From 8a5d36623e4fa261aea16731954c9987dcbb1ef1 Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Thu, 2 Nov 2023 20:38:10 +0000
Subject: Terrain mesh method for getting the height/position at a point on the
 surface

---
 game/terrain2.cpp     | 11 +++++++++++
 game/terrain2.h       |  2 ++
 test/test-terrain.cpp | 23 +++++++++++++++++++++++
 3 files changed, 36 insertions(+)

diff --git a/game/terrain2.cpp b/game/terrain2.cpp
index 8d29143..b129284 100644
--- a/game/terrain2.cpp
+++ b/game/terrain2.cpp
@@ -1,5 +1,7 @@
 #include "terrain2.h"
 #include <fstream>
+#include <glm/gtx/intersect.hpp>
+#include <maths.h>
 
 TerrainMesh::TerrainMesh(const std::filesystem::path & input)
 {
@@ -124,6 +126,15 @@ TerrainMesh::findPoint(glm::vec2 p, OpenMesh::FaceHandle f) const
 	return f;
 }
 
+glm::vec3
+TerrainMesh::positionAt(const PointFace & p) const
+{
+	glm::vec3 out {};
+	Triangle<3> t {this, fv_range(p.face(this))};
+	glm::intersectLineTriangle(p.point ^ 0.F, up, t[0], t[1], t[2], out);
+	return p.point ^ out[0];
+}
+
 void
 TerrainMesh::walk(const PointFace & from, const glm::vec2 to, const std::function<void(FaceHandle)> & op) const
 {
diff --git a/game/terrain2.h b/game/terrain2.h
index c2c3d19..f1d288e 100644
--- a/game/terrain2.h
+++ b/game/terrain2.h
@@ -58,6 +58,8 @@ public:
 	[[nodiscard]] FaceHandle findPoint(glm::vec2) const;
 	[[nodiscard]] FaceHandle findPoint(glm::vec2, FaceHandle start) const;
 
+	[[nodiscard]] glm::vec3 positionAt(const PointFace &) const;
+
 	void walk(const PointFace & from, const glm::vec2 to, const std::function<void(FaceHandle)> & op) const;
 	void walkUntil(const PointFace & from, const glm::vec2 to, const std::function<bool(FaceHandle)> & op) const;
 
diff --git a/test/test-terrain.cpp b/test/test-terrain.cpp
index bf73356..38cb51f 100644
--- a/test/test-terrain.cpp
+++ b/test/test-terrain.cpp
@@ -1,4 +1,5 @@
 #define BOOST_TEST_MODULE terrain
+#include "testHelpers.h"
 #include <boost/test/data/test_case.hpp>
 #include <boost/test/unit_test.hpp>
 #include <stream_support.h>
@@ -81,6 +82,28 @@ BOOST_DATA_TEST_CASE(findPointOnTerrain,
 	BOOST_CHECK_EQUAL(fh, fixedTerrtain.findPoint(p, TerrainMesh::FaceHandle(start)).idx());
 }
 
+using FindPositionData = std::tuple<glm::vec2, float>;
+
+BOOST_DATA_TEST_CASE(findPositionAt,
+		boost::unit_test::data::make<FindPositionData>({
+				// corners
+				{{310000, 490000}, 32.8F},
+				{{310050, 490050}, 33.0F},
+				{{310000, 490050}, 32.7F},
+				{{310050, 490000}, 33.2F},
+				{{310750, 490150}, 58.4F},
+				// midpoints
+				{{310025, 490025}, 32.9F},
+				{{310025, 490050}, 32.85F},
+				{{310000, 490025}, 32.75F},
+				// other
+				{{310751, 490152}, 58.326F},
+		}),
+		p, h)
+{
+	BOOST_CHECK_CLOSE_VEC(fixedTerrtain.positionAt(p), p ^ h);
+}
+
 using WalkTerrainData = std::tuple<glm::vec2, glm::vec2, std::vector<int>>;
 
 BOOST_DATA_TEST_CASE(walkTerrain,
-- 
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