opengl學習2

着色器

一般編程是頂點着色器。片段着色器。然後把這兩個鏈接一下。
可以寫成一個着色器類

#ifndef SHADER_H
#define SHADER_H

#include <string>
#include <fstream>
#include <sstream>
#include <iostream>
#include <GL/glew.h>

class Shader
{
public:
    GLuint Program;
    // Constructor generates the shader on the fly
    Shader(const GLchar* vertexPath, const GLchar* fragmentPath)
    {
        // 1. Retrieve the vertex/fragment source code from filePath
        std::string vertexCode;
        std::string fragmentCode;
        std::ifstream vShaderFile;
        std::ifstream fShaderFile;
        // ensures ifstream objects can throw exceptions:
        vShaderFile.exceptions(std::ifstream::badbit);
        fShaderFile.exceptions(std::ifstream::badbit);
        try
        {
            // Open files
            vShaderFile.open(vertexPath);
            fShaderFile.open(fragmentPath);
            std::stringstream vShaderStream, fShaderStream;
            // Read file's buffer contents into streams
            vShaderStream << vShaderFile.rdbuf();
            fShaderStream << fShaderFile.rdbuf();
            // close file handlers
            vShaderFile.close();
            fShaderFile.close();
            // Convert stream into string
            vertexCode = vShaderStream.str();
            fragmentCode = fShaderStream.str();
        }
        catch (std::ifstream::failure e)
        {
            std::cout << "ERROR::SHADER::FILE_NOT_SUCCESFULLY_READ" << std::endl;
        }
        const GLchar* vShaderCode = vertexCode.c_str();
        const GLchar * fShaderCode = fragmentCode.c_str();
        // 2. Compile shaders
        GLuint vertex, fragment;
        GLint success;
        GLchar infoLog[512];
        // Vertex Shader
        vertex = glCreateShader(GL_VERTEX_SHADER);
        glShaderSource(vertex, 1, &vShaderCode, NULL);
        glCompileShader(vertex);
        // Print compile errors if any
        glGetShaderiv(vertex, GL_COMPILE_STATUS, &success);
        if (!success)
        {
            glGetShaderInfoLog(vertex, 512, NULL, infoLog);
            std::cout << "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog << std::endl;
        }
        // Fragment Shader
        fragment = glCreateShader(GL_FRAGMENT_SHADER);
        glShaderSource(fragment, 1, &fShaderCode, NULL);
        glCompileShader(fragment);
        // Print compile errors if any
        glGetShaderiv(fragment, GL_COMPILE_STATUS, &success);
        if (!success)
        {
            glGetShaderInfoLog(fragment, 512, NULL, infoLog);
            std::cout << "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog << std::endl;
        }
        // Shader Program
        this->Program = glCreateProgram();
        glAttachShader(this->Program, vertex);
        glAttachShader(this->Program, fragment);
        glLinkProgram(this->Program);
        // Print linking errors if any
        glGetProgramiv(this->Program, GL_LINK_STATUS, &success);
        if (!success)
        {
            glGetProgramInfoLog(this->Program, 512, NULL, infoLog);
            std::cout << "ERROR::SHADER::PROGRAM::LINKING_FAILED\n" << infoLog << std::endl;
        }
        // Delete the shaders as they're linked into our program now and no longer necessery
        glDeleteShader(vertex);
        glDeleteShader(fragment);

    }
    // Uses the current shader
    void Use()
    {
        glUseProgram(this->Program);
    }
};

#endif

使用時類似這樣

 Shader ourShader("G:\\LearnOpenGLCN\\Project1\\x64\\Release\\shader.vs", "G:\\LearnOpenGLCN\\Project1\\x64\\Release\\shader.frag");

在gl循環中調用

ourShader.Use();

着色器語言

叫glsl
shader.vs

#version 330 core
layout (location = 0) in vec3 position;
layout (location = 1) in vec3 color;

out vec3 ourColor;

void main()
{
    gl_Position = vec4(position, 1.0f);
    ourColor = color;
}

shader.frag

#version 330 core
in vec3 ourColor;

out vec4 color;

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

例如上面的兩個着色器

#version 330 core
聲明glsl的語言版本，一般要與glfw版本匹配

vecn是一種類型聲明

類型	含義
vecn	包含n個float分量的默認向量
bvecn	包含n個bool分量的向量
ivecn	包含n個int分量的向量
uvecn	包含n個unsigned int分量的向量
dvecn	包含n個double分量的向量

其中n是可變的

着色器之間交流有輸入和輸出
關鍵字是 in 和 out
in代表從外部輸入的
out代表需要輸出的

比如上面ourColor就是從shader.vs傳到shader.frag的 需要注意的是，他們的變量名需要一致

layout (location = 0)設定了輸入變量的位置值，這個位置值是我們規定的,通過glEnableVertexAttribArray();

比如這樣的頂點數組

GLfloat vertices[] = {
        // 位置              // 顏色
         0.5f, -0.5f, 0.0f,  1.0f, 0.0f, 0.0f,   // 右下
        -0.5f, -0.5f, 0.0f,  0.0f, 1.0f, 0.0f,   // 左下
         0.0f,  0.5f, 0.0f,  0.0f, 0.0f, 1.0f    // 頂部
    };

前三個是位置，後三個是顏色

GLuint VBO, VAO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    // Bind the Vertex Array Object first, then bind and set vertex buffer(s) and attribute pointer(s).
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)0);
    glEnableVertexAttribArray(0);
    // 顏色屬性
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)(3 * sizeof(GLfloat)));
    glEnableVertexAttribArray(1);

    glBindBuffer(GL_ARRAY_BUFFER, 0); // Note that this is allowed, the call to glVertexAttribPointer registered VBO as the currently bound vertex buffer object so afterwards we can safely unbind

    glBindVertexArray(0); // Unbind VAO (it's always a good thing to unbind any buffer/array to prevent strange bugs)

glBufferData,將頂點數據初始化到緩衝，後面會有個地方使用glUseProgram正式在着色器中編譯這些數據。glDrawArray正式繪製
通過glVertexAttribPointer分割，通過glEnableVertexAttribArray規定

uniform一個比較特殊的關鍵字
Uniform是一種從CPU中的應用向GPU中的着色器發送數據的方式，但uniform和頂點屬性有些不同。首先，uniform是全局的(Global)。全局意味着uniform變量必須在每個着色器程序對象中都是獨一無二的，而且它可以被着色器程序的任意着色器在任意階段訪問。第二，無論你把uniform值設置成什麼，uniform會一直保存它們的數據，直到它們被重置或更新。

#version 330 core
out vec4 color;

uniform vec4 ourColor; // 在OpenGL程序代碼中設定這個變量

void main()
{
    color = ourColor;
}  

那代碼中怎麼改變這個值呢

比如這樣隨時間改變顏色

GLfloat timeValue = glfwGetTime();
GLfloat greenValue = (sin(timeValue) / 2) + 0.5;
GLint vertexColorLocation = glGetUniformLocation(shaderProgram, "ourColor");
glUseProgram(shaderProgram);
glUniform4f(vertexColorLocation, 0.0f, greenValue, 0.0f, 1.0f);

用glGetUniformLocation在 鏈接頂點着色器和片段的着色器的程序着色器中找到我們定義的uniform，然後給他賦值，需要注意的是一定要在glUseProgram後面給他賦值。