Drawing a Circle with OpenGL - c++

Question

I'm trying to manipulate some code to draw a circle instead of the triangle that is already being printed by the tutorial. I'm not super familiar with C++ or OpenGL, which is why I'm just trying it out.
Any suggestions or corrections to my code would be greatly appreciated.
I keep getting a breakpoint error in XCODE on this line:
glDrawArrays(GL_TRIANGLE_FAN, 0, numPoints); // draw the points and fill it in
and it says:
Thread 1: EXC_BAD_ACCESS(code=1, address=0x0)
Here is the original vector buffer for the triangle in the tutorial I'm following:
static const GLfloat g_vertex_buffer_data[] = {
-1.0f, -1.0f, 0.0f,
1.0f, -1.0f, 0.0f,
0.0f, 1.0f, 0.0f,
};
I'm pretty sure that my calculations are correct, but I don't know why it' not drawing a circle. Here is my manipulation:
// Make a circle
GLfloat x;
GLfloat y;
GLfloat z = 0.0f;
int theta = 0;
float radius = 50.0f;
int currentSize = 0;
int numPoints = 30;
GLfloat g_vertex_buffer_data[numPoints*3];
while (theta <= 360) {
x = (GLfloat) radius * cosf(theta);
y = (GLfloat) radius * sinf(theta);
g_vertex_buffer_data[currentSize++] = x;
g_vertex_buffer_data[currentSize++] = y;
g_vertex_buffer_data[currentSize++] = z;
/*
cout << "Theta: " << theta << endl;
for (int i = 0; i < currentSize; i++) {
cout << "g_vertex_buffer_data[" << g_vertex_buffer_data[i] << "]" << endl;
}
*/
theta = theta + (360/numPoints);
}
Here is the rest of the code in the .cpp file:
GLuint vertexbuffer;
glGenBuffers(1, &vertexbuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data),g_vertex_buffer_data, GL_STATIC_DRAW);
do{
// Clear the screen
glClear( GL_COLOR_BUFFER_BIT );
// Use our shader
glUseProgram(programID);
// 1rst attribute buffer : vertices
glEnableVertexAttribArray(vertexPosition_modelspaceID);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glVertexAttribPointer(
vertexPosition_modelspaceID, // The attribute we want to configure
numPoints, // size
GL_FLOAT, // type
GL_FALSE, // normalized?
0, // stride
(void*)0 // array buffer offset
);
// Draw the circle!
glDrawArrays(GL_TRIANGLE_FAN, 0, numPoints); // draw the points and fill it in
glDisableVertexAttribArray(vertexPosition_modelspaceID);
// Swap buffers
glfwSwapBuffers(window);
glfwPollEvents();
} // Check if the ESC key was pressed or the window was closed
while( glfwGetKey(window, GLFW_KEY_ESCAPE ) != GLFW_PRESS &&
glfwWindowShouldClose(window) == 0 );
// Cleanup VBO
glDeleteBuffers(1, &vertexbuffer);
glDeleteProgram(programID);
// Close OpenGL window and terminate GLFW
glfwTerminate();
return 0;

Answer 1

There are a few problems in this code. The most severe one that probably causes the crash is here:
glVertexAttribPointer(
vertexPosition_modelspaceID, // The attribute we want to configure
numPoints, // size
GL_FLOAT, // type
GL_FALSE, // normalized?
0, // stride
(void*)0 // array buffer offset
);
The second argument to glVertexAttribPointer() is the number of components per vertex. Since you have 3 floats (x, y and z) per vertex, the correct value is 3. So the call should be:
glVertexAttribPointer(
vertexPosition_modelspaceID, // The attribute we want to configure
3, // size
GL_FLOAT, // type
GL_FALSE, // normalized?
0, // stride
(void*)0 // array buffer offset
);
There is also a 1-off error where the points are created:
while (theta <= 360) {
If you include 360 in the range, you will effectively repeat the first vertex, and write one more vertex than the allocated space. This should be:
while (theta < 360) {
Also, the arguments to cosf() and sinf() are in radians. So you will have to convert the angles from degrees to radians for these functions:
x = (GLfloat) radius * cosf(theta * M_PI / 180.0f);
y = (GLfloat) radius * sinf(theta * M_PI / 180.0f);