基於PBR繪製球

參考：

https://learnopengl.com/PBR/Theory
https://learnopengl.com/PBR/Lighting

請先看下參考的兩個文章,特別是是第一個，看不懂看這個估計很費勁

重點在片段着色器

效果圖：

截圖工具不怎清楚

主要代碼實現
繪製球：

unsigned int sphereVAO = 0;
unsigned int indexCount;
void renderSphere()
{
    if (sphereVAO == 0)
    {
        glGenVertexArrays(1, &sphereVAO);

        unsigned int vbo, ebo;
        glGenBuffers(1, &vbo);
        glGenBuffers(1, &ebo);

        std::vector<glm::vec3> positions;
        std::vector<glm::vec2> uv;
        std::vector<glm::vec3> normals;
        std::vector<unsigned int> indices;

        const unsigned int X_SEGMENTS = 64;
        const unsigned int Y_SEGMENTS = 64;
        const float PI = 3.14159265359;
        for (unsigned int y = 0; y <= Y_SEGMENTS; ++y)
        {
            for (unsigned int x = 0; x <= X_SEGMENTS; ++x)
            {
                float xSegment = (float)x / (float)X_SEGMENTS;
                float ySegment = (float)y / (float)Y_SEGMENTS;
                float xPos = std::cos(xSegment * 2.0f * PI) * std::sin(ySegment * PI);
                float yPos = std::cos(ySegment * PI);
                float zPos = std::sin(xSegment * 2.0f * PI) * std::sin(ySegment * PI);

                positions.push_back(glm::vec3(xPos, yPos, zPos));
                uv.push_back(glm::vec2(xSegment, ySegment));
                normals.push_back(glm::vec3(xPos, yPos, zPos));
            }
        }

        bool oddRow = false;
        for (int y = 0; y < Y_SEGMENTS; ++y)
        {
            if (!oddRow) // even rows: y == 0, y == 2; and so on
            {
                for (int x = 0; x <= X_SEGMENTS; ++x)
                {
                    indices.push_back(y       * (X_SEGMENTS + 1) + x);
                    indices.push_back((y + 1) * (X_SEGMENTS + 1) + x);
                }
            }
            else
            {
                for (int x = X_SEGMENTS; x >= 0; --x)
                {
                    indices.push_back((y + 1) * (X_SEGMENTS + 1) + x);
                    indices.push_back(y       * (X_SEGMENTS + 1) + x);
                }
            }
            oddRow = !oddRow;
        }
        indexCount = indices.size();

        std::vector<float> data;
        for (int i = 0; i < positions.size(); ++i)
        {
            data.push_back(positions[i].x);
            data.push_back(positions[i].y);
            data.push_back(positions[i].z);
            if (uv.size() > 0)
            {
                data.push_back(uv[i].x);
                data.push_back(uv[i].y);
            }
            if (normals.size() > 0)
            {
                data.push_back(normals[i].x);
                data.push_back(normals[i].y);
                data.push_back(normals[i].z);
            }
        }
        glBindVertexArray(sphereVAO);
        glBindBuffer(GL_ARRAY_BUFFER, vbo);
        glBufferData(GL_ARRAY_BUFFER, data.size() * sizeof(float), &data[0], GL_STATIC_DRAW);
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo);
        glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(unsigned int), &indices[0], GL_STATIC_DRAW);
        float stride = (3 + 2 + 3) * sizeof(float);
        glEnableVertexAttribArray(0);
        glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, stride, (void*)0);
        glEnableVertexAttribArray(1);
        glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, stride, (void*)(3 * sizeof(float)));
        glEnableVertexAttribArray(2);
        glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, stride, (void*)(5 * sizeof(float)));
    }

    glBindVertexArray(sphereVAO);
    glDrawElements(GL_TRIANGLE_STRIP, indexCount, GL_UNSIGNED_INT, 0);
}

讀取着色器

bool ReadShaderFile(const char* pFileName, string& outFile)
{
    ifstream f(pFileName);
    bool ret = false;
    if (f.is_open()) {
        string line;
        while (getline(f, line)) {
            outFile.append(line);
            outFile.append("\n");
        }
        f.close();
        ret = true;
    }
    else {
        printf("read file error!");
    }
    return ret;
}

渲染函數：

static void RenderSceneCB()
{
    glEnable(GL_DEPTH_TEST);
    glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glUseProgram(render_prog);

    glUniform3f(glGetUniformLocation(render_prog, "albedo"),
                0.5f,0.0f,0.0f);
    glUniform1f(glGetUniformLocation(render_prog, "ao"), 0.5f);
//    static float metallic = 0.01;
//    if(metallic<1){
//        metallic += 0.00005;
//    }else{
//        metallic = 0.01;
//    }
    glUniform1f(glGetUniformLocation(render_prog, "metallic"), 0.1);
    static float roughness = 0.01;
    if(roughness<1){
        roughness += 0.00005;
    }else{
        roughness = 0.01;
    }
    glUniform1f(glGetUniformLocation(render_prog, "roughness"), roughness);
    glUniform3f(glGetUniformLocation(render_prog, "camPos"),
                0.0f, 0.0f, 10.0f);
    glUniform3f(glGetUniformLocation(render_prog, "lightPositions[0]"),
                -5.0f, 0.0f, 5.0f);
    glUniform3f(glGetUniformLocation(render_prog, "lightColors[0]"),
                300.0f, 300.0f, 300.0f);

    //  transform the box
    glm::mat4 projection = glm::perspective(100.0f, (GLfloat)(SCR_WIDTH/SCR_HEIGHT), 0.1f, 100.f);
    glm::mat4 view = glm::lookAt(glm::vec3(0.0f, 0.0f, 10.0f),
                     glm::vec3(0.0f, 0.0f, 0.0f),
                     glm::vec3(0.0f, 1.0f, 0.0f));
    glm::mat4 model = mat4(1.0f);

