使用FreeType在OpenGL中渲染文本出现黑屏问题求助

我尝试在OpenGL中使用cglm（C语言版glm）、glad和glfw显示简单文本，但程序运行后仅出现黑屏。程序编译运行无错误，代码如下：

#include <iostream>
#include <map>
#include <string>
#include <vendor/glad/glad.h>
#include <vendor/GLFW/glfw3.h>
#include <vendor/cglm/cglm.h>
#include <ft2build.h>
#include FT_FREETYPE_H


// settings
const unsigned int SCR_WIDTH = 800;
const unsigned int SCR_HEIGHT = 600;

const char *vertexShaderSource =
    "#version 330 core\n"
    "layout (location = 0) in vec4 vertex; // <vec2 pos, vec2 tex>\n"
    "out vec2 TexCoords;\n"
    "uniform mat4 projection;\n"
    "void main()\n"
    "{\n"
    "    gl_Position = projection * vec4(vertex.xy, 0.0, 1.0);\n"
    "    TexCoords = vertex.zw;\n"
    "};\n";
const char *fragmentShaderSource =
    "#version 330 core\n"
    "in vec2 TexCoords;\n"
    "out vec4 color;\n"
    "uniform sampler2D text;\n"
    "uniform vec3 textColor;\n"
    "void main()\n"
    "{\n"
    "    vec4 sampled = vec4(1.0, 1.0, 1.0, texture(text, TexCoords).r);\n"
    "    color = vec4(textColor, 1.0) * sampled;\n"
    "};\n";

typedef struct my_vec3
{
    float x;
    float y;
    float z;
} my_vec3;

typedef struct my_vec2
{
    int x;
    int y;
} my_vec2;

typedef struct Character
{
    unsigned int TextureID; // ID handle of the glyph texture
    my_vec2 Size;           // Size of glyph
    my_vec2 Bearing;        // Offset from baseline to left/top of glyph
    unsigned int Advance;   // Horizontal offset to advance to next glyph
} Character;


void framebuffer_size_callback(GLFWwindow *window, int width, int height);
void processInput(GLFWwindow *window);
void RenderText(GLuint shader, std::string text, float x, float y, float scale, my_vec3 color);
unsigned int VAO, VBO;
std::map<GLchar, Character> Characters;

int main()
{
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow *window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "LearnOpenGL", NULL, NULL);
    if (window == NULL)
    {
        printf("Failed to create GLFW window");
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        printf("Failed to initialize GLAD");
        return -1;
    }
    glEnable(GL_CULL_FACE);
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    unsigned int vertexShader = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
    glCompileShader(vertexShader);
    int success;
    char infoLog[512];
    glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);
        printf("ERROR::SHADER::VERTEX::COMPILATION_FAILED %s\n", infoLog);
    }
    unsigned int fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
    glCompileShader(fragmentShader);
    glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
    if (!success)
    {
        glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);
        printf("ERROR::SHADER::FRAGMENT::COMPILATION_FAILED %s\n", infoLog);
    }
    unsigned int shaderProgram = glCreateProgram();
    glAttachShader(shaderProgram, vertexShader);
    glAttachShader(shaderProgram, fragmentShader);
    glLinkProgram(shaderProgram);
    glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
    if (!success)
    {
        glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
        printf("ERROR::SHADER::PROGRAM::LINKING_FAILED %s\n", infoLog);
    }
    glDeleteShader(vertexShader);
    glDeleteShader(fragmentShader);


    mat4 projection;
    glm_mat4_identity(projection);

    glm_ortho(0.0, static_cast<float>(SCR_WIDTH), 0.0f, static_cast<float>(SCR_HEIGHT), 0.01, 100.0, projection);
    // glm_perspective(4.8, (float)SCR_WIDTH/(float)SCR_WIDTH, 0.01, 100.0, projection);
    glUseProgram(shaderProgram);
    glUniformMatrix4fv(glGetUniformLocation(shaderProgram, "projection"), 1, GL_FALSE, &projection[0][0]);

    FT_Library ft;
    if (FT_Init_FreeType(&ft))
    {
        std::cout << "ERROR::FREETYPE: Could not init FreeType Library" << std::endl;
        return -1;
    }

    std::string font_name = "/home/filip/Downloads/arial/arial.ttf";

