QT+OpenGL从顶点着色器传递颜色数据给片元着色器

一、概述

　　目的：

　　　　1.定义顶点数据和颜色数据，并将颜色数据通过顶点着色器传递给片元着色器

　　　　2.对定义的三角形动态进行：平移、颠倒、镜像

二、示例代码

　　1.定义顶点坐标和颜色值。此处颜色和顶点是放到一块的。需要告知GPU如何解析数据，如：哪些是顶点，哪些是颜色数据

float vertices[] = {
    // 位置              // 颜色
     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    // 顶部
};

　　2.定义顶点着色器和片元着色器

//顶点着色器
#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aColor;
uniform float mX;
uniform float mY;
uniform float mXT;
out vec3 ourColor;
void main(void)
{
    gl_Position = vec4(aPos.x*mX+mXT,aPos.y*mY,aPos.z,1.0f);
    ourColor = aColor;
}　

//片元着色器
#version 330 core
out vec4 FragColor;
in vec3 ourColor;
void main(void)
{
    FragColor = vec4(ourColor,1.0f);
}

　　3.编译、链接顶点着色器及片元着色器

GLuint BaseOpenGLWindow::buildAttachShaderAndReturnProgramId(QString vertexResPath, QString fragmentResPath) {
    //编译顶点着色器和片元着色器
    GLuint vertexShader = getShaderId(GL_VERTEX_SHADER, vertexResPath);
    GLuint fragmentShader = getShaderId(GL_FRAGMENT_SHADER, fragmentResPath);
    GLuint programId = glCreateProgram();
    glAttachShader(programId, vertexShader);
    glAttachShader(programId, fragmentShader);
    glLinkProgram(programId);
    getLinkProgramErrorInfo(programId);

    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);
    return programId;
}　

//小程序
programId = buildAttachShaderAndReturnProgramId(":/QtForOpenCV4Tool/shader/vertex_color.vert",
    ":/QtForOpenCV4Tool/shader/vertex_color.frag");

　　4.创建VAO、VBO，把VBO挂到VAO上，并将顶点数据一次性Copy到GPU中，方便后续使用（效率高）

glGenVertexArrays(1, &VAO);
glGenBuffers(1, &VBO);
glBindVertexArray(VAO);

//创建VBO并创建存放顶点数据的内存
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

　　5.告知显卡如何解析顶点数据（顶点和颜色）

//位置数据（参数1：位置对应aPos，参数2：顶点个数，参数3：顶点数据类型，参数4：是否标准化处理，false，参数5：步长，这里一行有6个数据交替，所以这里写6，参数6：偏移量）
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void*)0);
glEnableVertexAttribArray(0);//启用第一个顶点，即aPos
//颜色数据（参数1：位置对应aColor，参数5：步长同上，参数6：偏移量，这里颜色数据和顶点数据之间隔了3个float，所以是3*sizeof(float)）
glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(float), (void*)(3 * sizeof(float)));
glEnableVertexAttribArray(1);//启用第二个顶点即aColor

　　6.顶点着色器定义了三个uniform变量

uniform float mX;
uniform float mY;
uniform float mXT;

　　　　可以通过这三个值从CPU把数据传递给GPU

GLint mLocation = glGetUniformLocation(programId, "mX");
glUniform1f(mLocation, mX);
GLint mLocation1 = glGetUniformLocation(programId, "mY");
glUniform1f(mLocation1, mY);
GLint mLocation2 = glGetUniformLocation(programId, "mXT");
glUniform1f(mLocation2, mXT);

　　7.绘制出想要的三角形

void GLColorDataFromVertexWindow::paintGL() {
    glUseProgram(programId);
    GLint mLocation = glGetUniformLocation(programId, "mX");
    glUniform1f(mLocation, mX);
    GLint mLocation1 = glGetUniformLocation(programId, "mY");
    glUniform1f(mLocation1, mY);
    GLint mLocation2 = glGetUniformLocation(programId, "mXT");
    glUniform1f(mLocation2, mXT);
    switch (type) {
    case 1: {
        mX = -1.0f;
        GLint mLocation = glGetUniformLocation(programId, "mX");
        glUniform1f(mLocation, mX);
        break;
    }
    case 2: {
        mY = -1.0f;
        GLint mLocation = glGetUniformLocation(programId, "mY");
        glUniform1f(mLocation, mY);
    }
          break;
    case 3: {
        mXT = 0.25f;
        GLint mLocation = glGetUniformLocation(programId, "mXT");
        glUniform1f(mLocation, mXT);
    }
          break;
    }
    glBindVertexArray(VAO);
    glDrawArrays(GL_TRIANGLES, 0, 3);
}

　　8.增加点击事件动态给mX、mY、mXT赋值，从而改变三角形的状态

void GLColorDataFromVertexWindow::changeTriStatus(int type) {
    this->type = type;
    mY = 1.0f;
    mX = 1.0f;
    mXT = 0.0f;
    update();
}
QHBoxLayout* hLayout = new QHBoxLayout(this);
QPushButton* btnRed = new QPushButton(this);
btnRed->setText("Y轴颠倒");
QPushButton* btnGreen = new QPushButton(this);
btnGreen->setText("X轴颠倒");
QPushButton* btnBlue = new QPushButton(this);
btnBlue->setText("X轴平移");
hLayout->addWidget(btnRed);
hLayout->addWidget(btnGreen);
hLayout->addWidget(btnBlue);
hLayout->setAlignment(Qt::AlignTop);

connect(btnRed, &QPushButton::clicked, [=]() {
    changeTriStatus(1);
    });

connect(btnGreen, &QPushButton::clicked, [=]() {
    changeTriStatus(2);
    });

connect(btnBlue, &QPushButton::clicked, [=]() {
    changeTriStatus(3);
    });