Replaced perspective view matrix calculation

Replaced perspective view matrix calculation when given FOV and aspect
ratio. The new implementation does not calculate the view rectangle, so
the result matrix is still valid when znear = 0.

Author: sakumamayu

Source/SharpDX.Mathematics/Matrix.cs

@@ -2291,13 +2291,15 @@ public static Matrix PerspectiveRH(float width, float height, float znear, float
         /// <param name="result">When the method completes, contains the created projection matrix.</param>
         public static void PerspectiveFovLH(float fov, float aspect, float znear, float zfar, out Matrix result)
         {
-            float yScale = (float)(1.0 / Math.Tan(fov * 0.5f));
-            float xScale = yScale / aspect;
+            float yScale = (float)(1.0f / Math.Tan(fov * 0.5f));
+            float q = zfar / (zfar - znear);
 
-            float halfWidth = znear / xScale;
-            float halfHeight = znear / yScale;
-
-            PerspectiveOffCenterLH(-halfWidth, halfWidth, -halfHeight, halfHeight, znear, zfar, out result);
+            result = new Matrix();
+            result.M11 = yScale / aspect;
+            result.M22 = yScale;
+            result.M33 = q;
+            result.M34 = 1.0f;
+            result.M43 = -q * znear;
         }
 
         /// <summary>
@@ -2325,13 +2327,15 @@ public static Matrix PerspectiveFovLH(float fov, float aspect, float znear, floa
         /// <param name="result">When the method completes, contains the created projection matrix.</param>
         public static void PerspectiveFovRH(float fov, float aspect, float znear, float zfar, out Matrix result)
         {
-            float yScale = (float)(1.0 / Math.Tan(fov * 0.5f));
-            float xScale = yScale / aspect;
+            float yScale = (float)(1.0f / Math.Tan(fov * 0.5f));
+            float q = zfar / (znear - zfar);
 
-            float halfWidth = znear / xScale;
-            float halfHeight = znear / yScale;
-
-            PerspectiveOffCenterRH(-halfWidth, halfWidth, -halfHeight, halfHeight, znear, zfar, out result);
+            result = new Matrix();
+            result.M11 = yScale / aspect;
+            result.M22 = yScale;
+            result.M33 = q;
+            result.M34 = -1.0f;
+            result.M43 = q * znear;
         }
 
         /// <summary>