    FT_Face face;
    if (FT_New_Face(ft, font_name.c_str(), 0, &face))
    {
        std::cout << "ERROR::FREETYPE: Failed to load font" << std::endl;
        return -1;
    }
    else
    {
        FT_Set_Pixel_Sizes(face, 0, 48);

        glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

        for (unsigned char c = 0; c < 128; c++)
        {
            if (FT_Load_Char(face, c, FT_LOAD_RENDER))
            {
                std::cout << "ERROR::FREETYTPE: Failed to load Glyph" << std::endl;
                continue;
            }
            unsigned int texture;
            glGenTextures(1, &texture);
            glBindTexture(GL_TEXTURE_2D, texture);
            glTexImage2D(
                GL_TEXTURE_2D,
                0,
                GL_RED,
                face->glyph->bitmap.width,
                face->glyph->bitmap.rows,
                0,
                GL_RED,
                GL_UNSIGNED_BYTE,
                face->glyph->bitmap.buffer);
            glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
            glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
            glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
            glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
            my_vec2 size;
            size.x = face->glyph->bitmap.width;
            size.y = face->glyph->bitmap.rows;
            my_vec2 bearing;
            bearing.x = face->glyph->bitmap_left;
            bearing.y = face->glyph->bitmap_top;
            Character character = {
                texture,
                size,
                bearing,
                static_cast<unsigned int>(face->glyph->advance.x)};
            Characters.insert(std::pair<char, Character>(c, character));
        }
        glBindTexture(GL_TEXTURE_2D, 0);
    }
    FT_Done_Face(face);
    FT_Done_FreeType(ft);

    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glBindVertexArray(VAO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(float) * 6 * 4, NULL, GL_DYNAMIC_DRAW);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 4 * sizeof(float), 0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    while (!glfwWindowShouldClose(window))
    {
        processInput(window);

        glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);

        my_vec3 color1;
        color1.x = 0.5;
        color1.y = 0.8;
        color1.z = 0.2;

        my_vec3 color2;
        color2.x = 0.3;
        color2.y = 0.7;
        color2.z = 0.9;
        RenderText(shaderProgram, "This is sample text", 125.0f, 125.0f, 1.0f, color1);
        RenderText(shaderProgram, "(C) LearnOpenGL123.com", 370.0f, 370.0f, 0.5f, color2);
        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glfwTerminate();
    return 0;
}

void processInput(GLFWwindow *window)
{
    if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
        glfwSetWindowShouldClose(window, true);
}

void framebuffer_size_callback(GLFWwindow *window, int width, int height)
{
    glViewport(0, 0, width, height);
}

void RenderText(GLuint shader, std::string text, float x, float y, float scale, my_vec3 color)
{
    glUseProgram(shader);
    glUniform3f(glGetUniformLocation(shader, "textColor"), color.x, color.y, color.z);
    glActiveTexture(GL_TEXTURE0);
    glBindVertexArray(VAO);

    std::string::const_iterator c;
    for (c = text.begin(); c != text.end(); c++)
    {
        Character ch = Characters[*c];

        float xpos = x + ch.Bearing.x * scale;
        float ypos = y - (ch.Size.y - ch.Bearing.y) * scale;

        float w = ch.Size.x * scale;
        float h = ch.Size.y * scale;
        float vertices[6][4] = {
            {xpos, ypos + h, 0.0f, 0.0f},
            {xpos, ypos, 0.0f, 1.0f},
            {xpos + w, ypos, 1.0f, 1.0f},

            {xpos, ypos + h, 0.0f, 0.0f},
            {xpos + w, ypos, 1.0f, 1.0f},
            {xpos + w, ypos + h, 1.0f, 0.0f}};
        glBindTexture(GL_TEXTURE_2D, ch.TextureID);
        glBindBuffer(GL_ARRAY_BUFFER, VBO);
        glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(vertices), vertices); // be sure to use glBufferSubData and not glBufferData

        glBindBuffer(GL_ARRAY_BUFFER, 0);

        glDrawArrays(GL_TRIANGLES, 0, 6);
        x += (ch.Advance >> 6) * scale; // bitshift by 6 to get value in pixels (2^6 = 64 (divide amount of 1/64th pixels by 64 to get amount of pixels))
    }
    glBindVertexArray(0);
    glBindTexture(GL_TEXTURE_2D, 0);
}

问题排查与解决 #

• 正交矩阵方向错误：当前glm_ortho的y轴范围是从0到窗口高度，而OpenGL的NDC坐标系中y轴向上，FreeType的字形基线在下方，这会导致文本被渲染到视口外。修正正交矩阵的y轴范围：

  glm_ortho(0.0f, static_cast<float>(SCR_WIDTH), static_cast<float>(SCR_HEIGHT), 0.0f, 0.01f, 100.0f, projection);
  

• 纹理采样器未绑定：片段着色器中的sampler2D text默认使用纹理单元0，但需要显式设置该uniform的值。在设置投影矩阵后添加：

  glUniform1i(glGetUniformLocation(shaderProgram, "text"), 0);
  

• 字体路径验证：确认/home/filip/Downloads/arial/arial.ttf路径正确，若FreeType加载字体失败，会跳过字形纹理生成，导致无内容可渲染。可在FT_New_Face成功后添加日志输出验证。

• 渲染状态检查：确保GL_BLEND已正确启用（你的代码中已启用），否则文本的透明通道无法正常显示，可能导致文本被黑屏覆盖。

内容的提问来源于stack exchange，提问作者Filip Vuković