Drawing a circle in c++ using openGL - c++

Question

I've been trying to draw a circle in c++ using openGL. So far i have a compresses circle and it just has a random line going across the screen.
This is the function I'm using to get this shape.
void Sprite::init(int x, int y, int width, int height, Type mode, float scale) {
_x = x;
_y = y;
_width = width;
_height = height;
//generate buffer if it hasn't been generated
if (_vboID == 0) {
glGenBuffers(1, &_vboID);
}
Vertex vertexData[360];
if (mode == Type::CIRCLE) {
float rad = 3.14159;
for (int i = 0; i < 359; i++) {
vertexData[i].setPosition((rad * scale) * cos(i), (rad * scale) * sin(i));
}
}
//Tell opengl to bind our vertex buffer object
glBindBuffer(GL_ARRAY_BUFFER, _vboID);
//Upload the data to the GPU
glBufferData(GL_ARRAY_BUFFER, sizeof(vertexData), vertexData, GL_STATIC_DRAW);
//Unbind the buffer
glBindBuffer(GL_ARRAY_BUFFER, 0);
}
What is causing the line? Why is my circle being compressed?
Sorry if this is a dumb question or if this question doesn't belong on this website I'm very new to both c++ as well as this website.

Answer 1

It is difficult to be sure without testing the code myself, but I'll guess anyway.
Your weird line is probably caused by the buffer not being fully initialized. This is wrong:
Vertex vertexData[360];
for (int i = 0; i < 359; i++) {
It should be:
for (int i = 0; i < 360; i++) {
or else the position at vertexData[359] is left uninitialized and contains some far away point.
About the ellipse instead of a circle, that is probably caused by your viewport not having the same scale horizontally and vertically. If you configure the viewport plus transformation matrices to have a viewing frustum of X=-10..10, Y=-10..10, but the actual viewport is X=0..800 and the Y=0..600, for example, then the scale would be different and you'll get your image distorted.
The solution would be one of:
Create a square viewport instead of rectangular. Check your arguments to glViewport().
Define a view matrix to consider the same ratio your viewport has. You don't show how you set the view/world matrix, maybe you are not even using matrices... If that is the case, you should probably use one.

Answer 2

I don't understand, exactly, what you want obtain but... cos() and sin() receive a radiant argument; so, instead of cos(i) and sin(i), I suppose you need cos((2*rad*i)/360.0)) and sin((2*rad*i)/360.0)) or, semplified, cos((rad*i)/180.0)) and cos((rad*i)/180.0))
And what about the center and the radious of the circle?
(x, y) should be the center of the circle?
scale is the radious?
In this case, I suppose you should write something like (caution: not tested)
Vertex vertexData[360];
float rad = 3.14159;
if (mode == Type::CIRCLE) {
for (int i = 0; i < 359; ++i) {
float angle = (rad / 180) * i; // (thanks Rodrigo)
vertexData[i].setPosition(x + scale * cos(angle), y + scale * sin(angle));
}
}
or, loosing precision but avoidind some moltiplication,
Vertex vertexData[360];
float rad = 3.14159;
float angIncr = rad / 180.0;
if (mode == Type::CIRCLE) {
for (int i = 0, float angle = 0.0; i < 359; ++i, angle += angIncr) {
vertexData[i].setPosition(x + scale * cos(angle), y + scale * sin(angle));
}
}
But what about width and heigth?
p.s.: sorry for my bad English.
--- modified with suggestion from Rodrigo --