I am trying to figure out how indices work in Stage3D to fix some bugs with starling graphics library.

I want to implement different joint type between 2 line segments when you are drawing with lineTo in starling graphics extension and for that I need to add new vertices.

This is the sketch of what I am trying to do:

Each vertex is setup to use 9 elements which are all numbers. Inside starling graphics library there is a method that creates vertexes and indices for (for GPU) for each point that you add with lineTo method. In that method there is a for loop like this:

private static var i2:int;
protected static function createPolyLinePreAlloc(strokeData:Vector.<StrokeVertex>, vertices:Vector.<Number>,  indices:Vector.<uint>, _hasDegenerates:Boolean):void
{
        numVertices = strokeData.length;
	lastVerticeIndex = numVertices - 1;
	
	for(i = 0; i < numVertices; i++)
	{
                idx = i;

                isFirstVertice = idx == 0;
		isLastVertice = idx == lastVerticeIndex; 
		
                idx0 = isFirstVertice ? idx : ( idx - 1 );
		idx2 = isLastVertice ? idx : ( idx + 1 );
				
		v0 = strokeData[idx0];
		v1 = strokeData[idx];
		v2 = strokeData[idx2];
				
		// Point i from the sketch
		vertices[vertCounter++] = v1xPos;
		vertices[vertCounter++] = v1yPos;
		vertices[vertCounter++] = 0;
		vertices[vertCounter++] = v1.r2;
		vertices[vertCounter++] = v1.g2;
		vertices[vertCounter++] = v1.b2;
		vertices[vertCounter++] = v1.a2;
		vertices[vertCounter++] = v1.u;
		vertices[vertCounter++] = 0;
		
		
		// point j from the sketch
		vertices[vertCounter++] = v1xNeg;
		vertices[vertCounter++] = v1yNeg;
		vertices[vertCounter++] = 0;
		vertices[vertCounter++] = v1.r1;
		vertices[vertCounter++] = v1.g1;
		vertices[vertCounter++] = v1.b1;
		vertices[vertCounter++] = v1.a1;
		vertices[vertCounter++] = v1.u;
		vertices[vertCounter++] = 0;
		
		
		if(!isLastVertice)
		{
			// tried to add point B from the sketch
			vertices[vertCounter++] = v2EndxPos;
			vertices[vertCounter++] = v2EndyPos;
			vertices[vertCounter++] = 0;
			vertices[vertCounter++] = v1.r2;
			vertices[vertCounter++] = v1.g2;
			vertices[vertCounter++] = v1.b2;
			vertices[vertCounter++] = v1.a2;
			vertices[vertCounter++] = v1.u;
			vertices[vertCounter++] = 0;
			
			// tried to add point C from the sketch
			vertices[vertCounter++] = v2EndxNegPos;
			vertices[vertCounter++] = v2EndyNegPos;
			vertices[vertCounter++] = 0;
			vertices[vertCounter++] = v1.r1;
			vertices[vertCounter++] = v1.g1;
			vertices[vertCounter++] = v1.b1;
			vertices[vertCounter++] = v1.a1;
			vertices[vertCounter++] = v1.u;
			vertices[vertCounter++] = 0;
		}
					
	        // Adding indices here
		if(i < numVertices - 1)
		{
			i2 = i << 1; // why is this?
			indices[indiciesCounter++] = i2;
			indices[indiciesCounter++] = i2+2; // why is this 2?
			indices[indiciesCounter++] = i2+1;
			indices[indiciesCounter++] = i2+1;  // why is this 1?
			indices[indiciesCounter++] = i2+2;
			indices[indiciesCounter++] = i2+3;  // why is this 3?
		}

		// tried to add more indices because of point new added point C but it is not working
		if(!isLastVertice)
		{
			i2 = (i + 2) << 1;
			indices[indiciesCounter++] = i2;
			indices[indiciesCounter++] = i2+2;
			indices[indiciesCounter++] = i2+1;
			indices[indiciesCounter++] = i2+1;
			indices[indiciesCounter++] = i2+2;
			indices[indiciesCounter++] = i2+3;	
		}
	}
}

So for each two sets of vertices there is one set of indices added. But I do not understand what all this numbers that are added to indices represent. And how many more indices I need to add if I want to add more vertices.

I want to implement different joint types between 2 line segments (miter, round or bevel) when drawing with lineTo method. I have already solved the bug where line is drawn thinner when there is sharp angle but for implementing bevel joint type I need to add more vertices.

In current implementation vertices between two points are reused but I need to introduce a third point for example for bevel or miter joint type.

I do not have issue calculating x, y for the position of each point but I don't know how to add new point properly to vertices and indices.

Everything works fine when I am just reusing points A and B from the image but the issue is when I try to add new point C.