minetest/source/Irrlicht/CGLTFMeshFileLoader.cpp

#include "CGLTFMeshFileLoader.h"
#include "CMeshBuffer.h"
#include "coreutil.h"
#include "IAnimatedMesh.h"
#include "ILogger.h"
#include "IReadFile.h"
#include "irrTypes.h"
#include "os.h"
#include "path.h"
#include "S3DVertex.h"
#include "SAnimatedMesh.h"
#include "SColor.h"
#include "SMesh.h"
#include "vector3d.h"
#include <cstddef>
#include <cstring>
#include <memory>
#include <optional>
#include <stdexcept>
#include <string>
#include <utility>
#include <variant>
#include <vector>

/* Notes on the coordinate system.
 *
 * glTF uses a right-handed coordinate system where +Z is the
 * front-facing axis, and Irrlicht uses a left-handed coordinate
 * system where -Z is the front-facing axis.
 * We convert between them by reflecting the mesh across the X axis.
 * Doing this correctly requires negating the Z coordinate on
 * vertex positions and normals, and reversing the winding order
 * of the vertex indices.
 */

namespace irr {

namespace scene {

CGLTFMeshFileLoader::BufferOffset::BufferOffset(
		const std::vector<unsigned char>& buf,
		const std::size_t offset)
	: m_buf(buf)
	, m_offset(offset)
{
}

CGLTFMeshFileLoader::BufferOffset::BufferOffset(
		const CGLTFMeshFileLoader::BufferOffset& other,
		const std::size_t fromOffset)
	: m_buf(other.m_buf)
	, m_offset(other.m_offset + fromOffset)
{
}

/**
 * Get a raw unsigned char (ubyte) from a buffer offset.
*/
unsigned char CGLTFMeshFileLoader::BufferOffset::at(
		const std::size_t fromOffset) const
{
	return m_buf.at(m_offset + fromOffset);
}

CGLTFMeshFileLoader::CGLTFMeshFileLoader() noexcept
{
}

/**
 * The most basic portion of the code base. This tells irllicht if this file has a .gltf extension.
*/
bool CGLTFMeshFileLoader::isALoadableFileExtension(
		const io::path& filename) const
{
	return core::hasFileExtension(filename, "gltf");
}

/**
 * Entry point into loading a GLTF model.
*/
IAnimatedMesh* CGLTFMeshFileLoader::createMesh(io::IReadFile* file)
{
	if (file->getSize() <= 0) {
		return nullptr;
	}
	std::optional<tiniergltf::GlTF> model = tryParseGLTF(file);
	if (!model.has_value()) {
		return nullptr;
	}

	if (!(model->buffers.has_value()
			&& model->bufferViews.has_value()
			&& model->accessors.has_value()
			&& model->meshes.has_value()
			&& model->nodes.has_value())) {
		return nullptr;
	}

	MeshExtractor parser(std::move(model.value()));
	SMesh* baseMesh(new SMesh {});
	loadPrimitives(parser, baseMesh);

	SAnimatedMesh* animatedMesh(new SAnimatedMesh {});
	animatedMesh->addMesh(baseMesh);
	baseMesh->drop();

	return animatedMesh;
}


/**
 * Load up the rawest form of the model. The vertex positions and indices.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#meshes
 * If material is undefined, then a default material MUST be used.
*/
void CGLTFMeshFileLoader::loadPrimitives(
		const MeshExtractor& parser,
		SMesh* mesh)
{
	for (std::size_t i = 0; i < parser.getMeshCount(); ++i) {
		for (std::size_t j = 0; j < parser.getPrimitiveCount(i); ++j) {
			auto indices = parser.getIndices(i, j);
			auto vertices = parser.getVertices(i, j);

			SMeshBuffer* meshbuf(new SMeshBuffer {});
			meshbuf->append(vertices.data(), vertices.size(),
				indices.data(), indices.size());
			mesh->addMeshBuffer(meshbuf);
			meshbuf->drop();
		}
	}
}

CGLTFMeshFileLoader::MeshExtractor::MeshExtractor(
		const tiniergltf::GlTF& model) noexcept
	: m_model(model)
{
}

