GLFW环境下OpenGL三角形首个顶点未按指定坐标渲染问题

问题描述 #

跟随YouTube上The Cherno的OpenGL教程学习着色器，用C++实现后，渲染三角形时第一个顶点未显示在指定的(-0.5f, 0.5f)坐标，而是出现在(0.0f, 0.0f)。曾怀疑是着色器未正确处理顶点数据，但排查后排除该可能。

错误渲染效果：

使用的GLFW版本为4.6.0 - Build 31.0.101.2111，代码如下：

#include <iostream>
#include <GL/glew.h>
#include <GLFW/glfw3.h>


static unsigned int CompileShader(unsigned int type, const std::string& source) {
    unsigned int id = glCreateShader(type);
    const char* src = source.c_str();
    glShaderSource(id, 1, &src, nullptr);
    glCompileShader(id);

    int result;
    glGetShaderiv(id, GL_COMPILE_STATUS, &result);

    if (!result) {
        int length;
        glGetShaderiv(id, GL_INFO_LOG_LENGTH, &length);
        char* message = (char*) alloca(length * sizeof(char));
        glGetShaderInfoLog(id, length, &length, message);
        std::cerr << "Failed to compile " <<
            (type == GL_VERTEX_SHADER ? "vertex" : type == GL_FRAGMENT_SHADER ? "fragment" : "")
            << " shader!" << '\n';
        std::cerr << message << '\n';
        glDeleteShader(id);
        return 0;
    }

    return id;
}

static unsigned int CreateShader(const std::string& vertexShader, const std::string& fragmentShader) {
    unsigned int program = glCreateProgram();
    unsigned int vs = CompileShader(GL_VERTEX_SHADER, vertexShader);
    unsigned int fs = CompileShader(GL_FRAGMENT_SHADER, fragmentShader);

    glAttachShader(program, vs);
    glAttachShader(program, fs);

    glLinkProgram(program);

    glValidateProgram(program);

    glDeleteShader(vs);
    glDeleteShader(fs);

    return program;
}

int main(void) {
    GLFWwindow* window;

    if (!glfwInit()) return -1;

    window = glfwCreateWindow(840, 680, "Window", NULL, NULL);
    if (!window) {
        glfwTerminate();
        return -1;
    }

    glfwMakeContextCurrent(window);

    if (glewInit() != GLEW_OK) return -1;
    std::cout << glGetString(GL_VERSION) << '\n';

    float positions[6] = {
        -0.5f, 0.5f,
        0.0f, 0.5f,
        0.5f, -0.5f
    };
    unsigned int buffer;
    glGenBuffers(1, &buffer);
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
    glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(float), positions, GL_STATIC_DRAW);

    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), 0);

    std::string vertexShader = R"(
        #version 330 core

        layout(location = 0) in vec4 position;

        void main() {
            gl_Position = position;
        }
    )";

    std::string fragmentShader =R"(
        #version 330 core

        layout(location = 0) out vec4 color;

        void main() {
            color = vec4(1.0, 0.0, 0.0, 1.0);
        }
    )";
    unsigned int shader = CreateShader(vertexShader, fragmentShader);
    glUseProgram(shader);

    while (!glfwWindowShouldClose(window)) {
        glClear(GL_COLOR_BUFFER_BIT);

        glDrawArrays(GL_TRIANGLES, 1, 3);

        glfwSwapBuffers(window);

        glfwPollEvents();
    }

    glfwTerminate();
    return 0;
}

问题原因与解决方法 #

问题出在glDrawArrays的调用参数：

glDrawArrays(GL_TRIANGLES, 1, 3);

第二个参数是起始顶点索引，第三个是要绘制的顶点数量。你的顶点数组只有3个有效顶点（索引0、1、2），从索引1开始取3个顶点时，第三个顶点属于越界访问，OpenGL会用默认的(0.0f, 0.0f, 0.0f, 1.0f)填充这个无效顶点，这就是出现(0,0)顶点的原因。

只需修改glDrawArrays的参数，从索引0开始取3个顶点即可：

glDrawArrays(GL_TRIANGLES, 0, 3);

修改后，OpenGL会正确使用你定义的三个顶点，三角形就能按预期渲染在指定坐标上。

内容的提问来源于stack exchange，提问作者Konstei