C++解析obj模型文件方法介绍

2022-11-13 16:11:42 模型 文件 解析

一、前言

tinyobjloader地址:

传送门

而tinyobjloader库只有一个头文件,可以很方便的读取obj文件。支持材质,不过不支持骨骼动画,vulkan官方教程便是使用的它。不过没有骨骼动画还是有很大的局限性,这里只是分享一下怎么读取材质和拆分网格。

二、中间文件

我抽象了一个ModelObject类表示模型数据,而一个ModelObject包含多个Sub模型,每个Sub模型使用同一材质(有的人称为图元Primitive或DrawCall)。最后我将其保存为文件,这样我的引擎便可直接解析ModelObject文件,而不是再去读obj、fbx等其他文件了。

这一节可以跳过,下一节是真正使用tinyobjloader库。

//一个文件会有多个ModelObject,一个ModelObject根据材质分为多个ModelSub
//注意ModelSub为一个材质,需要读取时合并网格
class ModelObject
{
	friend class VK;
public:
	//从源文件加载模型
	static vector<ModelObject*> Create(string_view path_name);
	void Load(string_view path_name);
	//保存到文件
	void SaveToFile(string_view path_name);
private:
	vector<ModelObjectSub> _allSub; //下标减1 为材质,0为没有材质
	vector<Vertex> _allVertex;//顶点缓存
	vector<uint32_t> _allIndex;//索引缓存
	vector<ModelObjectMaterial> _allMaterial;//所有材质
	//------------------不同格式加载实现--------------------------------
	//obj
	static vector<ModelObject*> _load_obj(string_view path_name);
	static vector<ModelObject*> _load_obj_2(string_view path_name);
};

ModelObjectSub只是表示在索引缓存的一段范围:

//模型三角形范围
struct ModelTriangleRange
{
	ModelTriangleRange() :
		_countTriangle{ 0 },
		_offsetIndex{ 0 }
	{}
	size_t _countTriangle;
	size_t _offsetIndex;
};
//子模型对象 范围
struct ModelObjectSub
{
	ModelTriangleRange _range;
};

而ModelObjectMaterial表示模型材质:

//! 材质
struct Material
{
	glm::vec4 _diffuseAlbedo;//漫反射率
	glm::vec3 _fresnelR0;	//菲涅耳系数
	float _roughness;		//粗糙度
};
//模型对象 材质
struct ModelObjectMaterial
{
	//最后转为Model时,变为可以用的着色器资源
	Material _material;
	string _materialName;
	//路径为空,则表示没有(VK加载时会返回0)
	string _pathTexDiffuse;
	string _pathTexNORMal;
};

三、使用

首先引入头文件:

#define TINYOBJLOADER_IMPLEMENTATioN
#include <tiny_obj_loader.h>

接口原型,将obj文件变为多个ModelObject:

vector<ModelObject*> ModelObject::_load_obj_2(string_view path_name);

取得文件名,和文件所在路径(会自动加载路径下的同名mtl文件,里面包含了材质):

string str_path = string{ path_name };
string str_base = String::EraseFilename(path_name);
const char* filename = str_path.c_str();
const char* basepath = str_base.c_str();

基本数据:

debug(format("开始加载obj文件:{},{}", filename, basepath));
bool triangulate = true;//三角化
tinyobj::attrib_t attrib; // 所有的数据放在这里
std::vector<tinyobj::shape_t> shapes;//子模型
std::vector<tinyobj::material_t> materials;//材质
std::string warn;
std::string err;

加载并打印一些信息:

bool b_read = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err, filename,
	basepath, triangulate);
//打印错误
if (!warn.empty())
	debug_warn(warn);
if (!err.empty()) 
	debug_err(err);
if (!b_read)
{
	debug_err(format("读取obj文件失败:{}", path_name));
	return {};
}
debug(format("顶点数:{}", attrib.vertices.size() / 3));
debug(format("法线数:{}", attrib.normals.size() / 3));
debug(format("UV数:{}", attrib.texcoords.size() / 2));
debug(format("子模型数:{}", shapes.size()));
debug(format("材质数:{}", materials.size()));

