在OpenGL中垂直翻转FreeType文本

Question

我正在做一个科学项目，我们必须将照相机颠倒过来，然后翻转其他所有东西。但是我似乎找不到垂直翻转Freetype文本的方法。这是现在的样子：

这就是我创建文本的方式：

// Text.cpp
#include "Text.h"

using namespace OpenGL::Rendering::Models;

Text::Text(const std::string& text, OpenGL::Container::Position position,
           int font_size, OpenGL::Container::Color color) {
    m_font_size = font_size;
    m_scale = 1.0;
    m_text = text;

    this->color.r = color.r;
    this->color.g = color.g;
    this->color.b = color.b;
    this->color.a = color.a;

    this->position.x = position.x;
    this->position.y = position.y;
    this->position.z = position.z;

    if (FT_Init_FreeType(&font)) {
        Log()->critical("Could not initalize Freetype library for fonts.");
    }

    if (FT_New_Face(font, "/usr/share/fonts/truetype/ubuntu/Ubuntu-R.ttf", 0,
                    &face)) {
        Log()->critical("Could not load font. File is missing maybe?");
    }

    FT_Set_Char_Size(face, 0, m_font_size * 64, 300, 300);
    FT_Set_Pixel_Sizes(face, 0, m_font_size);
    if (FT_Load_Char(face, 'X', FT_LOAD_RENDER)) {
        Log()->critical(
            "Could not load a test glyph. The font is corrupted maybe?");
    }

    float angle = 0;
    matrix.xx = (FT_Fixed)(cos(angle) * 0x10000L);
    matrix.xy = (FT_Fixed)(-sin(angle) * 0x10000L);
    matrix.yx = (FT_Fixed)(sin(angle) * 0x10000L);
    matrix.yy = (FT_Fixed)(cos(angle) * 0x10000L);
    FT_Set_Transform(face, &matrix, 0);

    for (GLubyte c = 0; c < 128; ++c) {
        if (FT_Load_Char(face, c, FT_LOAD_RENDER)) {
            Log()->critical("Could not load glyph \"{}\"", c);
            continue;
        }

        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);

        glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

        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);

        Character character = {
            texture,
            glm::ivec2(face->glyph->bitmap.width, face->glyph->bitmap.rows),
            glm::ivec2(face->glyph->bitmap_left, face->glyph->bitmap_top),
            face->glyph->advance.x};

        characters.insert(std::pair<GLchar, Character>(c, character));
    }

    FT_Done_Face(face);
    FT_Done_FreeType(font);
}

void Text::create() {
    GLuint vao;
    GLuint vbo;

    glGenVertexArrays(1, &vao);
    glGenBuffers(1, &vbo);

    glBindVertexArray(vao);
    glBindBuffer(GL_ARRAY_BUFFER, vbo);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Container::Vertex) * 6, NULL,
                 GL_DYNAMIC_DRAW);

    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Container::Vertex),
                          (void*)0);

    glEnableVertexAttribArray(1);
    glVertexAttribPointer(
        1, 2, GL_FLOAT, GL_FALSE, sizeof(Container::Vertex),
        (void*)(offsetof(Container::Vertex, Container::Vertex::m_texcoord)));

    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    this->vao = vao;
    this->vbos.push_back(vbo);
    this->set_program(OpenGL::Managers::ShaderManager::get_program("text"));
    this->set_position(position.x, position.y, position.z);
    //this->set_rotation(180, 0, 0, 1);
}

void Text::draw() {
    glUseProgram(this->program);
    glUniformMatrix4fv(glGetUniformLocation(this->program, "model_matrix"), 1,
                       false, &model_matrix[0][0]);
    glUniform4f(glGetUniformLocation(this->program, "text_color"), color.r,
                color.g, color.b, color.a);
    glActiveTexture(GL_TEXTURE0);
    glBindVertexArray(this->vao);

