AssimpModelWriter.hpp

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#pragma once
 
#include "MayaFlux/IO/ModelWriter.hpp"
 
namespace MayaFlux::IO {
 
/**
 * @class AssimpModelWriter
 * @brief ModelWriter implementation backed by Assimp::Exporter.
 *
 * Packs one or more Kakshya::MeshData into an aiScene (one aiMesh per
 * MeshData, one aiMaterial per aiMesh sourced from the MeshData's submesh
 * Region: name, material_name, diffuse_path) and calls Assimp::Exporter.
 *
 * Vertex bytes must be in the canonical 60-byte MeshVertex layout
 * (MeshData::layout.stride_bytes == sizeof(Kakshya::MeshVertex)), the same
 * assumption ModelReader/MeshInsertion make on the way in. A MeshData with
 * any other stride is rejected rather than misinterpreted.
 *
 * Mesh-only: this writer targets triangle geometry exclusively. It does not
 * attempt point/line export. Assimp's own topology support for those is
 * format-dependent and, for two formats (STL, Collada), silently drops the
 * geometry entirely rather than erroring. That case is out of scope here by
 * design.
 *
 * format_id resolution (ModelWriteOptions::format_id):
 * Assimp dispatches by an internal format id string, not by extension;
 * several extensions are genuinely ambiguous (.obj with or without a
 * sidecar .mtl; .fbx binary vs. ascii both use the same extension). When
 * format_id is empty, this table supplies a default:
 *
 *   .dae  -> "collada"
 *   .obj  -> "obj"      (with .mtl; pass "objnomtl" explicitly to omit it)
 *   .stl  -> "stlb"      (binary; pass "stl" explicitly for ascii)
 *   .ply  -> "ply"        (ascii; pass "plyb" explicitly for binary)
 *   .3ds  -> "3ds"
 *   .gltf -> "gltf2"      (not the legacy v1 "gltf" format id)
 *   .glb  -> "glb2"
 *   .fbx  -> "fbx"        (binary; pass "fbxa" explicitly for ascii)
 *   .x3d  -> "x3d"
 *   .json -> "assjson"    (Assimp's own scene dump, round-trips through
 *                          Assimp faithfully; not a general DCC target)
 *
 * Textures: only a diffuse texture path is written (AI_MATKEY_TEXTURE,
 * aiTextureType_DIFFUSE), taken verbatim from the source submesh Region's
 * diffuse_path attribute. No image bytes are embedded and no path
 * resolution or copying happens. The caller is responsible for the
 * referenced file existing relative to the exported model, same contract
 * ModelReader expects on the way back in via its TextureResolver.
 */
class MAYAFLUX_API AssimpModelWriter : public ModelWriter {
public:
    AssimpModelWriter() = default;
    ~AssimpModelWriter() override = default;
 
    [[nodiscard]] bool can_write(const std::string& filepath) const override;
 
    bool write(
        const std::string& filepath,
        const std::vector<Kakshya::MeshData>& meshes,
        const ModelWriteOptions& options = {}) override;
 
    [[nodiscard]] std::vector<std::string> get_supported_extensions() const override;
    [[nodiscard]] std::string get_last_error() const override { return m_last_error; }
 
    /**
     * @brief Register this writer with the ModelWriterRegistry.
     *
     * Called from engine/subsystem init. Idempotent.
     */
    static void register_with_registry();
 
private:
    mutable std::string m_last_error;
 
    void set_error(std::string msg) const { m_last_error = std::move(msg); }
};
 
} // namespace MayaFlux::IO