Kav/Geometry/SpriteMesh.cs

using System.Collections.Generic;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

namespace Kav
{
    public class SpriteMesh : ICullable, IIndexDrawable
    {
        public enum FlipOptions
        {
            None,
            Horizontal,
            Vertical,
            Both
        }

        private static readonly short[] Indices = new short[]
        {
            0,
            1,
            2,
            1,
            3,
            2
        };

        public IndexBuffer IndexBuffer { get; }
        public VertexBuffer VertexBuffer { get; }
        public BoundingBox BoundingBox { get; }

        public SpriteMesh(
            GraphicsDevice graphicsDevice,
            int width,
            int height,
            FlipOptions flipOptions
        ) {
			IndexBuffer = new IndexBuffer(
				graphicsDevice,
				IndexElementSize.SixteenBits,
				6,
				BufferUsage.WriteOnly
			);
            IndexBuffer.SetData(Indices);

            var vertexArray = GenerateVertexArray(width, height, flipOptions);

            VertexBuffer = new VertexBuffer(
                graphicsDevice,
                typeof(VertexPositionNormalTexture),
                4,
                BufferUsage.WriteOnly
            );
            VertexBuffer.SetData(vertexArray);

            BoundingBox = BoundingBox.CreateFromPoints(Positions(vertexArray));
        }

        private static VertexPositionNormalTexture[] GenerateVertexArray(int pixelWidth, int pixelHeight, FlipOptions flipOptions)
        {
            var width = pixelWidth / (float)40;
            var height = pixelHeight / (float)40;

            VertexPositionNormalTexture[] result = new VertexPositionNormalTexture[4];

            var xLeft = 0;
            var xRight = 1;

            var yTop = 0;
            var yBottom = 1;

            if (flipOptions == FlipOptions.Horizontal || flipOptions == FlipOptions.Both)
            {
                xLeft = 1;
                xRight = 0;
            }
            if (flipOptions == FlipOptions.Vertical || flipOptions == FlipOptions.Both)
            {
                yTop = 1;
                yBottom = 0;
            }

            result[0].Position = new Vector3(-width / 2, height / 2, 0);
            result[0].Normal = new Vector3(0, 0, -1);
            result[0].TextureCoordinate = new Vector2(xLeft, yTop);

            result[1].Position = new Vector3(width / 2, height / 2, 0);
            result[1].Normal = new Vector3(0, 0, -1);
            result[1].TextureCoordinate = new Vector2(xRight, yTop);

            result[2].Position = new Vector3(-width / 2, -height / 2, 0);
            result[2].Normal = new Vector3(0, 0, -1);
            result[2].TextureCoordinate = new Vector2(xLeft, yBottom);

            result[3].Position = new Vector3(width / 2, -height / 2, 0);
            result[3].Normal = new Vector3(0, 0, -1);
            result[3].TextureCoordinate = new Vector2(xRight, yBottom);

            return result;
        }

        private static IEnumerable<Vector3> Positions(IEnumerable<VertexPositionNormalTexture> vertices)
        {
            foreach (var vertex in vertices)
            {
                yield return vertex.Position;
            }
        }
    }
}