Drawing a circle with a sector cut out in OpenGL ES 1.1 - opengl-es

Question

I'm trying to draw the following shape using OpenGL ES 1.1. And well, I'm stuck, I don't really know how to go about it.
My game currently uses Android's Canvas API, which isn't hardware accelerated, so I'm rewriting it with OpenGL ES. The Canvas class has a method called drawArc which makes drawing this shape very very easy; Canvas.drawArc
Any advice/hints on doing the same with OpenGL ES?
Thank you for reading.

Answer 1

void gltDrawArc(unsigned int const segments, float angle_start, float angle_stop)
{
int i;
float const angle_step = (angle_stop - angle_start)/segments;
GLfloat *arc_vertices;
arc_vertices = malloc(2*sizeof(GLfloat) * (segments+2));
arc_vertices[0] = arc_vertices[1] = 0.
for(i=0; i<segments+1; i++) {
arc_vertices[2 + 2*i ] = cos(angle_start + i*angle_step);
arc_vertices[2 + 2*i + 1] = sin(angle_start + i*angle_step);
}
glVertexPointer(2, GL_FLOAT, 0, arc_vertices);
glEnableClientState(GL_VERTEX_ARRAY);
glDrawArrays(GL_TRIANGLE_FAN, 0, segments+2);
free(arc_vertices);
}

Answer 2

What about just sampling the circle at discrete angles and drawing a GL_TRIANGLE_FAN?
EDIT: Something like this will just draw a sector of a unit circle around the origin in 2D:
glBegin(GL_TRIANGLE_FAN);
glVertex2f(0.0f, 0.0f);
for(angle=startAngle; angle<=endAngle; ++angle)
glVertex2f(cos(angle), sin(angle));
glEnd();
Actually take this more as pseudocode, as sin and cos usually work on radians and I'm using degrees, but you should get the point.

Answer 3

I am new to android programming so I am sure there is probably a better way to do this. But I was following the OpenGL ES 1.0 tutorial on the android developers site http://developer.android.com/resources/tutorials/opengl/opengl-es10.html which walks you through drawing a green triangle. You can follow the link and you will see most of the code I used there. I wanted to draw a circle on the triangle. The code I added is based on the above example posted by datenwolf. And is shown in snippets below:
public class HelloOpenGLES10Renderer implements GLSurfaceView.Renderer {
// the number small triangles used to make a circle
public int segments = 100;
public float mAngle;
private FloatBuffer triangleVB;
// array to hold the FloatBuffer for the small triangles
private FloatBuffer [] segmentsArray = new FloatBuffer[segments];
private void initShapes(){
.
.
.
// stuff to draw holes in the board
int i = 0;
float angle_start = 0.0f;
float angle_stop = 2.0f * (float) java.lang.Math.PI;
float angle_step = (angle_stop - angle_start)/segments;
for(i=0; i<segments; i++) {
float[] holeCoords;
FloatBuffer holeVB;
holeCoords = new float [ 9 ];
// initialize vertex Buffer for triangle
// (# of coordinate values * 4 bytes per float)
ByteBuffer vbb2 = ByteBuffer.allocateDirect(holeCoords.length * 4);
vbb2.order(ByteOrder.nativeOrder());// use the device hardware's native byte order
holeVB = vbb2.asFloatBuffer(); // create a floating point buffer from the ByteBuffer
float x1 = 0.05f * (float) java.lang.Math.cos(angle_start + i*angle_step);
float y1 = 0.05f * (float) java.lang.Math.sin(angle_start + i*angle_step);
float z1 = 0.1f;
float x2 = 0.05f * (float) java.lang.Math.cos(angle_start + i+1*angle_step);
float y2 = 0.05f * (float) java.lang.Math.sin(angle_start + i+1*angle_step);
float z2 = 0.1f;
holeCoords[0] = 0.0f;
holeCoords[1] = 0.0f;
holeCoords[2] = 0.1f;
holeCoords[3] = x1;
holeCoords[4] = y1;
holeCoords[5] = z1;
holeCoords[6] = x2;
holeCoords[7] = y2;
holeCoords[8] = z2;
holeVB.put(holeCoords); // add the coordinates to the FloatBuffer
holeVB.position(0); // set the buffer to read the first coordinate
segmentsArray[i] = holeVB;
}
}
.
.
.
public void onDrawFrame(GL10 gl) {
.
.
.
// Draw hole
gl.glColor4f( 1.0f - 0.63671875f, 1.0f - 0.76953125f, 1.0f - 0.22265625f, 0.0f);
for ( int i=0; i<segments; i++ ) {
gl.glVertexPointer(3, GL10.GL_FLOAT, 0, segmentsArray[i]);
gl.glDrawArrays(GL10.GL_TRIANGLES, 0, 3);
}
}