    GLuint projectionidx = glGetUniformLocation(render_prog, "projection");
    glUniformMatrix4fv(projectionidx, 1, GL_FALSE, &projection[0][0]);
    GLuint viewidx = glGetUniformLocation(render_prog, "view");
    glUniformMatrix4fv(viewidx, 1, GL_FALSE, &view[0][0]);
    GLuint modelidx = glGetUniformLocation(render_prog, "model");
    glUniformMatrix4fv(modelidx, 1, GL_FALSE, &model[0][0]);
    renderSphere();

    glutSwapBuffers();
}

頂點着色器：

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec2 aTexCoords;
layout (location = 2) in vec3 aNormal;

out vec2 TexCoords;
out vec3 WorldPos;
out vec3 Normal;

uniform mat4 projection;
uniform mat4 view;
uniform mat4 model;

void main()
{
    TexCoords = aTexCoords;
    WorldPos = vec3(model * vec4(aPos, 1.0));
    Normal = mat3(model) * aNormal;   

    gl_Position =  projection * view * vec4(WorldPos, 1.0);
}

片段着色器：

#version 330 core
out vec4 FragColor;
in vec2 TexCoords;
in vec3 WorldPos;
in vec3 Normal;

// material parameters
uniform vec3 albedo;
uniform float metallic;
uniform float roughness;
uniform float ao;

// lights
uniform vec3 lightPositions[4];
uniform vec3 lightColors[4];

uniform vec3 camPos;

const float PI = 3.14159265359;
// ----------------------------------------------------------------------------
float DistributionGGX(vec3 N, vec3 H, float roughness)
{
    float a = roughness*roughness;
    float a2 = a*a;
    float NdotH = max(dot(N, H), 0.0);
    float NdotH2 = NdotH*NdotH;

    float nom   = a2;
    float denom = (NdotH2 * (a2 - 1.0) + 1.0);
    denom = PI * denom * denom;

    return nom / max(denom, 0.001); // prevent divide by zero for roughness=0.0 and NdotH=1.0
}
// ----------------------------------------------------------------------------
float GeometrySchlickGGX(float NdotV, float roughness)
{
    float r = (roughness + 1.0);
    float k = (r*r) / 8.0;

    float nom   = NdotV;
    float denom = NdotV * (1.0 - k) + k;

    return nom / denom;
}
// ----------------------------------------------------------------------------
float GeometrySmith(vec3 N, vec3 V, vec3 L, float roughness)
{
    float NdotV = max(dot(N, V), 0.0);
    float NdotL = max(dot(N, L), 0.0);
    float ggx2 = GeometrySchlickGGX(NdotV, roughness);
    float ggx1 = GeometrySchlickGGX(NdotL, roughness);

    return ggx1 * ggx2;
}
// ----------------------------------------------------------------------------
vec3 fresnelSchlick(float cosTheta, vec3 F0)
{
    return F0 + (1.0 - F0) * pow(1.0 - cosTheta, 5.0);
}
// ----------------------------------------------------------------------------
void main()
{		
    vec3 N = normalize(Normal);
    vec3 V = normalize(camPos - WorldPos);

    // calculate reflectance at normal incidence; if dia-electric (like plastic) use F0 
    // of 0.04 and if it's a metal, use the albedo color as F0 (metallic workflow)    
    vec3 F0 = vec3(0.04); 
    F0 = mix(F0, albedo, metallic);

    // reflectance equation
    vec3 Lo = vec3(0.0);
    for(int i = 0; i < 1; ++i) 
    {
        // calculate per-light radiance
        vec3 L = normalize(lightPositions[i] - WorldPos);
        vec3 H = normalize(V + L);
        float distance = length(lightPositions[i] - WorldPos);
        float attenuation = 1.0 / (distance * distance);
        vec3 radiance = lightColors[i] * attenuation;

        // Cook-Torrance BRDF
        float NDF = DistributionGGX(N, H, roughness);   
        float G   = GeometrySmith(N, V, L, roughness);      
        vec3 F    = fresnelSchlick(clamp(dot(H, V), 0.0, 1.0), F0);
           
        vec3 nominator    = NDF * G * F; 
        float denominator = 4 * max(dot(N, V), 0.0) * max(dot(N, L), 0.0);
        vec3 specular = nominator / max(denominator, 0.001); // prevent divide by zero for NdotV=0.0 or NdotL=0.0
        
        // kS is equal to Fresnel
        vec3 kS = F;
        // for energy conservation, the diffuse and specular light can't
        // be above 1.0 (unless the surface emits light); to preserve this
        // relationship the diffuse component (kD) should equal 1.0 - kS.
        vec3 kD = vec3(1.0) - kS;
        // multiply kD by the inverse metalness such that only non-metals 
        // have diffuse lighting, or a linear blend if partly metal (pure metals
        // have no diffuse light).
        kD *= 1.0 - metallic;	  

        // scale light by NdotL
        float NdotL = max(dot(N, L), 0.0);        

        // add to outgoing radiance Lo
        Lo += (kD * albedo / PI + specular) * radiance * NdotL;  // note that we already multiplied the BRDF by the Fresnel (kS) so we won't multiply by kS again
    }   
    
    // ambient lighting (note that the next IBL tutorial will replace 
    // this ambient lighting with environment lighting).
    vec3 ambient = vec3(0.03) * albedo * ao;

    vec3 color = ambient + Lo;

    // HDR tonemapping
    color = color / (color + vec3(1.0));
    // gamma correct
    color = pow(color, vec3(1.0/2.2)); 

    FragColor = vec4(color, 1.0);
}