绘制n点星形的opengl C ++

Question

So I am trying to draw a n-pointed star using opengl and I'm running across a problem when drawing stars that have multiple of 3 points.

The output for any other looks something like this eleven point star:

The output for something like a nine point star is nine point star:

The code I'm using to generate vertices is below. could really use the help.

srand(time(nullptr));
numVertices = 6 * (rand() % 15 + 5);

int numPoints = numVertices / 3;
float r = 0.95;
drawMode = GL_FILL;
vPos = 0;
vColor = 1;
const float M_PI = 3.1415926535897;
float DegToRad;
cout<<numPoints<<endl;
//if(((numPoints/2)%3) !=0)
DegToRad = 2*M_PI / numPoints;
//  else
//    DegToRad = 2*M_PI / (numPoints);

glClearColor(0,0,0,1);
cout<<"Graphics initialized"<<"\n";
VertexData* vertices = (VertexData*)malloc((numVertices+1) * sizeof(VertexData));

for(int i = 0; i < numVertices; i++)
{
    float DegInRad = i * DegToRad;

    if(i % 3 !=0 )
    {
        if(i % 2 == 0)
        {
            cout<<"Drawing triangle point "<<i<<" at "<<r*cos(DegInRad+(M_PI/2))<<","<<r*sin(DegInRad+(M_PI/2))<<endl;
            vertices[i] = {{1.00,0.00,0.00}, {r*cos(DegInRad+(M_PI/2)), r*sin(DegInRad+(M_PI/2))}};
        }else {
            cout<<"Drawing triangle point "<<i<<" at "<<r/3*cos(DegInRad+(M_PI/2))<<","<<r/3*sin(DegInRad+(M_PI/2))<<endl;
            vertices[i] = {{1.00,1.00,1.00}, {r/3*cos(DegInRad+(M_PI/2)), r/3*sin(DegInRad+(M_PI/2))}};
        }

    }else{
        cout<<"Drawing triangle point to origin "<<i<<endl;
        vertices[i] = {{1.00,1.00,1.00}, {0.0, 0.0}};
    }
}

glEnable(GL_BLEND);
glGenVertexArrays(1, &VAO);

glBindVertexArray(VAO);

// Generate and bind (turn on) a buffer (storage location).
glGenBuffers(1, &Buffer);
glBindBuffer(GL_ARRAY_BUFFER, Buffer);

// Transfer the vertex information to the buffer.
glBufferData(GL_ARRAY_BUFFER, numVertices * sizeof(VertexData), vertices, GL_STATIC_DRAW);

// Setup color data position information. Pointer to data on graphics card itself specifically the color.
//((position tag, number of colors, data-type identifier, to normalize or not, stride the number of bytes to the next color, where to start))
glVertexAttribPointer(vColor, 3, GL_FLOAT, GL_TRUE, sizeof(VertexData), BUFFER_OFFSET(0));

// Setup vertex data position information. Never Normalize the position
//vPosition is 0, number of data-types, data-type, to normalize, stride, start (number of bytes to) of the first position data
glVertexAttribPointer(vPos, 2, GL_FLOAT, GL_FALSE, sizeof(VertexData),BUFFER_OFFSET(sizeof(vertices[0].rgb)));

// Set position indexes for shader streams. Turn on vPosition and vColor.
glEnableVertexAttribArray(vPos);
glEnableVertexAttribArray(vColor);

Answer 1

numVertices = 6 * (rand() % 15 + 5); 

You divide that in integer by three, so number of "points" may not match?

why you generate number of vertices? Shouldn't you generate number of star points and calculate vertices from there?. Number of unique vertices for star 2*(star_points + 1) .

Why it so? Outline of star contains 2*(star_points) + 1 , three per each lobe, two of each shared between lobes. And you have center point.

OpenGL allows several approaches:

1. push every triangle separately into vertex buffer, you'll use more data if a lot of vertices are shared: in your case you'll have 6*star_points vertices;
2. generate every vertex once, then use index buffer to create triangles or quads from them;
3. create a triangle strip or fan (fan is good choice here, you have the center).

You have wrong angle step, eg for 3-point star triangles are drawn in wrong order:

Drawing triangle point to origin 0
Drawing triangle point 1 at -0.866025,0.5
Drawing triangle point 2 at -2.59808,-1.5
Drawing triangle point to origin 3
Drawing triangle point 4 at 2.59808,-1.5
Drawing triangle point 5 at 0.866025,0.5
Drawing triangle point to origin 6
Drawing triangle point 7 at -0.866025,0.5
Drawing triangle point 8 at -2.59808,-1.5
Drawing triangle point to origin 9
Drawing triangle point 10 at 2.59808,-1.5
Drawing triangle point 11 at 0.866025,0.5
Drawing triangle point to origin 12
Drawing triangle point 13 at -0.866025,0.5
Drawing triangle point 14 at -2.59807,-1.5
Drawing triangle point to origin 15
Drawing triangle point 16 at 2.59808,-1.5
Drawing triangle point 17 at 0.866025,0.500001

This doesn't look right

DegToRad = 2 *M_PI / numPoints;

perhaps

DegToRad = M_PI / numPoints;

because each triangle is half of a lobe. The loop itself is wrong too by design. it leaves gaps between consecutive triangles, it increments angle during "drawing to origin" step. but next it would circle around the 360 degrees and begin fill those gaps..or not, if you have number of ends as a multiple of 3.