Flip FreeType text vertically in OpenGL

Question

I'm working on a scientific project, where we have to make the camera upside down, and flip everything else. But I can't seem to find a way to flip Freetype text vertically. Here's how it looks like now:

And this is how I create my texts:

// 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(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 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;
}

The ideal solution is to flip each glyph while keeping bearing and stuff, but I have no idea how to do that. My coordinates are all in pixels, and I'm using OpenGL 3.3 core profile.

Flip FreeType text vertically in OpenGL

Answers (1)

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