这将打印以下数据:

由于obj文件只产生一个ModelObject,我们如下添加一个,并返回顶点、索引、材质等引用,用于后面填充:

//obj只有一个ModelObject
vector<ModelObject*> ret;
ModelObject* model_object = new ModelObject;
std::vector<Vertex>& mo_vertices = model_object->_allVertex;
std::vector<uint32_t>& mo_indices = model_object->_allIndex;
vector<ModelObjectMaterial>& mo_material = model_object->_allMaterial;
ret.push_back(model_object);

首先记录材质信息:

//------------------获取材质-------------------
mo_material.resize(materials.size());
for (size_t i = 0; i < materials.size(); ++i)
{
	tinyobj::material_t m = materials[i];
	debug(format("材质:{},{}", i, m.name));
	ModelObjectMaterial& material = model_object->_allMaterial[i];
	material._materialName = m.name;
	material._material._diffuseAlbedo = { m.diffuse[0], m.diffuse[1], m.diffuse[2], 1.0f };
	material._material._fresnelR0 = { m.specular[0], m.specular[1], m.specular[2] };
	material._material._roughness = ShininessToRoughness(m.shininess);
	if(!m.diffuse_texname.empty())
		material._pathTexDiffuse = str_base + m.diffuse_texname;
	if (!m.normal_texname.empty())
		material._pathTexNormal = str_base + m.normal_texname;
}

这将产生以下输出:

然后遍历shape,按材质记录顶点。这里需要注意的是,一个obj文件有多个shape,每个shape由n个三角面组成。而每个三角形拥有独立的材质编号,所以这里按材质分别记录,而不是一般的合并为整体:

//------------------获取模型-------------------
//按 材质 放入面的顶点
vector<vector<tinyobj::index_t>> all_sub;
all_sub.resize(1 + materials.size());//0为默认
for (size_t i = 0; i < shapes.size(); i++) 
{//每一个子shape
	tinyobj::shape_t& shape = shapes[i];
	size_t num_index = shape.mesh.indices.size();
	size_t num_face = shape.mesh.num_face_vertices.size();
	debug(format("读取子模型:{},{}", i, shape.name));
	debug(format("索引数:{};面数:{}", num_index, num_face));
	//当前mesh下标(每个面递增3)
	size_t index_offset = 0;
	//每一个面
	for (size_t j = 0; j < num_face; ++j)
	{
		int index_mat = shape.mesh.material_ids[j];//每个面的材质
		vector<tinyobj::index_t>& sub_idx = all_sub[1 + index_mat];
		sub_idx.push_back(shape.mesh.indices[index_offset++]);
		sub_idx.push_back(shape.mesh.indices[index_offset++]);
		sub_idx.push_back(shape.mesh.indices[index_offset++]);
	}
}

按材质记录顶点的索引(tinyobj::index_t)后,接下来就是读取顶点的实际数据,并防止重复读取:

//生成子模型,并填入顶点
std::unordered_map<tinyobj::index_t, size_t, hash_idx, equal_idx>
	uniqueVertices;//避免重复插入顶点
size_t i = 0;
for (vector<tinyobj::index_t>& sub_idx : all_sub)
{
	ModelObjectSub sub;
	sub._range._offsetIndex = i;
	sub._range._countTriangle = sub_idx.size() / 3;
	model_object->_allSub.push_back(sub);
	for (tinyobj::index_t& idx : sub_idx)
	{
		auto iter = uniqueVertices.find(idx);
		if (iter == uniqueVertices.end())
		{
			Vertex v;
			//v
			v._pos[0] = attrib.vertices[idx.vertex_index * 3 + 0];
			v._pos[1] = attrib.vertices[idx.vertex_index * 3 + 1];
			v._pos[2] = attrib.vertices[idx.vertex_index * 3 + 2];
			// vt
			v._texCoord[0] = attrib.texcoords[idx.texcoord_index * 2 + 0];
			v._texCoord[1] = attrib.texcoords[idx.texcoord_index * 2 + 1];
			v._texCoord[1] = 1.0f - v._texCoord[1];
			uniqueVertices[idx] = mo_vertices.size();
			mo_indices.push_back((uint32_t)mo_vertices.size());
			mo_vertices.push_back(v);
		}
		else
		{
			mo_indices.push_back((uint32_t)iter->second);
		}
		++i;
	}
}
debug(format("解析obj模型完成:v{},i{}", mo_vertices.size(), mo_indices.size()));
return ret;

