linearly around the planet until we reach the point we started. Unfortunately the point we started has a texture coordinate which is about zero so we get a steep drop in coordinates here. What we wanted was to wrap around from one edge of the image to the other, but OpenGL doesn''t know this and instead draws nearly the entire texture ''backwards''. The solution is to spot the seam and give the initial vertex different texture coords according to which polygon it is part of. For the first polygon you give it the normal X coordinate. For the final polygon you give it the X coordinate + 1.
Without going through your source codes more thoroughly than I''ve had time to thus far, I can''t say how to implement this in you class.
As for the second seam - It shouldn''t be there! something has gone wrong! I''ll have a look at it and hopefully I''ll have an answer sometime tomorrow.
Enigma
514
September 27, 2002 06:28 PM
My modification should be used with Enigma''s previous method.
Anyhow I''m glad that the current method is correct (well, almost).
Now that the seams are here, it''s probably the most difficult part. To be honest, I don''t know a "simple" method. The only one I would dare using is detecting the polygons that are responsible of the seams and apply specific texture coordinates on them.
Anyhow I''m glad that the current method is correct (well, almost).
Now that the seams are here, it''s probably the most difficult part. To be honest, I don''t know a "simple" method. The only one I would dare using is detecting the polygons that are responsible of the seams and apply specific texture coordinates on them.
169
September 27, 2002 06:32 PM
That looks so dang cool 
Post a pic of the final product when it lines up the way you like it!
[edited by - Waverider on September 27, 2002 7:32:50 PM]
Post a pic of the final product when it lines up the way you like it!
[edited by - Waverider on September 27, 2002 7:32:50 PM]
It's not what you're taught, it's what you learn.
September 27, 2002 07:16 PM
Enigma: Please ignore my "square texture" tangent, I''m totally confusing the issue and inventing problems where there are none. The reason for the zigzag seam is obvious (now you''ve pointed it out!) - I''ll work on fixing that.
Waverider: I will I think it looks pretty cool in its messed-up incarnations 
There is a point to all this, incidentally - eventually this thing won''t even be texturemapped, but to do what I want to I need a way of reliably getting UVs onto a two-dimensional grid, so perfecting it with something as visual as a texture seemed the way to go.
From that you''ve probably figured out I''m working on procedural planets and the 2D grid is actually a noise function that returns the same values when given the same inputs (so no storage of the noise required).
The Icosahedron class below is a simplification of what I''m actually using, which already includes LOD, but the tex coord generation still applies.
Thanks again all,
I.
Waverider: I will
I think it looks pretty cool in its messed-up incarnations There is a point to all this, incidentally - eventually this thing won''t even be texturemapped, but to do what I want to I need a way of reliably getting UVs onto a two-dimensional grid, so perfecting it with something as visual as a texture seemed the way to go.
From that you''ve probably figured out I''m working on procedural planets and the 2D grid is actually a noise function that returns the same values when given the same inputs (so no storage of the noise required).
The Icosahedron class below is a simplification of what I''m actually using, which already includes LOD, but the tex coord generation still applies.
Thanks again all,
I.
[size="2"]www.coldcity.com code, art, life
1,410
September 27, 2002 07:24 PM
Well, here''s a (pretty horrible) hack that''ll fix the seams:
It could be optimised, but since it''s such a hack there''s no point. The value 0.2 is just a value < 1 that is guarenteed to be greater that the difference between any two correct texture coordinates.
It does work - I''ve just spent the last hour hacking various bits of old code together to get a simple OpenGL app to display it. It fixes both problems - I still don''t know what caused the second one but nevermind!
Enigma
void Icosahedron::DrawTriangle(float *v1, float *v2, float *v3) { Vector3f point; GLfloat texU, texV; GLfloat texU1, texU2, texU3; GLfloat texAdd1 = 0; GLfloat texAdd2 = 0; GLfloat texAdd3 = 0; // determine if triangle spans point.Set (v1[0], v1[1], v1[2]); GetTextureCoord (&point, &texU1, &texV); point.Set (v2[0], v2[1], v2[2]); GetTextureCoord (&point, &texU2, &texV); point.Set (v3[0], v3[1], v3[2]); GetTextureCoord (&point, &texU3, &texV); if (texU2 - texU1 > 0.2 || texU3 - texU1 > 0.2){ texAdd1 = 1; } if (texU1 - texU2 > 0.2 || texU3 - texU2 > 0.2){ texAdd2 = 1; } if (texU1 - texU3 > 0.2 || texU2 - texU3 > 0.2){ texAdd3 = 1; } glBegin(GL_TRIANGLES); point.Set (v1[0], v1[1], v1[2]); glNormal3f (point.x, point.y, point.z); // (Points are already normalised) GetTextureCoord (&point, &texU, &texV); glTexCoord2f(texU + texAdd1, texV); glVertex3f (point.x, point.y, point.z); point.Set (v2[0], v2[1], v2[2]); glNormal3f (point.x, point.y, point.z); GetTextureCoord (&point, &texU, &texV); glTexCoord2f(texU + texAdd2, texV); glVertex3f (point.x, point.y, point.z); point.Set (v3[0], v3[1], v3[2]); glNormal3f (point.x, point.y, point.z); GetTextureCoord (&point, &texU, &texV); glTexCoord2f(texU + texAdd3, texV); glVertex3f (point.x, point.y, point.z); glEnd();} It could be optimised, but since it''s such a hack there''s no point. The value 0.2 is just a value < 1 that is guarenteed to be greater that the difference between any two correct texture coordinates.
It does work - I''ve just spent the last hour hacking various bits of old code together to get a simple OpenGL app to display it. It fixes both problems - I still don''t know what caused the second one but nevermind!
Enigma
1,410
September 28, 2002 07:14 AM
Just spotted what was causing the second seam. In the GetTextureCoord method, this:
should have been:
The first if will set targetU to +/- PI/2. The -PI/2 should then have been transformed to 3PI/2 by adding 2PI, but I put the code to add 2PI inside the else clause so for this one case it didn''t get executed.
Enigma
if (normalisedZ == 0){ *targetU = ((normalisedX * PI) / 2);}else { *targetU = atan(normalisedX / normalisedZ); if (normalisedZ < 0){ *targetU += PI; } if (*targetU < 0){ *targetU += 2 * PI; }} should have been:
if (normalisedZ == 0){ *targetU = ((normalisedX * PI) / 2);}else { *targetU = atan(normalisedX / normalisedZ); if (normalisedZ < 0){ *targetU += PI; }}if (*targetU < 0){ *targetU += 2 * PI;} The first if will set targetU to +/- PI/2. The -PI/2 should then have been transformed to 3PI/2 by adding 2PI, but I put the code to add 2PI inside the else clause so for this one case it didn''t get executed.
Enigma
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