From c20c5167510758353d5a62cfff24fd5494b9c5a4 Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan.goodliffe@octal.co.uk>
Date: Tue, 13 Dec 2022 12:14:43 +0000
Subject: Add Camera method to get the extents of the view frustrum at some
 distance

---
 gfx/gl/camera.cpp    | 18 ++++++++++++++++++
 gfx/gl/camera.h      |  2 ++
 test/test-render.cpp | 17 +++++++++++++++++
 3 files changed, 37 insertions(+)

diff --git a/gfx/gl/camera.cpp b/gfx/gl/camera.cpp
index 69cfe35..31be9a6 100644
--- a/gfx/gl/camera.cpp
+++ b/gfx/gl/camera.cpp
@@ -32,3 +32,21 @@ Camera::upFromForward(const glm::vec3 & forward)
 	const auto right = glm::cross(forward, ::down);
 	return glm::cross(forward, right);
 }
+
+std::array<glm::vec3, 4>
+Camera::extentsAtDist(const float dist) const
+{
+	const auto depth = -(2.f * (dist - near) * far) / (dist * (near - far)) - 1.f;
+	static constexpr const std::array extents {-1.F, 1.F};
+	std::array<glm::vec3, 4> out {};
+	auto outitr = out.begin();
+	for (auto x : extents) {
+		for (auto y : extents) {
+			const glm::vec4 in {x, y, depth, 1.f};
+
+			const auto out = inverseViewProjection * in;
+			*outitr++ = out / out.w;
+		}
+	}
+	return out;
+}
diff --git a/gfx/gl/camera.h b/gfx/gl/camera.h
index 2d937a0..1ded3e0 100644
--- a/gfx/gl/camera.h
+++ b/gfx/gl/camera.h
@@ -61,6 +61,8 @@ public:
 		return position;
 	}
 
+	std::array<glm::vec3, 4> extentsAtDist(float) const;
+
 	static glm::vec3 upFromForward(const glm::vec3 & forward);
 
 private:
diff --git a/test/test-render.cpp b/test/test-render.cpp
index 77501ac..1150a67 100644
--- a/test/test-render.cpp
+++ b/test/test-render.cpp
@@ -10,6 +10,7 @@
 #include <gfx/models/texture.h>
 #include <lib/glArrays.h>
 #include <maths.h>
+#include <stream_support.hpp>
 #include <ui/applicationBase.h>
 #include <ui/window.h>
 
@@ -95,6 +96,22 @@ class TestScene : public SceneProvider {
 BOOST_GLOBAL_FIXTURE(ApplicationBase);
 BOOST_GLOBAL_FIXTURE(TestMainWindow);
 
+BOOST_DATA_TEST_CASE(cam,
+		boost::unit_test::data::xrange(0.5F, 30.F, 1.3F) * boost::unit_test::data::xrange(0.5F, 10.F, 0.3F)
+				* boost::unit_test::data::xrange(50.F, 500.F, 70.F),
+		dist, near, far)
+{
+	static constexpr glm::vec3 pos {-10, -10, 60};
+	Camera cam {pos, half_pi, 1.f, near, far};
+
+	const auto e = cam.extentsAtDist(dist);
+
+	BOOST_CHECK_CLOSE_VEC(e[0], pos + glm::vec3(-dist, dist, -dist));
+	BOOST_CHECK_CLOSE_VEC(e[1], pos + glm::vec3(-dist, dist, dist));
+	BOOST_CHECK_CLOSE_VEC(e[2], pos + glm::vec3(dist, dist, -dist));
+	BOOST_CHECK_CLOSE_VEC(e[3], pos + glm::vec3(dist, dist, dist));
+}
+
 BOOST_FIXTURE_TEST_SUITE(w, TestRenderOutput);
 
 BOOST_AUTO_TEST_CASE(basic)
-- 
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