上面用到的哈希函数:

struct equal_idx
{
	bool operator()(const tinyobj::index_t& a, const tinyobj::index_t& b) const
	{
		return a.vertex_index == b.vertex_index
			&& a.texcoord_index == b.texcoord_index
			&& a.normal_index == b.normal_index;
	}
};
struct hash_idx 
{
	size_t operator()(const tinyobj::index_t& a) const
	{
		return ((a.vertex_index
			^ a.texcoord_index << 1) >> 1)
			^ (a.normal_index << 1);
	}
};

最后打印出来的数据如下:

对于材质的处理,漫反射贴图即是基本贴图,而法线(凹凸)贴图、漫反射率、菲涅耳系数、光滑度等需要渲染管线支持并与光照计算产生效果。

四、完整代码

可以此处获取最新的源码(我会改用Assimp,并添加骨骼动画、Blinn-Phong光照模型),也可以用后面的:传送门

如果有用,欢迎点赞、收藏、关注,我将更新更多c++相关的文章。

#define TINYOBJLOADER_IMPLEMENTATION
#include <tiny_obj_loader.h>
struct equal_idx
{
	bool operator()(const tinyobj::index_t& a, const tinyobj::index_t& b) const
	{
		return a.vertex_index == b.vertex_index
			&& a.texcoord_index == b.texcoord_index
			&& a.normal_index == b.normal_index;
	}
};
struct hash_idx 
{
	size_t operator()(const tinyobj::index_t& a) const
	{
		return ((a.vertex_index
			^ a.texcoord_index << 1) >> 1)
			^ (a.normal_index << 1);
	}
};
float ShininessToRoughness(float Ypoint)
{
	float a = -1;
	float b = 2;
	float c;
	c = (Ypoint / 100) - 1;
	float D;
	D = b * b - (4 * a * c);
	float x1;
	x1 = (-b + sqrt(D)) / (2 * a);
	return x1;
}
vector<ModelObject*> ModelObject::_load_obj_2(string_view path_name)
{
	string str_path = string{ path_name };
	string str_base = String::EraseFilename(path_name);
	const char* filename = str_path.c_str();
	const char* basepath = str_base.c_str();
	bool triangulate = true;
	debug(format("开始加载obj文件:{},{}", filename, basepath));
	tinyobj::attrib_t attrib; // 所有的数据放在这里
	std::vector<tinyobj::shape_t> shapes;//子模型
	std::vector<tinyobj::material_t> materials;
	std::string warn;
	std::string err;
	bool b_read = tinyobj::LoadObj(&attrib, &shapes, &materials, &warn, &err, filename,
		basepath, triangulate);
	//打印错误
	if (!warn.empty())
		debug_warn(warn);
	if (!err.empty()) 
		debug_err(err);
	if (!b_read)
	{
		debug_err(format("读取obj文件失败:{}", path_name));
		return {};
	}
	debug(format("顶点数:{}", attrib.vertices.size() / 3));
	debug(format("法线数:{}", attrib.normals.size() / 3));
	debug(format("UV数:{}", attrib.texcoords.size() / 2));
	debug(format("子模型数:{}", shapes.size()));
	debug(format("材质数:{}", materials.size()));
	//obj只有一个ModelObject
	vector<ModelObject*> ret;
	ModelObject* model_object = new ModelObject;
	std::vector<Vertex>& mo_vertices = model_object->_allVertex;
	std::vector<uint32_t>& mo_indices = model_object->_allIndex;
	vector<ModelObjectMaterial>& mo_material = model_object->_allMaterial;
	ret.push_back(model_object);
	//------------------获取材质-------------------
	mo_material.resize(materials.size());
	for (size_t i = 0; i < materials.size(); ++i)
