From 771801d826b9050498d2a4dee410b552f9770b55 Mon Sep 17 00:00:00 2001
From: Anders Jenbo <anders@jenbo.dk>
Date: Wed, 4 Jun 2025 04:48:30 +0200
Subject: [PATCH] Implement InverseTransform()

---
 miniwin/src/d3drm/d3drmviewport.cpp       | 49 ++++++++++++++++++++++-
 miniwin/src/internal/d3drmviewport_impl.h |  1 +
 2 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/miniwin/src/d3drm/d3drmviewport.cpp b/miniwin/src/d3drm/d3drmviewport.cpp
index 39108ba2..55184c17 100644
--- a/miniwin/src/d3drm/d3drmviewport.cpp
+++ b/miniwin/src/d3drm/d3drmviewport.cpp
@@ -446,10 +446,17 @@ void Direct3DRMViewportImpl::UpdateProjectionMatrix()
 		{0, 0, m_back / depth, 1},
 		{0, 0, (-m_front * m_back) / depth, 0},
 	};
-
 	memcpy(m_projectionMatrix, projection, sizeof(D3DRMMATRIX4D));
 
 	m_renderer->SetProjection(projection, m_front, m_back);
+
+	D3DRMMATRIX4D inverseProjectionMatrix = {
+		{1.0f / f, 0, 0, 0},
+		{0, 1.0f / (f * aspect), 0, 0},
+		{0, 0, 0, depth / (-m_front * m_back)},
+		{0, 0, 1, -(m_back / depth) * depth / (-m_front * m_back)},
+	};
+	memcpy(m_inverseProjectionMatrix, inverseProjectionMatrix, sizeof(D3DRMMATRIX4D));
 }
 
 D3DVALUE Direct3DRMViewportImpl::GetField()
@@ -506,7 +513,45 @@ HRESULT Direct3DRMViewportImpl::Transform(D3DRMVECTOR4D* screen, D3DVECTOR* worl
 
 HRESULT Direct3DRMViewportImpl::InverseTransform(D3DVECTOR* world, D3DRMVECTOR4D* screen)
 {
-	MINIWIN_NOT_IMPLEMENTED();
+	// Convert to screen coordinates
+	float screenX = screen->x / screen->z;
+	float screenY = screen->y / screen->w;
+
+	// Convert screen coordinates to NDC
+	float ndcX = screenX / m_width * 2.0f - 1.0f;
+	float ndcY = 1.0f - (screenY / m_height) * 2.0f;
+
+	float clipVec[4] = {ndcX * screen->w, ndcY * screen->w, screen->z, screen->w};
+
+	float viewVec[4] = {0.0f, 0.0f, 0.0f, 0.0f};
+	for (int i = 0; i < 4; i++) {
+		for (int j = 0; j < 4; j++) {
+			viewVec[j] += m_inverseProjectionMatrix[i][j] * clipVec[i];
+		}
+	}
+
+	float invViewMatrix[4][4];
+	D3DRMMatrixInvertOrthogonal(invViewMatrix, m_viewMatrix);
+
+	float worldVec[4] = {0.0f, 0.0f, 0.0f, 0.0f};
+	for (int i = 0; i < 4; i++) {
+		for (int j = 0; j < 4; j++) {
+			worldVec[j] += invViewMatrix[i][j] * viewVec[i];
+		}
+	}
+
+	// Perspective divide
+	if (worldVec[3] != 0.0f) {
+		world->x = worldVec[0] / worldVec[3];
+		world->y = worldVec[1] / worldVec[3];
+		world->z = worldVec[2] / worldVec[3];
+	}
+	else {
+		world->x = worldVec[0];
+		world->y = worldVec[1];
+		world->z = worldVec[2];
+	}
+
 	return DD_OK;
 }
 
diff --git a/miniwin/src/internal/d3drmviewport_impl.h b/miniwin/src/internal/d3drmviewport_impl.h
index 900c40b5..162230d1 100644
--- a/miniwin/src/internal/d3drmviewport_impl.h
+++ b/miniwin/src/internal/d3drmviewport_impl.h
@@ -43,6 +43,7 @@ struct Direct3DRMViewportImpl : public Direct3DRMObjectBaseImpl<IDirect3DRMViewp
 	DWORD m_height;
 	D3DRMMATRIX4D m_viewMatrix;
 	D3DRMMATRIX4D m_projectionMatrix;
+	D3DRMMATRIX4D m_inverseProjectionMatrix;
 	IDirect3DRMFrame* m_rootFrame = nullptr;
 	IDirect3DRMFrame* m_camera = nullptr;
 	D3DVALUE m_front = 1.f;