Question

I want to draw polygons with triangle fan. I get the polygons as a data structure with a count of the number of edges followed by an array of coordinates. I figured out that it should work something like this:

-(void) fillarea:(int16_t) count vertices:(int16_t*) pxyarray { 
    int valueCount = count*2;
    GLfloat vertexBuffer[valueCount];
    for (int i=0; i<valueCount; i++) {
        vertexBuffer[i] = pxyarray[i];
    }
    glVertexPointer(2, GL_FLOAT, 0, vertexBuffer);
    glDrawArrays(GL_TRIANGLE_FAN, 0, valueCount);
    [context presentRenderbuffer:GL_RENDERBUFFER_OES];
}

It seems to work perfect with triangles, however as soon as I use polygons with more edges (squares, pentagons,…) they all draw another triangle to the origin at 0,0. Can someone explain to me what is happening here.

If it helps some examples for polygons I defined to be drawn with this method:

int16_t verticesTriangle[6] = {50,50,100,50,100,100};
[self fillarea:3 vertices:verticesTriangle];
int16_t verticesSquare[8] = {100,100,150,100,150,150,100,150};
[self fillarea:4 vertices:verticesSquare];
int16_t vertices5[10] = {150,50,175,25,200,50,200,100,150,100};
[self fillarea:5 vertices:vertices5];
int16_t vertices6[12] = {250,50,275,25,300,50,300,75,275,100,250,75};
[self fillarea:6 vertices:vertices6];

Answer 1

The answer to the problem is actually very simple. glDrawArrays wants to know the number of vertices not the number of values handed over to it. So instead of writing:

glDrawArrays(GL_TRIANGLE_FAN, 0, valueCount); // valueCount = 6 for a triangle

I need to write:

glDrawArrays(GL_TRIANGLE_FAN, 0, count); // count = 3 for a triangle