Drawing the top and bottom of a cylinder - c++

Question

I'm trying to create a class that can procedurally create prisms (or cylinders if the precision is high enough) but only the sides of the 3d model are showing (not the top and bottom). This is using openGL and c++. Not going for efficiency, just modifying a previous class that made a sphere.
#define numSlices 2
Prism::Prism() {
init(3);
}
Prism::Prism(int prec) {
init(prec);
}
float Prism::toRadians(float degrees) { return (degrees * 2.0f * 3.14159f) / 360.0f; }
void Prism::init(int prec) {
prec = (prec < 3) ? 3 : prec;
numVertices = (prec + 1) * (numSlices+1);
numIndices = prec * numSlices * 6;
for (int i = 0; i < numVertices; i++) { vertices.push_back(glm::vec3()); }
for (int i = 0; i < numVertices; i++) { texCoords.push_back(glm::vec2()); }
for (int i = 0; i < numVertices; i++) { normals.push_back(glm::vec3()); }
for (int i = 0; i < numVertices; i++) { tangents.push_back(glm::vec3()); }
for (int i = 0; i < numIndices; i++) { indices.push_back(0); }
// calculate triangle vertices
for (int i = 0; i <= numSlices; i++) {
for (int j = 0; j <= prec; j++) {
float y = i;
float x = -(float)cos(toRadians(j * 360.0f / (float)prec));
float z = (float)sin(toRadians(j * 360.0f / (float)prec));
vertices[i * (prec + 1) + j] = glm::vec3(x, y, z);
texCoords[i * (prec + 1) + j] = glm::vec2(((float)j / prec), ((float)i / numSlices));
}
}
// calculate triangle indices
for (int i = 0; i < numSlices; i++) {
for (int j = 0; j < prec; j++) {
indices[6 * (i * prec + j) + 0] = i * (prec + 1) + j;
indices[6 * (i * prec + j) + 1] = i * (prec + 1) + j + 1;
indices[6 * (i * prec + j) + 2] = (i + 1) * (prec + 1) + j;
indices[6 * (i * prec + j) + 3] = i * (prec + 1) + j + 1;
indices[6 * (i * prec + j) + 4] = (i + 1) * (prec + 1) + j + 1;
indices[6 * (i * prec + j) + 5] = (i + 1) * (prec + 1) + j;
}
}
}
Any tips or solutions that stick closely to the code already written would much appreciated.

Answer 1

To render the top and bottom of the cylinder, you can create a "triangle fan" that starts from a vertex at the center of the top/bottom of the cylinder and creates one triangle for every side.
Adapting your code: (untested, I may have made mistakes against winding order)
int bottom_center = vertices.length(); vertices.push_back(glm::vec3(0,0,0));
int top_center = vertices.length(); vertices.push_back(glm::vec3(0,numSlices,0));
// Bottom
for (int j = 0; j < prec; j++) {
int base = 0;
indices.push_back(bottom_center);
indices.push_back(base+j);
indices.push_back(base+j+1);
}
// Top
for (int j = 0; j < prec; j++) {
int base = numSlices * (prec+1);
indices.push_back(top_center);
indices.push_back(base+j);
indices.push_back(base+j+1);
}
See http://www.songho.ca/opengl/gl_cylinder.html for a more worked-out example.