【Overload游戏引擎分析】画场景网格的Shader

Overload引擎地址： GitHub - adriengivry/Overload: 3D Game engine with editor

一、栅格绘制基本原理

Overload Editor启动之后，场景视图中有栅格线，这个在很多软件中都有。刚开始我猜测它应该是通过绘制线实现的。阅读代码发现，这个栅格的几何网格只有两个三角形面片组成的正方形，使用特殊Shader绘制出来的。

绘制栅格的代码在EditorRenderer.cpp中，代码如下：

cpp 复制代码

void OvEditor::Core::EditorRenderer::RenderGrid(const OvMaths::FVector3& p_viewPos, const OvMaths::FVector3& p_color)
{
    constexpr float gridSize = 5000.0f; // 栅格的总的大小
 
    FMatrix4 model = FMatrix4::Translation({ p_viewPos.x, 0.0f, p_viewPos.z }) * FMatrix4::Scaling({ gridSize * 2.0f, 1.f, gridSize * 2.0f }); // 栅格的模型矩阵
	m_gridMaterial.Set("u_Color", p_color); // 栅格的颜色
	m_context.renderer->DrawModelWithSingleMaterial(*m_context.editorResources->GetModel("Plane"), m_gridMaterial, &model); // 绘制栅格

    // 绘制坐标轴的三条线
    m_context.shapeDrawer->DrawLine(OvMaths::FVector3(-gridSize + p_viewPos.x, 0.0f, 0.0f), OvMaths::FVector3(gridSize + p_viewPos.x, 0.0f, 0.0f), OvMaths::FVector3(1.0f, 0.0f, 0.0f), 1.0f);
    m_context.shapeDrawer->DrawLine(OvMaths::FVector3(0.0f, -gridSize + p_viewPos.y, 0.0f), OvMaths::FVector3(0.0f, gridSize + p_viewPos.y, 0.0f), OvMaths::FVector3(0.0f, 1.0f, 0.0f), 1.0f);
    m_context.shapeDrawer->DrawLine(OvMaths::FVector3(0.0f, 0.0f, -gridSize + p_viewPos.z), OvMaths::FVector3(0.0f, 0.0f, gridSize + p_viewPos.z), OvMaths::FVector3(0.0f, 0.0f, 1.0f), 1.0f);
}

从中看出，先将面片平移到视点的前方，使得三角形始终在视锥体范围内，同时将三角形进行缩放，总的尺寸缩放到10000。然后使用m_gridMaterial材质进行绘制。所谓的材质就是Shader的封装。最后再绘制坐标轴的三条线。

可以使用RenderDoc抓帧，可以验证确实是这么实现的。

二、栅格绘制的Shader代码

绘制栅格的Vertex Shader代码如下：

cpp 复制代码

#version 430 core

layout (location = 0) in vec3 geo_Pos;
layout (location = 1) in vec2 geo_TexCoords;
layout (location = 2) in vec3 geo_Normal;

layout (std140) uniform EngineUBO
{
    mat4    ubo_Model;
    mat4    ubo_View;
    mat4    ubo_Projection;
    vec3    ubo_ViewPos;
    float   ubo_Time;
};

out VS_OUT
{
    vec3 FragPos;
    vec2 TexCoords;
} vs_out;

void main()
{
    vs_out.FragPos      = vec3(ubo_Model * vec4(geo_Pos, 1.0)); // 计算顶点世界坐标系坐标
    vs_out.TexCoords    = vs_out.FragPos.xz;  // 对应的纹理坐标，取对应的世界坐标

    gl_Position = ubo_Projection * ubo_View * vec4(vs_out.FragPos, 1.0); // 计算NDC坐标
}

Vertex Shader的代码相对较简单，有效的输入只有geo_Pos。EngineUBO是OpenGL的UBO变量，传入了模型、视图、投影矩阵。main方法中，计算了三角形的世界坐标系坐标、纹理坐标、输出gl_Position变量。