CGLTFMeshFileLoader::MeshExtractor::MeshExtractor(
		const tiniergltf::GlTF&& model) noexcept
	: m_model(model)
{
}

/**
 * Extracts GLTF mesh indices into the irrlicht model.
*/
std::vector<u16> CGLTFMeshFileLoader::MeshExtractor::getIndices(
		const std::size_t meshIdx,
		const std::size_t primitiveIdx) const
{
	// FIXME this need not exist. What do we do if it doesn't?
	const auto accessorIdx = getIndicesAccessorIdx(meshIdx, primitiveIdx);
	
	const auto& buf = getBuffer(accessorIdx.value());

	// FIXME check accessor type (which could also be u8 or u32).
	std::vector<u16> indices{};
	const auto count = getElemCount(accessorIdx.value());
	for (std::size_t i = 0; i < count; ++i) {
		std::size_t elemIdx = count - i - 1;
		indices.push_back(readPrimitive<u16>(
			BufferOffset(buf, elemIdx * sizeof(u16))));
	}

	return indices;
}

/**
 * Create a vector of video::S3DVertex (model data) from a mesh & primitive index.
*/
std::vector<video::S3DVertex> CGLTFMeshFileLoader::MeshExtractor::getVertices(
		const std::size_t meshIdx,
		const std::size_t primitiveIdx) const
{
	const auto positionAccessorIdx = getPositionAccessorIdx(
			meshIdx, primitiveIdx);
	std::vector<vertex_t> vertices{};
	vertices.resize(getElemCount(positionAccessorIdx));
	copyPositions(positionAccessorIdx, vertices);

	const auto normalAccessorIdx = getNormalAccessorIdx(
			meshIdx, primitiveIdx);
	if (normalAccessorIdx.has_value()) {
		copyNormals(normalAccessorIdx.value(), vertices);
	}

	const auto tCoordAccessorIdx = getTCoordAccessorIdx(
			meshIdx, primitiveIdx);
	if (tCoordAccessorIdx.has_value()) {
		copyTCoords(tCoordAccessorIdx.value(), vertices);
	}

	return vertices;
}

/**
 * Get the amount of meshes that a model contains.
*/
std::size_t CGLTFMeshFileLoader::MeshExtractor::getMeshCount() const
{
	return m_model.meshes->size();
}

/**
 * Get the amount of primitives that a mesh in a model contains.
*/
std::size_t CGLTFMeshFileLoader::MeshExtractor::getPrimitiveCount(
		const std::size_t meshIdx) const
{
	return m_model.meshes->at(meshIdx).primitives.size();
}

/**
 * Templated buffer reader. Based on type width.
 * This is specifically used to build upon to read more complex data types.
 * It is also used raw to read arrays directly.
 * Basically we're using the width of the type to infer 
 * how big of a gap we have from the beginning of the buffer.
*/
template <typename T>
T CGLTFMeshFileLoader::MeshExtractor::readPrimitive(
		const BufferOffset& readFrom)
{
	unsigned char d[sizeof(T)]{};
	for (std::size_t i = 0; i < sizeof(T); ++i) {
		d[i] = readFrom.at(i);
	}
	T dest;
	std::memcpy(&dest, d, sizeof(dest));
	return dest;
}

/**
 * Read a vector2df from a buffer at an offset.
 * @return vec2 core::Vector2df
*/
core::vector2df CGLTFMeshFileLoader::MeshExtractor::readVec2DF(
		const CGLTFMeshFileLoader::BufferOffset& readFrom)
{
	return core::vector2df(readPrimitive<float>(readFrom),
		readPrimitive<float>(BufferOffset(readFrom, sizeof(float))));

}

/**
 * Read a vector3df from a buffer at an offset.
 * @return vec3 core::Vector3df
*/
core::vector3df CGLTFMeshFileLoader::MeshExtractor::readVec3DF(
		const BufferOffset& readFrom,
		const core::vector3df scale = {1.0f,1.0f,1.0f})
{
	return core::vector3df(
		scale.X * readPrimitive<float>(readFrom),
		scale.Y * readPrimitive<float>(BufferOffset(readFrom, sizeof(float))),
		-scale.Z * readPrimitive<float>(BufferOffset(readFrom, 2 *
		sizeof(float))));
}