	{
		tinyobj::material_t m = materials[i];
		debug(format("材质:{},{}", i, m.name));
		ModelObjectMaterial& material = model_object->_allMaterial[i];
		material._materialName = m.name;
		material._material._diffuseAlbedo = { m.diffuse[0], m.diffuse[1], m.diffuse[2], 1.0f };
		material._material._fresnelR0 = { m.specular[0], m.specular[1], m.specular[2] };
		material._material._roughness = ShininessToRoughness(m.shininess);
		if(!m.diffuse_texname.empty())
			material._pathTexDiffuse = str_base + m.diffuse_texname;
		if (!m.normal_texname.empty())//注意这里凹凸贴图(bump_texname)更常见
			material._pathTexNormal = str_base + m.normal_texname;
	}
	//------------------获取模型-------------------
	//按 材质 放入面的顶点
	vector<vector<tinyobj::index_t>> all_sub;
	all_sub.resize(1 + materials.size());//0为默认
	for (size_t i = 0; i < shapes.size(); i++) 
	{//每一个子shape
		tinyobj::shape_t& shape = shapes[i];
		size_t num_index = shape.mesh.indices.size();
		size_t num_face = shape.mesh.num_face_vertices.size();
		debug(format("读取子模型:{},{}", i, shape.name));
		debug(format("索引数:{};面数:{}", num_index, num_face));
		//当前mesh下标(每个面递增3)
		size_t index_offset = 0;
		//每一个面
		for (size_t j = 0; j < num_face; ++j)
		{
			int index_mat = shape.mesh.material_ids[j];//每个面的材质
			vector<tinyobj::index_t>& sub_idx = all_sub[1 + index_mat];
			sub_idx.push_back(shape.mesh.indices[index_offset++]);
			sub_idx.push_back(shape.mesh.indices[index_offset++]);
			sub_idx.push_back(shape.mesh.indices[index_offset++]);
		}
	}
	//生成子模型,并填入顶点
	std::unordered_map<tinyobj::index_t, size_t, hash_idx, equal_idx>
		uniqueVertices;//避免重复插入顶点
	size_t i = 0;
	for (vector<tinyobj::index_t>& sub_idx : all_sub)
	{
		ModelObjectSub sub;
		sub._range._offsetIndex = i;
		sub._range._countTriangle = sub_idx.size() / 3;
		model_object->_allSub.push_back(sub);
		for (tinyobj::index_t& idx : sub_idx)
		{
			auto iter = uniqueVertices.find(idx);
			if (iter == uniqueVertices.end())
			{
				Vertex v;
				//v
				v._pos[0] = attrib.vertices[idx.vertex_index * 3 + 0];
				v._pos[1] = attrib.vertices[idx.vertex_index * 3 + 1];
				v._pos[2] = attrib.vertices[idx.vertex_index * 3 + 2];
				// vt
				v._texCoord[0] = attrib.texcoords[idx.texcoord_index * 2 + 0];
				v._texCoord[1] = attrib.texcoords[idx.texcoord_index * 2 + 1];
				v._texCoord[1] = 1.0f - v._texCoord[1];
				uniqueVertices[idx] = mo_vertices.size();
				mo_indices.push_back((uint32_t)mo_vertices.size());
				mo_vertices.push_back(v);
			}
			else
			{
				mo_indices.push_back((uint32_t)iter->second);
			}
			++i;
		}
	}
	debug(format("解析obj模型完成:v{},i{}", mo_vertices.size(), mo_indices.size()));
	return ret;
}

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