Fragment Shader的代码如下：

cpp 复制代码

#version 430 core

out vec4 FRAGMENT_COLOR;

layout (std140) uniform EngineUBO
{
    mat4    ubo_Model;
    mat4    ubo_View;
    mat4    ubo_Projection;
    vec3    ubo_ViewPos;
    float   ubo_Time;
};

in VS_OUT
{
    vec3 FragPos;
    vec2 TexCoords;
} fs_in;

uniform vec3 u_Color;

float MAG(float p_lp)
{
  const float lineWidth = 1.0f;

  const vec2 coord       = fs_in.TexCoords / p_lp;
  const vec2 grid        = abs(fract(coord - 0.5) - 0.5) / fwidth(coord);
  const float line       = min(grid.x, grid.y);
  const float lineResult = lineWidth - min(line, lineWidth);

  return lineResult;
}

float Grid(float height, float a, float b, float c)
{
  const float cl   = MAG(a);
  const float ml   = MAG(b);
  const float fl   = MAG(c);

  const float cmit =  10.0f;
  const float cmet =  40.0f;
  const float mfit =  80.0f;
  const float mfet =  160.0f;

  const float df   = clamp((height - cmit) / (cmet - cmit), 0.0f, 1.0f);
  const float dff  = clamp((height - mfit) / (mfet - mfit), 0.0f, 1.0f);

  const float inl  = mix(cl, ml, df);
  const float fnl  = mix(inl, fl, dff);

  return fnl;
}

void main()
{
  const float height = distance(ubo_ViewPos.y, fs_in.FragPos.y);

  const float gridA = Grid(height, 1.0f, 4.0f, 8.0f);
  const float gridB = Grid(height, 4.0f, 16.0f, 32.0f);

  const float grid  = gridA * 0.5f + gridB;

  // const vec2  viewdirW    = ubo_ViewPos.xz - fs_in.FragPos.xz;
  // const float viewdist    = length(viewdirW);
  
  FRAGMENT_COLOR = vec4(u_Color, grid);
}

Fragment shader的代码没有看太明白，需要的时候再分析吧。

三、绘制坐标轴线Shader

相比之下，绘制坐标轴线的Shader就简单太多了。线的顶点使用两个uniform变量传入线的两个顶点，根据gl_VertexID判断使用哪个顶点。FS直接给出颜色。

cpp 复制代码

############ Vertex Shader ###########

#version 430 core

uniform vec3 start;
uniform vec3 end;
uniform mat4 viewProjection;

void main()
{
	vec3 position = gl_VertexID == 0 ? start : end;
    gl_Position = viewProjection * vec4(position, 1.0);
}


########  Fragment Shader #############
#version 430 core

uniform vec3 color;

out vec4 FRAGMENT_COLOR;

void main()
{
	FRAGMENT_COLOR = vec4(color, 1.0);
}

对应CPU端的代码：

cpp 复制代码

void OvRendering::Core::ShapeDrawer::DrawLine(const OvMaths::FVector3& p_start, const OvMaths::FVector3& p_end, const OvMaths::FVector3& p_color, float p_lineWidth)
{
    // 绑定line Shader
	m_lineShader->Bind();

	m_lineShader->SetUniformVec3("start", p_start); // 线的起点
	m_lineShader->SetUniformVec3("end", p_end);     // 线的终点
	m_lineShader->SetUniformVec3("color", p_color); // 线的颜色

    // 绘制线
	m_renderer.SetRasterizationMode(OvRendering::Settings::ERasterizationMode::LINE);
	m_renderer.SetRasterizationLinesWidth(p_lineWidth);
    // 掉Draw call
	m_renderer.Draw(*m_lineMesh, Settings::EPrimitiveMode::LINES);
	m_renderer.SetRasterizationLinesWidth(1.0f);
	m_renderer.SetRasterizationMode(OvRendering::Settings::ERasterizationMode::FILL);

	m_lineShader->Unbind();
}

这里有个m_lineMesh对象，其包含两个随意的顶点即可，只是为了启动两次顶点着色器，真实的顶点坐标是靠uniform传入的。Overload将其全部初始化为0：

cpp 复制代码

std::vector<Geometry::Vertex> vertices;
	vertices.push_back
	({
		0, 0, 0,// 坐标
		0, 0,   // 纹理
		0, 0, 0,// 法线
		0, 0, 0,
		0, 0, 0
	});
	vertices.push_back
	({
		0, 0, 0,
		0, 0,
		0, 0, 0,
		0, 0, 0,
		0, 0, 0
	});

	m_lineMesh = new Resources::Mesh(vertices, { 0, 1 }, 0);