    GLfloat temp_x = 0;
    GLfloat temp_y = 0;

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

        GLfloat xpos = temp_x + ch.bearing.x * m_scale;
        GLfloat ypos = temp_y - (ch.size.y - ch.bearing.y) * m_scale;
        GLfloat zpos = position.z;

        GLfloat w = ch.size.x * m_scale;
        GLfloat h = ch.size.y * m_scale;

        std::vector<Container::Vertex> vertices;
        vertices.push_back(Container::Vertex(glm::vec3(xpos, ypos + h, zpos),
                                             glm::vec2(0.0, 0.0)));
        vertices.push_back(Container::Vertex(glm::vec3(xpos, ypos, zpos),
                                             glm::vec2(0.0, 1.0)));
        vertices.push_back(Container::Vertex(glm::vec3(xpos + w, ypos, zpos),
                                             glm::vec2(1.0, 1.0)));

        vertices.push_back(Container::Vertex(glm::vec3(xpos, ypos + h, zpos),
                                             glm::vec2(0.0, 0.0)));
        vertices.push_back(Container::Vertex(glm::vec3(xpos + w, ypos, zpos),
                                             glm::vec2(1.0, 1.0)));
        vertices.push_back(Container::Vertex(
            glm::vec3(xpos + w, ypos + h, zpos), glm::vec2(1.0, 0.0)));

        glBindTexture(GL_TEXTURE_2D, ch.texture_id);
        glBindBuffer(GL_ARRAY_BUFFER, this->vbos[0]);
        glBufferSubData(GL_ARRAY_BUFFER, 0, sizeof(Container::Vertex) * vertices.size(), &vertices[0]);
        glBindBuffer(GL_ARRAY_BUFFER, 0);
        glDrawArrays(GL_TRIANGLES, 0, 6);
        temp_x += (ch.advance >> 6) * m_scale;
    }

    glBindVertexArray(0);
    glBindTexture(GL_TEXTURE_2D, 0);
}

void Text::set_text(const std::string& a_text) {
    if (!a_text.empty()) {
        m_text = a_text;
    } else {
        Log()->info("Cannot set the text. Input seems to be empty.");
    }
}

std::string Text::get_text() { return m_text; }

void Text::set_color(const Container::Color a_color) {
    color.r = a_color.r;
    color.g = a_color.g;
    color.b = a_color.b;
    color.a = a_color.a;
}

理想的解决方案是在保持方位和填充物的同时翻转每个字形，但是我不知道该怎么做。我的坐标全部以像素为单位，并且我使用的是OpenGL 3.3核心配置文件。

Answer 1

看一下代码，您将每个字符渲染为两个纹理三角形，因此翻转文本非常容易。所需要做的就是更改纹理的UV坐标：

//Flip UV's second value
vertices.push_back(Container::Vertex(glm::vec3(xpos, ypos + h, zpos),
                                         glm::vec2(0.0, 1.0)));
vertices.push_back(Container::Vertex(glm::vec3(xpos, ypos, zpos),
                                         glm::vec2(0.0, 0.0)));
vertices.push_back(Container::Vertex(glm::vec3(xpos + w, ypos, zpos),
                                         glm::vec2(1.0, 0.0)));

vertices.push_back(Container::Vertex(glm::vec3(xpos, ypos + h, zpos),
                                         glm::vec2(0.0, 1.0)));
vertices.push_back(Container::Vertex(glm::vec3(xpos + w, ypos, zpos),
                                         glm::vec2(1.0, 0.0)));
vertices.push_back(Container::Vertex(glm::vec3(xpos + w, ypos + h, zpos), 
                                         glm::vec2(1.0, 1.0)));

除此之外，这不是一种渲染大量文本的有效方法。通常，它是通过将所有字符打包到一个缓冲区中并将字形收集到一个纹理集来完成的。因此，只有一个绘制调用和一个纹理绑定，然后使用UV将特定字形映射到三角形。