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POOptimizing a formula to create a triangle from arbitrary points
 

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  1. POOptimizing a formula to create a triangle from arbitrary points
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    <p>I think a little background will help before I get into my question. What I'm doing is creating my own small 2D physics library in xna, for fun nonetheless. This is also my first independent xna project, and my first time working with the 3D tools, so I may be doing things all wacky. Anyway, I'm currently making a triangle class in which the constructor takes three arbitrary points in the form of <code>Vector2s</code>. In the constructor I have to put these points in clockwise order (so they're not culled) and then find the texture coordinates they should correspond to (since I'm using <code>VertexPositionTextures</code> as my vertices). What I've got works, but it seems very long and complicated. I'm looking for any ways to shorten/simplify the code, which is this:</p> <pre><code>public PTriangle(Vector2 a, Vector2 b, Vector2 c) : base() { //set up vertices VertexPositionTexture[] vertices = new VertexPositionTexture[3]; //center vertices around origin Vector2 center = new Vector2((a.X + b.X + c.X) / 3, (a.Y + b.Y + c.Y) / 3); Vector2 newA = a - center; Vector2 newB = b - center; Vector2 newC = c - center; //get angle of each vertex (clockwise from -x axis) double angleA = MathHelper.Pi - Math.Atan((double)(newA.Y / newA.X)); double angleB = MathHelper.Pi - Math.Atan((double)(newB.Y / newB.X)); double angleC = MathHelper.Pi - Math.Atan((double)(newC.Y / newC.X)); if (newA.X &lt; 0) { if (newA.Y &lt; 0) { angleA += MathHelper.Pi; } else { angleA -= MathHelper.Pi; } } if (newB.X &lt; 0) { if (newB.Y &lt; 0) { angleB += MathHelper.Pi; } else { angleB -= MathHelper.Pi; } } if (newC.X &lt; 0) { if (newC.Y &lt; 0) { angleC += MathHelper.Pi; } else { angleC -= MathHelper.Pi; } } //order vertices by angle Vector2[] newVertices = new Vector2[3]; if (angleA &lt; angleB &amp;&amp; angleA &lt; angleC) { newVertices[0] = newA; if (angleB &lt; angleC) { newVertices[1] = newB; newVertices[2] = newC; } else { newVertices[1] = newC; newVertices[2] = newB; } } else if (angleB &lt; angleA &amp;&amp; angleB &lt; angleC) { newVertices[0] = newB; if (angleA &lt; angleC) { newVertices[1] = newA; newVertices[2] = newC; } else { newVertices[1] = newC; newVertices[2] = newA; } } else { newVertices[0] = newC; if (angleA &lt; angleB) { newVertices[1] = newA; newVertices[2] = newB; } else { newVertices[1] = newB; newVertices[2] = newA; } } //set positions of vertices vertices[0].Position = new Vector3(newVertices[0] + center, 0); vertices[1].Position = new Vector3(newVertices[1] + center, 0); vertices[2].Position = new Vector3(newVertices[2] + center, 0); //get width and height of triangle float minX = 0; float minY = 0; float maxX = 0; float maxY = 0; foreach (Vector2 vertex in newVertices) { if (vertex.X &lt; minX) { minX = vertex.X; } else if (vertex.X &gt; maxX) { maxX = vertex.X; } if (vertex.Y &lt; minY) { minY = vertex.Y; } else if (vertex.Y &gt; maxY) { maxY = vertex.Y; } } float width = maxX - minX; float height = maxY - minY; //shift triangle so fits in quadrant IV, and set texture coordinates for (int index = 0; index &lt; newVertices.Length; ++index) { newVertices[index].X -= minX; newVertices[index].Y -= minY; vertices[index].TextureCoordinate = new Vector2( newVertices[index].X / width, 1 - (newVertices[index].Y / height)); } this.Vertices = vertices; //set up indices this.Indices = new short[] { 0, 1, 2 }; } </code></pre>
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    1. COYou can use CCW test (http://en.wikipedia.org/wiki/Graham_scan#Pseudocode) to check the direction of the triangle. If the turn is correct, just use it, if the turn is not correct, just swap the last 2 points.
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    2. COCool, that would probably work. Although I realized I used my repositioned, clockwise-ordered points to determine my texture coordinates. I might be able to work that in though.
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    3. COYou can make it CW-ordered or CCW-ordered. The function doesn't restrict you on that aspect.
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