How to draw a circle using VBO in ES2.0

Question

I am trying to develop an ES 2.0 application in Linux environment. My target GPU is Fujitsu ruby MB86298 . To optimize the performance I have decided to use the VBO concept. I am very new to VBOs. I rendered basic primitives like triangle and quads using VBO where I have less no vertices . For rendering crown using a VBO, I computed all the vertices(more than 200). Now I am finding difficulty in sending this data of 200 vertices to the VBO.I cannot manually enter the all the vertex data and store in an array and pass it to VBO. Is there any way to send that vertex data of each for loop( used in computation of vertices of crown) to the VBO? Can any one share the code snippet of drawing an arc or circle in ES 2.0 using VBO's?

Answer 1

Here's some code fragments for rendering a circle. I haven't compiled or run this code, so there's a possibility of (hopefully minor) typos.

To prepare the VBO, which would be done once:

// Number of segments the circle is divided into.
const unsigned DIV_COUNT = 32;

// Will use a triangle fan rooted at the origin to draw the circle. So one additional
// point is needed for the origin, and another one because the first point is repeated
// as the last one to close the circle.
GLfloat* coordA = new GLfloat[(DIV_COUNT + 2) * 2];

// Origin.
unsigned coordIdx = 0;
coordA[coordIdx++] = 0.0f;
coordA[coordIdx++] = 0.0f;

// Calculate angle increment from point to point, and its cos/sin.
float angInc = 2.0f * M_PI / static_cast<float>(DIV_COUNT);
float cosInc = cos(angInc);
float sinInc = sin(angInc);

// Start with vector (1.0f, 0.0f), ...
coordA[coordIdx++] = 1.0f;
coordA[coordIdx++] = 0.0f;

// ... and then rotate it by angInc for each point.
float xc = 1.0f;
float yc = 0.0f;
for (unsigned iDiv = 1; iDiv < DIV_COUNT; ++iDiv) {
    float xcNew = cosInc * xc - sinInc * yc;
    yc = sinInc * xc + cosInc * yc;
    xc = xcNew;

    coordA[coordIdx++] = xc;
    coordA[coordIdx++] = yc;
}

// Repeat first point as last point to close circle.
coordA[coordIdx++] = 1.0f;
coordA[coordIdx++] = 0.0f;

GLuint vboId = 0;
glGenBuffers(1, &circVboId);
glBindBuffer(GL_ARRAY_BUFFER, circVboId);
glBufferData(GL_ARRAY_BUFFER, (DIV_COUNT + 2) * 2 * sizeof(GLfloat), coordA, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);

delete[] coordA;

Then to draw, with posLoc being the location of the vertex attribute for the position:

glBindBuffer(GL_ARRAY_BUFFER, circVboId);
glVertexAttribPointer(posLoc, 2, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(posLoc);

glDrawArrays(GL_TRIANGLE_FAN, 0, DIV_COUNT + 2);

glBindBuffer(GL_ARRAY_BUFFER, 0);