/**
 * Streams vertex positions raw data into usable buffer via reference.
 * Buffer: ref Vector<video::S3DVertex>
*/
void CGLTFMeshFileLoader::MeshExtractor::copyPositions(
		const std::size_t accessorIdx,
		std::vector<vertex_t>& vertices) const
{

	const auto& buffer = getBuffer(accessorIdx);
	const auto count = getElemCount(accessorIdx);
	const auto byteStride = getByteStride(accessorIdx);

	for (std::size_t i = 0; i < count; i++) {
		const auto v = readVec3DF(BufferOffset(buffer,
			(byteStride * i)), getScale());
		vertices[i].Pos = v;
	}
}

/**
 * Streams normals raw data into usable buffer via reference.
 * Buffer: ref Vector<video::S3DVertex>
*/
void CGLTFMeshFileLoader::MeshExtractor::copyNormals(
		const std::size_t accessorIdx,
		std::vector<vertex_t>& vertices) const
{
	const auto& buffer = getBuffer(accessorIdx);
	const auto count = getElemCount(accessorIdx);
	
	for (std::size_t i = 0; i < count; i++) {
		const auto n = readVec3DF(BufferOffset(buffer,
			3 * sizeof(float) * i));
		vertices[i].Normal = n;
	}
}

/**
 * Streams texture coordinate raw data into usable buffer via reference.
 * Buffer: ref Vector<video::S3DVertex>
*/
void CGLTFMeshFileLoader::MeshExtractor::copyTCoords(
		const std::size_t accessorIdx,
		std::vector<vertex_t>& vertices) const
{

	const auto& buffer = getBuffer(accessorIdx);
	const auto count = getElemCount(accessorIdx);

	for (std::size_t i = 0; i < count; ++i) {
		const auto t = readVec2DF(BufferOffset(buffer,
			2 * sizeof(float) * i));
		vertices[i].TCoords = t;
	}
}

/**
 * Gets the scale of a model's node via a reference Vector3df.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-node
 * Type: number[3] (tinygltf: vector<double>)
 * Required: NO
 * @returns: core::vector2df
*/
core::vector3df CGLTFMeshFileLoader::MeshExtractor::getScale() const
{
	core::vector3df buffer{1.0f,1.0f,1.0f};
	// FIXME this just checks the first node
	const auto &node = m_model.nodes->at(0);
	// FIXME this does not take the matrix into account
	// (fix: properly map glTF -> Irrlicht node hierarchy)
	if (std::holds_alternative<tiniergltf::Node::TRS>(node.transform)) {
		const auto &trs = std::get<tiniergltf::Node::TRS>(node.transform);
		buffer.X = static_cast<float>(trs.scale[0]);
		buffer.Y = static_cast<float>(trs.scale[1]);
		buffer.Z = static_cast<float>(trs.scale[2]);
	}
	return buffer;
}

/**
 * The number of elements referenced by this accessor, not to be confused with the number of bytes or number of components.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_accessor_count
 * Type: Integer
 * Required: YES
*/
std::size_t CGLTFMeshFileLoader::MeshExtractor::getElemCount(
		const std::size_t accessorIdx) const
{
	return m_model.accessors->at(accessorIdx).count;
}

/**
 * The stride, in bytes, between vertex attributes.
 * When this is not defined, data is tightly packed.
 * When two or more accessors use the same buffer view, this field MUST be defined.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_bufferview_bytestride
 * Required: NO
*/
std::size_t CGLTFMeshFileLoader::MeshExtractor::getByteStride(
		const std::size_t accessorIdx) const
{
	const auto& accessor = m_model.accessors->at(accessorIdx);
	// FIXME this does not work with sparse / zero-initialized accessors
	const auto& view = m_model.bufferViews->at(accessor.bufferView.value());
	return view.byteStride.value_or(accessor.elementSize());
}

/**
 * Specifies whether integer data values are normalized (true) to [0, 1] (for unsigned types) 
 * or to [-1, 1] (for signed types) when they are accessed. This property MUST NOT be set to
 * true for accessors with FLOAT or UNSIGNED_INT component type.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_accessor_normalized
 * Required: NO
*/
bool CGLTFMeshFileLoader::MeshExtractor::isAccessorNormalized(
	const std::size_t accessorIdx) const
{
	const auto& accessor = m_model.accessors->at(accessorIdx);
	return accessor.normalized;
}

