From 80e0790a639551f54b7a9dec2d7c6f7883497c03 Mon Sep 17 00:00:00 2001
From: Lightling <contactlightling@gmail.com>
Date: Sun, 3 Apr 2022 16:05:25 -0400
Subject: [PATCH] calculate tangents

---
 Defines.hlsli |  1 +
 Mesh.cpp      | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++
 Mesh.h        |  5 +++
 Vertex.h      |  1 +
 4 files changed, 98 insertions(+)

diff --git a/Defines.hlsli b/Defines.hlsli
index e31b1ae..7d3be7e 100644
--- a/Defines.hlsli
+++ b/Defines.hlsli
@@ -29,6 +29,7 @@ struct VertexShaderInput
 	//  v    v                v
 	float3 localPosition	: POSITION;
 	float3 normal			: NORMAL;
+	float3 tangent			: TANGENT;
 	float2 uv				: TEXCOORD;
 };
 
diff --git a/Mesh.cpp b/Mesh.cpp
index f1a0bbd..78e5fa1 100644
--- a/Mesh.cpp
+++ b/Mesh.cpp
@@ -230,9 +230,100 @@ Mesh::Mesh(const char* _file, Microsoft::WRL::ComPtr<ID3D11Device> _device, Micr
 	//    sophisticated model loading library like TinyOBJLoader or AssImp (yes, that's its name)
 
 	/// END OBJLOADER ///
+	CalculateTangents(&verts[0], verts.size(), &indices[0], indices.size());
 	CreateMesh(&verts[0], verts.size(), &indices[0], indices.size(), _device, _context);
 }
 
+// --------------------------------------------------------
+// Author: Chris Cascioli
+// Purpose: Calculates the tangents of the vertices in a mesh
+// 
+// - You are allowed to directly copy/paste this into your code base
+//   for assignments, given that you clearly cite that this is not
+//   code of your own design.
+//
+// - Code originally adapted from: http://www.terathon.com/code/tangent.html
+//   - Updated version now found here: http://foundationsofgameenginedev.com/FGED2-sample.pdf
+//   - See listing 7.4 in section 7.5 (page 9 of the PDF)
+//
+// - Note: For this code to work, your Vertex format must
+//         contain an XMFLOAT3 called Tangent
+//
+// - Be sure to call this BEFORE creating your D3D vertex/index buffers
+// --------------------------------------------------------
+void Mesh::CalculateTangents(Vertex* verts, int numVerts, unsigned int* indices, int numIndices)
+{
+	// Reset tangents
+	for (int i = 0; i < numVerts; i++)
+	{
+		verts[i].Tangent = XMFLOAT3(0, 0, 0);
+	}
+
+	// Calculate tangents one whole triangle at a time
+	for (int i = 0; i < numIndices;)
+	{
+		// Grab indices and vertices of first triangle
+		unsigned int i1 = indices[i++];
+		unsigned int i2 = indices[i++];
+		unsigned int i3 = indices[i++];
+		Vertex* v1 = &verts[i1];
+		Vertex* v2 = &verts[i2];
+		Vertex* v3 = &verts[i3];
+
+		// Calculate vectors relative to triangle positions
+		float x1 = v2->Position.x - v1->Position.x;
+		float y1 = v2->Position.y - v1->Position.y;
+		float z1 = v2->Position.z - v1->Position.z;
+
+		float x2 = v3->Position.x - v1->Position.x;
+		float y2 = v3->Position.y - v1->Position.y;
+		float z2 = v3->Position.z - v1->Position.z;
+
+		// Do the same for vectors relative to triangle uv's
+		float s1 = v2->UV.x - v1->UV.x;
+		float t1 = v2->UV.y - v1->UV.y;
+
+		float s2 = v3->UV.x - v1->UV.x;
+		float t2 = v3->UV.y - v1->UV.y;
+
+		// Create vectors for tangent calculation
+		float r = 1.0f / (s1 * t2 - s2 * t1);
+
+		float tx = (t2 * x1 - t1 * x2) * r;
+		float ty = (t2 * y1 - t1 * y2) * r;
+		float tz = (t2 * z1 - t1 * z2) * r;
+
+		// Adjust tangents of each vert of the triangle
+		v1->Tangent.x += tx;
+		v1->Tangent.y += ty;
+		v1->Tangent.z += tz;
+
+		v2->Tangent.x += tx;
+		v2->Tangent.y += ty;
+		v2->Tangent.z += tz;
+
+		v3->Tangent.x += tx;
+		v3->Tangent.y += ty;
+		v3->Tangent.z += tz;
+	}
+
+	// Ensure all of the tangents are orthogonal to the normals
+	for (int i = 0; i < numVerts; i++)
+	{
+		// Grab the two vectors
+		XMVECTOR normal = XMLoadFloat3(&verts[i].Normal);
+		XMVECTOR tangent = XMLoadFloat3(&verts[i].Tangent);
+
+		// Use Gram-Schmidt orthonormalize to ensure
+		// the normal and tangent are exactly 90 degrees apart
+		tangent = XMVector3Normalize(
+			tangent - normal * XMVector3Dot(normal, tangent));
+
+		// Store the tangent
+		XMStoreFloat3(&verts[i].Tangent, tangent);
+	}
+}
+
 void Mesh::CreateMesh(Vertex* _vertices, int _vertexCount, unsigned int* _indices, int _indexCount, Microsoft::WRL::ComPtr<ID3D11Device> _device, Microsoft::WRL::ComPtr<ID3D11DeviceContext> _context)
 {
 	// Create the VERTEX BUFFER description
diff --git a/Mesh.h b/Mesh.h
index ad90b4d..c912c4a 100644
--- a/Mesh.h
+++ b/Mesh.h
@@ -31,6 +31,11 @@ private:
 	Microsoft::WRL::ComPtr<ID3D11DeviceContext>     deviceContext;
 	int                                             countIndex;
 
+	void											CalculateTangents(
+														Vertex*										_verts,
+														int											_numVerts,
+														unsigned int*								_indices,
+														int											_numIndices);
 	void											CreateMesh(
 														Vertex*										_vertices,
 														int                                         _vertexCount,
diff --git a/Vertex.h b/Vertex.h
index fe94c18..eef37ea 100644
--- a/Vertex.h
+++ b/Vertex.h
@@ -11,5 +11,6 @@ struct Vertex
 {
 	DirectX::XMFLOAT3 Position;
 	DirectX::XMFLOAT3 Normal;
+	DirectX::XMFLOAT3 Tangent;
 	DirectX::XMFLOAT2 UV;
 };
\ No newline at end of file