feat(track): rails and ballast blended into those of the tracks joined, where their textures differ

world/TrackBlend.h

@@ -0,0 +1,70 @@
+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+#pragma once
+
+#include <array>
+
+#include <glm/glm.hpp>
+
+#include "utilities/Classes.h"
+#include "interfaces/ITexture.h"
+#include "rendering/geometrybank.h"
+
+class TTrack;
+
+// the rails and the ballast of a track blended into those of the tracks joined at its ends, where their textures differ: near
+// an end the vertices carry the weight of the track joined there (y, z, w of their user data for the ends p1, p2 and p4 of a
+// switch) and the track is drawn with a material blending its texture with theirs. the ballast comes in as grains, the rails
+// fade into each other. a plain track blends its rails and its ballast, a switch the trackbed made for it
+namespace track_blend {
+
+// how far from an end the textures blend, by default
+float constexpr default_length { 3.f };
+
+enum class kind {
+    rails,
+    ballast
+};
+
+struct blending {
+    struct end {
+        bool active { false };
+        glm::dvec3 point { 0.0 }; // relative to the origin of the track
+        glm::dvec3 inward { 0.0 }; // unit vector from the end into the track
+        double length { 0.0 };
+        float peak { 0.f }; // the weight at the end: a half where the track joined there blends back, whole where it doesn't
+    };
+    std::array<end, 3> ends;
+    material_handle material { null_handle }; // the material the blending is drawn with, null where nothing's blended
+
+    bool any() const {
+        return material != null_handle; }
+    // the weights of the ends put in the user data of the vertices (relative to the origin of the track), made as long as the
+    // vertices where it's shorter; what the user data hold at x is kept
+    void weigh( gfx::vertex_array const &Vertices, gfx::userdata_array &Userdata ) const;
+};
+
+// how the rails or the ballast of the track blend into those of the tracks joined to it, nothing for the tracks which don't
+blending setup( TTrack const &Track, kind const Kind );
+
+// while one is alive, the chunks of geometry inserted with track_rails::insert_chunk() get the weights of the blending
+class weighing {
+public:
+    explicit weighing( blending const *Blending );
+    ~weighing();
+    weighing( weighing const & ) = delete;
+    weighing &operator=( weighing const & ) = delete;
+private:
+    blending const *m_previous { nullptr };
+};
+// the blending the chunks inserted now are weighed by, null when they aren't
+blending const *weighed();
+
+} // namespace track_blend