C++ 使用 for 循环绘制三角形带 - 顶点、索引

Question

I'm trying to draw a plane using triangle strips. I understand how to do it manually, but I'm really struggling to do it using a for loop. So far, the code below draws two triangles.

//vertices for triangle strip
vertices.push_back(Point3(0,0,0));
vertices.push_back(Point3(1,0,0));
vertices.push_back(Point3(1,1,0));
vertices.push_back(Point3(0,1,0));
vertices.push_back(Point3(-1,1,0));
vertices.push_back(Point3(-1,0,0));


// indices into the arrays above for the first triangle
indices.push_back(0);
indices.push_back(1);
indices.push_back(2);    
// indices for the second triangle
indices.push_back(0);
indices.push_back(2);
indices.push_back(3);    
//indices for the third triangle
indices.push_back(5);
indices.push_back(0);
indices.push_back(3);    
//indices for the fourth triangle
indices.push_back(5);
indices.push_back(3);
indices.push_back(4);

I have to draw 100 going from -pi to pi on the x, and -pi/2 to pi/2 on the y. Is there an easier way to loop through these values and get the vertices and indices? Thank you for any help!

Edited to add: I went left to right manually, but it doesn't matter either way.

Answer 1

First, you generate your coordinates in a regular grid in the interval [-pi.. +pi], [-pi/2.. +pi/2]

size_t sizeX = 10; 
size_t sizeY = 10;

//Generate 10x10 coordinates in interval [-pi .. +pi] , [-pi/2 .. +pi/2] , 0
std::vector<Point3>vertices(sizeX * sizeY);
for (size_t j=0; j< sizeY; j++)
   for (size_t i=0; i< sizeX; i++)
      vertices[j*sizeX+i] = Point3(-pi + 2*i*pi/(sizeX-1), -pi/2 + j*pi/(sizeY-1), 0);

In this grid of vertex, each row has sizeX elements. This mean that for a vertex with index IDX:

• The vertex at its right will have index IDX+1
• The vertex over it, will have index IDX+sizeX.

Now let's go for the triangles. It is easier to use a schema based in quads. A quad with lower-left corner in vertex IDX will connect to vertex IDX+1, IDX+sizeX, IDX+sizeX+1.

After understanding this, you divide the quad into two triangles using same 4 corners. So you program a nested for to iterate over quads, but you create two triangles instead of one quad. Beware of triangle orientation.

//Generate triangles. Use a schema based in quads
std::vector<size_t>indices;
indices.reserve(2*(sizeX-1) * (sizeY-1));

for (size_t j=0; j< sizeY-1; j++)
{
    //Set idx to point at first vertex of row j
    size_t idx=j*sizeX

    for (size_t i=0; i< sizeX-1; i++)
    {        
        //Bottom triangle of the quad
        indices.push_back(idx);
        indices.push_back(idx+1);
        indices.push_back(idx+sizeX);
        //Top triangle of the quad
        indices.push_back(idx+1);
        indices.push_back(idx+sizeX+1);
        indices.push_back(idx+sizeX);
        //Move one vertex to the right
        idx++;
    }
}