问题
I'm trying to implement a ray caster and I'm starting out with simple .obj files (utah-teapot) and currently I only made classes for Spheres and Triangles, I basically have all the functions for the intersections, generating view rays, etc.. all ready but I just can't seem to be able to parse the .obj file into triangles (three vectors each) so I can have the ray casting possible on custom .obj files instead of just spheres.
This is my current .obj file parser (didn't include the full working code here)
char lineHeader[512];
// read the first word of the line
int res = fscanf(file, "%s", lineHeader);
if (res == EOF)
break; // EOF
// else : parse lineHeader
if (strcmp(lineHeader, "v") == 0) {
glm::vec3 vertex;
fscanf(file, "%f %f %f\n", &vertex.x, &vertex.y, &vertex.z);
vertex.x *= scale;
vertex.y *= scale;
vertex.z *= scale;
temp_vertices.push_back(vertex);
}
else if (strcmp(lineHeader, "vt") == 0) {
glm::vec2 uv;
fscanf(file, "%f %f\n", &uv.x, &uv.y);
uv.y = -uv.y; // Invert V coordinate since we will only use DDS texture, which are inverted. Remove if you want to use TGA or BMP loaders.
temp_uvs.push_back(uv);
}
else if (strcmp(lineHeader, "vn") == 0) {
glm::vec3 normal;
fscanf(file, "%f %f %f\n", &normal.x, &normal.y, &normal.z);
temp_normals.push_back(normal);
}
else if (strcmp(lineHeader, "f") == 0) {
std::string vertex1, vertex2, vertex3;
unsigned int vertexIndex[3] = { 0 }, uvIndex[3] = { 0 }, normalIndex[3] = { 0 };
char stupidBuffer[1024];
fgets(stupidBuffer, 1024, file);
int matches = sscanf(stupidBuffer, "%d/%d/%d %d/%d/%d %d/%d/%d\n", &vertexIndex[0], &uvIndex[0], &normalIndex[0], &vertexIndex[1], &uvIndex[1], &normalIndex[1], &vertexIndex[2], &uvIndex[2], &normalIndex[2]);
if (matches != 9) {
vertexIndex[3] = { 0 }, uvIndex[3] = { 0 }, normalIndex[3] = { 0 };
matches = sscanf(stupidBuffer, "%d//%d %d//%d %d//%d\n", &vertexIndex[0], &normalIndex[0], &vertexIndex[1], &normalIndex[1], &vertexIndex[2], &normalIndex[2]);
if (matches != 6) {
vertexIndex[3] = { 0 }, uvIndex[3] = { 0 }, normalIndex[3] = { 0 };
matches = sscanf(stupidBuffer, "%d %d %d\n", &vertexIndex[0], &vertexIndex[1], &vertexIndex[2]);
if (matches != 3) {
printf("File can't be read \n");
fclose(file);
return false;
}
}
}
}
This is my triangle class
class Triangle {
public:
Vector p0, p1, p2;
Vector color;
Vector normal(void);
};
I can't figure out how to parse the info from the .obj file into triangles that consist of three 3d vectors (points). I don't need code, I just need to understand how (if possible?) to parse all that info into triangles. Any other ideas are welcome. I want to make a simple puzzle game on the long run but I'm just taking it a step at a time.
回答1:
You're 90% of the way there. In your face element parser use the parsed position/normal/texcoord indexes of each face-vertex to grab info from the temp_* vectors. If a face element has three vertices you can emit a triangle as-is, otherwise for 4+ vertices I've generally assumed the the resulting polygon is convex & co-planar, in which case you can triangulate by pretending it's a triangle fan.
All together:
struct Vertex
{
glm::vec3 position;
glm::vec2 texcoord;
glm::vec3 normal;
};
struct VertRef
{
VertRef( int v, int vt, int vn ) : v(v), vt(vt), vn(vn) { }
int v, vt, vn;
};
std::vector< Vertex > LoadOBJ( std::istream& in )
{
std::vector< Vertex > verts;
std::vector< glm::vec4 > positions( 1, glm::vec4( 0, 0, 0, 0 ) );
std::vector< glm::vec3 > texcoords( 1, glm::vec3( 0, 0, 0 ) );
std::vector< glm::vec3 > normals( 1, glm::vec3( 0, 0, 0 ) );
std::string lineStr;
while( std::getline( in, lineStr ) )
{
std::istringstream lineSS( lineStr );
std::string lineType;
lineSS >> lineType;
// vertex
if( lineType == "v" )
{
float x = 0, y = 0, z = 0, w = 1;
lineSS >> x >> y >> z >> w;
positions.push_back( glm::vec4( x, y, z, w ) );
}
// texture
if( lineType == "vt" )
{
float u = 0, v = 0, w = 0;
lineSS >> u >> v >> w;
texcoords.push_back( glm::vec3( u, v, w ) );
}
// normal
if( lineType == "vn" )
{
float i = 0, j = 0, k = 0;
lineSS >> i >> j >> k;
normals.push_back( glm::normalize( glm::vec3( i, j, k ) ) );
}
// polygon
if( lineType == "f" )
{
std::vector< VertRef > refs;
std::string refStr;
while( lineSS >> refStr )
{
std::istringstream ref( refStr );
std::string vStr, vtStr, vnStr;
std::getline( ref, vStr, '/' );
std::getline( ref, vtStr, '/' );
std::getline( ref, vnStr, '/' );
int v = atoi( vStr.c_str() );
int vt = atoi( vtStr.c_str() );
int vn = atoi( vnStr.c_str() );
v = ( v >= 0 ? v : positions.size() + v );
vt = ( vt >= 0 ? vt : texcoords.size() + vt );
vn = ( vn >= 0 ? vn : normals.size() + vn );
refs.push_back( VertRef( v, vt, vn ) );
}
// triangulate, assuming n>3-gons are convex and coplanar
for( size_t i = 1; i+1 < refs.size(); ++i )
{
const VertRef* p[3] = { &refs[0], &refs[i], &refs[i+1] };
// http://www.opengl.org/wiki/Calculating_a_Surface_Normal
glm::vec3 U( positions[ p[1]->v ] - positions[ p[0]->v ] );
glm::vec3 V( positions[ p[2]->v ] - positions[ p[0]->v ] );
glm::vec3 faceNormal = glm::normalize( glm::cross( U, V ) );
for( size_t j = 0; j < 3; ++j )
{
Vertex vert;
vert.position = glm::vec3( positions[ p[j]->v ] );
vert.texcoord = glm::vec2( texcoords[ p[j]->vt ] );
vert.normal = ( p[j]->vn != 0 ? normals[ p[j]->vn ] : faceNormal );
verts.push_back( vert );
}
}
}
}
return verts;
}
See the full program here.
来源：https://stackoverflow.com/questions/52824956/how-can-i-parse-a-simple-obj-file-into-triangles