# Tangent Calculation

Hi, I’m having trouble figuring out the tangent vector for bsp code in the project I’m working in. I’m trying to rewrite my Simple Tangent Calculation using the following verts. I need help on how to write a CalculateTangent function with this vert array to get a tangent vector. I’m only 17 so please be patient with me

``````		// Simple Tangent Calculation Example
// v[0...2] vertex position
// t[0...2] texcoords
// tan.xyz = (v1-v0).xyz * (t2-t0).y - (v2-v0).xyz * (t1-t0).y;

// Write above into a CalculateTangent() func using
// Variables described below:

// #define VERTEXSIZE 9 //xyz s1t1 s2t2 s3t3
// where xyz = vert coords; s1t1 = normal tex coords;
// s2t2 = lightmap tex coords; s3t2 = detail tex coords

// s-&gt;polys-&gt;verts (float verts[4][9]) - variable sized (xyz s1t1 s2t2)

// s-&gt;texinfo-&gt;vecs (float vecs[2][4]) - [s/t][xyz offset]
// s-&gt;plane-&gt;normal (float normal[3]) - plane normal

// Pass the Tangent to the Vertex Shader using an attribute 3fv:
// attribute3fv("tangent", tan);
``````

// BSP Render Code:

``````
for ( ; s; s = s->texturechain) {
v = s->polys->verts[0];
glBegin(GL_POLYGON);
for (k = 0; k < s->polys->numverts; k++, v += VERTEXSIZE) {
glMultiTexCoord2fARB (GL_TEXTURE0_ARB, v[3], v[4]);

if (s->flags & SURF_PLANEBACK)
glNormal3fv(-s->plane->normal);
else
glNormal3fv(s->plane->normal);

glVertex3fv (v);
}
glEnd ();
}

``````

Any help is greatly appreciated

This is the best I could come up with so far. Part of the problem I see with this method is there is often more than 3 verts per polygon of which I’m not sure how I account for. Can anyone give me some direction or let me know if I’m on the right track please.

``````
void TangentForPoly(msurface_t *s) {

//vec3_t stvecs [3];
float *v;
float *v0, *v1, *v2;
float s0, s1, s2;
float t0, t1, t2;
vec3_t vec1, vec2;
vec3_t planes[3];
int i;

v = s->polys->verts[0];

v0 = v;
v1 = v+VERTEXSIZE;
v2 = v+(VERTEXSIZE*2);
s0 = v[3];
t0 = v[4];
s1 = v1[3];
t1 = v1[4];
s2 = v2[3];
t2 = v2[4];

for (i=0; i<3; i++) {
vec1[0] = v1[i]-v0[i];
vec1[1] = s1-s0;
vec1[2] = t1-t0;
vec2[0] = v2[i]-v0[i];
vec2[1] = s2-s0;
vec2[2] = t2-t0;
VectorNormalize(vec1);
VectorNormalize(vec2);
CrossProduct(vec1,vec2,planes[i]);
}

s->tangent[0] = -planes[0][1]/planes[0][0];
s->tangent[1] = -planes[1][1]/planes[1][0];
s->tangent[2] = -planes[2][1]/planes[2][0];
s->binormal[0] = -planes[0][2]/planes[0][0];
s->binormal[1] = -planes[1][2]/planes[1][0];
s->binormal[2] = -planes[2][2]/planes[2][0];
VectorNormalize(s->tangent); //is this needed?
VectorNormalize(s->binormal);
}
// inside draw bsp function
for ( ; s; s = s->texturechain) {

v = s->polys->verts[0];

// Calculate Tangent & Binormal Space for polygon
TangentForPoly(s);

glBegin(GL_POLYGON);
for (k = 0; k < s->polys->numverts; k++, v += VERTEXSIZE) {
glMultiTexCoord2fARB (GL_TEXTURE0_ARB, v[3], v[4]);

if (s->flags & SURF_PLANEBACK)
glNormal3fv(-s->plane->normal);
else
glNormal3fv(s->plane->normal);

glVertex3fv (v);
}
glEnd ();
}

``````