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Help! Tangent Calculation

Started by JasonGillespi Mar 19, 2009 at 1:08 PM 0 replies 800+ views

Original Post

Mar 19, 2009 at 1:08 PM

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. // 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->polys->verts (float verts[4][9]) - variable sized (xyz s1t1 s2t2) // Additional Variables of Interest: // s->texinfo->vecs (float vecs[2][4]) - [s/t][xyz offset] // s->plane->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 :) [Edited by - JasonGillespi on March 19, 2009 9:15:50 PM]

JasonGillespi

Mar 20, 2009 at 1:21 PM

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) {	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-v0;		vec1[1] = s1-s0;		vec1[2] = t1-t0;		vec2[0] = v2-v0;		vec2[1] = s2-s0;		vec2[2] = t2-t0;		VectorNormalize(vec1);		VectorNormalize(vec2);		CrossProduct(vec1,vec2,planes);	}	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 functionfor ( ; 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 ();}

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