如何使用 OpenGL、SDL 和 C++ 绘制文本？

要使用OpenGL、SDL和C++绘制文本，您需要使用一种名为FreeType的库。FreeType是一个用于处理字体文件的常用库，可以轻松地将字体转换为可在OpenGL和SDL中使用的文本纹理。以下是一个简单的示例，说明如何使用这些库绘制文本：

首先，确保您已经安装了FreeType库。如果没有，请访问FreeType官方网站（https://www.freetype.org/）以获取有关如何安装和使用库的详细信息。
在您的C++项目中，包含必要的头文件：

#include <GL/glew.h>
#include <SDL.h>
#include <SDL_opengl.h>
#include <freetype2/ft2build.h>
#include FT_FREETYPE_H

初始化SDL和OpenGL窗口：

SDL_Init(SDL_INIT_VIDEO);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);

SDL_Window* window = SDL_CreateWindow("Text Rendering", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, SDL_WINDOW_OPENGL | SDL_WINDOW_SHOWN);
SDL_GLContext glContext = SDL_GL_CreateContext(window);

GLenum status = glewInit();
if (status != GLEW_OK) {
    printf("GLEW initialization failed\n");
}

初始化FreeType库并加载字体：

FT_Library ft;
if (FT_Init_FreeType(&ft)) {
    std::cout << "Error: Could not initialize FreeType library"<< std::endl;
}

FT_Face face;
if (FT_New_Face(ft, "path/to/font/file.ttf", 0, &face)) {
    std::cout << "Error: Failed to load font"<< std::endl;
}

FT_Set_Pixel_Sizes(face, 0, 48);

创建一个函数来加载字体的纹理：

GLuint load_texture(FT_Face face) {
    GLuint 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);

    return texture;
}

创建一个函数来渲染文本：

void render_text(std::string text, GLuint texture, FT_Face face) {
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    glBindTexture(GL_TEXTURE_2D, texture);

    glColor3f(1.0f, 1.0f, 1.0f);

    float x = 0.0f;
    float y = 0.0f;

    for (char c : text) {
        if (c == '\n') {
            y -= 1.0f;
            x = 0.0f;
        } else {
            FT_Load_Char(face, c, FT_LOAD_RENDER);
            GLuint glyph_texture = load_texture(face);

            float x2 = x + face->glyph->bitmap_left;
            float y2 = y - face->glyph->bitmap_top;
            float w = face->glyph->bitmap.width;
            float h = face->glyph->bitmap.rows;

            glBegin(GL_QUADS);
            glTexCoord2f(0, 0); glVertex2f(x2, y2);
            glTexCoord2f(1, 0); glVertex2f(x2 + w, y2);
            glTexCoord2f(1, 1); glVertex2f(x2 + w, y2 + h);
            glTexCoord2f(0, 1); glVertex2f(x2, y2 + h);
            glEnd();

            x += face->glyph->advance.x >> 6;
            y += face->glyph->advance.y >> 6;
        }
    }

    glDisable(GL_BLEND);
}

在主循环中调用渲染文本函数：

bool running = true;
SDL_Event event;

while (running) {
    while (SDL_PollEvent(&event)) {
        if (event.type == SDL_QUIT) {
            running = false;
        }
    }

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

    render_text("Hello, World!", load_texture(face), face);

    SDL_GL_SwapWindow(window);
}