/**
 * Walk through the complex chain of the model to extract the required buffer.
 * Accessor -> BufferView -> Buffer
*/
CGLTFMeshFileLoader::BufferOffset CGLTFMeshFileLoader::MeshExtractor::getBuffer(
		const std::size_t accessorIdx) const
{
	const auto& accessor = m_model.accessors->at(accessorIdx);
	// FIXME this does not work with sparse / zero-initialized accessors
	const auto& view = m_model.bufferViews->at(accessor.bufferView.value());
	const auto& buffer = m_model.buffers->at(view.buffer);

	return BufferOffset(buffer.data, view.byteOffset);
}

/**
 * The index of the accessor that contains the vertex indices. 
 * When this is undefined, the primitive defines non-indexed geometry. 
 * When defined, the accessor MUST have SCALAR type and an unsigned integer component type.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_mesh_primitive_indices
 * Type: Integer
 * Required: NO
*/
std::optional<std::size_t> CGLTFMeshFileLoader::MeshExtractor::getIndicesAccessorIdx(
		const std::size_t meshIdx,
		const std::size_t primitiveIdx) const
{
	return m_model.meshes->at(meshIdx).primitives[primitiveIdx].indices;
}

/**
 * The index of the accessor that contains the POSITIONs.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#meshes-overview
 * Type: VEC3 (Float)
 * ! Required: YES (Appears so, needs another pair of eyes to research.)
 * Second pair of eyes says: "When positions are not specified, client implementations SHOULD skip primitive’s rendering"
*/
std::size_t CGLTFMeshFileLoader::MeshExtractor::getPositionAccessorIdx(
		const std::size_t meshIdx,
		const std::size_t primitiveIdx) const
{
	// FIXME position-less primitives should be skipped.
	return m_model.meshes->at(meshIdx).primitives[primitiveIdx]
		.attributes.position.value();
}

/**
 * The index of the accessor that contains the NORMALs.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#meshes-overview
 * Type: VEC3 (Float)
 * ! Required: NO (Appears to not be, needs another pair of eyes to research.)
*/
std::optional<std::size_t> CGLTFMeshFileLoader::MeshExtractor::getNormalAccessorIdx(
		const std::size_t meshIdx,
		const std::size_t primitiveIdx) const
{
	return m_model.meshes->at(meshIdx).primitives[primitiveIdx].attributes.normal;
}

/**
 * The index of the accessor that contains the TEXCOORDs.
 * Documentation: https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#meshes-overview
 * Type: VEC3 (Float)
 * ! Required: YES (Appears so, needs another pair of eyes to research.)
*/
std::optional<std::size_t> CGLTFMeshFileLoader::MeshExtractor::getTCoordAccessorIdx(
		const std::size_t meshIdx,
		const std::size_t primitiveIdx) const
{
	const auto& texcoords = m_model.meshes->at(meshIdx).primitives[primitiveIdx].attributes.texcoord;
	if (!texcoords.has_value())
		return std::nullopt;
	return texcoords->at(0);
}

/**
 * This is where the actual model's GLTF file is loaded and parsed by tiniergltf.
*/
std::optional<tiniergltf::GlTF> CGLTFMeshFileLoader::tryParseGLTF(io::IReadFile* file)
{
	auto size = file->getSize();
	auto buf = std::make_unique<char[]>(size + 1);
	file->read(buf.get(), size);
	// We probably don't need this, but add it just to be sure.
	buf[size] = '\0';
	Json::CharReaderBuilder builder;
    const std::unique_ptr<Json::CharReader> reader(builder.newCharReader());
	Json::Value json;
	JSONCPP_STRING err;
    if (!reader->parse(buf.get(), buf.get() + size, &json, &err)) {
      return std::nullopt;
    }
	try {
		return tiniergltf::GlTF(json);
	}  catch (const std::runtime_error &e) {
		return std::nullopt;
	} catch (const std::out_of_range &e) {
		return std::nullopt;
	}
}

} // namespace scene

} // namespace irr