Generated glyphs from FreeType containing segments of memory

Question

Implementing text rendering into my game engine using FreeType (2.10.1), I am encountering odd looking glyphs, containing the letter repeated four times, mirrored on the x axis and turned upside down.

Up above the desired letter there seems to be neighboring memory interpreted as glyph, which changes every run and causes a segfault on some launches.

This is what I get when I try to render the word "sphinx" .

Here is the full example sentence "sphinx of black quartz, judge my vow" flipped horizontally and rotated 180 degrees.

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I have compiled this code to rule out my MinGW environment being erroneous .

I'm very sure my usage of OpenGL is not the issue, since my texture uploading and rendering code works for other images.

Currently I'm wrapping the important bits of the FT_Glyph_Slot in a struct called Letter and caching that struct. Removing the wrapping and caching did not fix the error.

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Here are the relevant code snippets:

FreeType initialization.

// src/library/services/graphics/font/FreeType.cpp

void FreeType::initialize() {
    Logger::info("Initializing FreeType");

    if (FT_Init_FreeType(&m_library)) {
        Logger::error("Could not initialize FreeType");

        return;
    }
}

void FreeType::useFont(const std::string& fontName, const unsigned int fontSize = 42) {
    Logger::info("Loading font " + fontName);

    if (FT_New_Face(m_library, fontName.c_str(), 0, &m_currentFace)) {
        Logger::error("Could not open font " + fontName);

        return;
    }

    FT_Set_Pixel_Sizes(m_currentFace, 0, fontSize);
}

The code using FreeType to create a Letter .

// src/library/services/graphics/font/FreeType.cpp

std::shared_ptr<Letter> FreeType::getLetter(unsigned long character) {
    // Try loading from cache
    if (std::shared_ptr<Letter> letter = m_letters.get(std::to_string(character))) {
        return letter;
    }

    return loadLetter(character);
}

std::shared_ptr<Letter> FreeType::loadLetter(unsigned long character) {
    if (FT_Load_Char(m_currentFace, character, FT_LOAD_RENDER)) {
        Logger::error("Could not load character " + std::string(1, character));

        return std::shared_ptr<Letter>();
    }

    FT_GlyphSlot& glyph = m_currentFace->glyph;

    Letter letter = {
            .id = character,
            .textureId = 0,
            .bitmap = {
                    .buffer = glyph->bitmap.buffer,
                    .width = glyph->bitmap.width,
                    .height = glyph->bitmap.rows
            },
            .offset = {
                    .x = glyph->bitmap_left,
                    .y = glyph->bitmap_top
            },
            .advance = {
                    .x = glyph->advance.x,
                    .y = glyph->advance.y
            }
    };

    std::shared_ptr<Letter> sharedLetter = std::make_shared<Letter>(letter);

    cache(sharedLetter);

    return sharedLetter;
}

void FreeType::cache(std::shared_ptr<Letter> letter) {
    m_letters.add(std::to_string(letter->id), letter);
}

The graphics system initializing FreeType

// src/library/services/graphics/opengl/OpenGLGraphics.cpp

void OpenGLGraphics::initialize(int windowWidth, int windowHeight) {
    // ... OpenGL initialization

    m_freeType.initialize();
    m_freeType.useFont("../../../src/library/assets/fonts/OpenSans-Regular.ttf");
}

The code getting the Letter in the text renderer.

// src/library/services/graphics/opengl/OpenGLGraphics.cpp

void OpenGLGraphics::drawText(const std::string &text, Vector2f location) {
    for (auto iterator = text.begin(); iterator < text.end(); ++iterator) {
        std::shared_ptr<Letter> letter = m_freeType.getLetter(*iterator);

        if (!letter->textureId) {
            std::shared_ptr<Texture> tex = 
                ImageLoader::loadFromCharArray(
                    letter->bitmap.buffer, 
                    letter->bitmap.width, 
                    letter->bitmap.height
                );

            letter->textureId = tex->id;
            m_freeType.cache(letter);
        }

    // ... OpenGL text rendering
    }
}

The code for generating a Texture from the bitmap->buffer .

// src/library/services/graphics/opengl/util/ImageLoader.cpp

std::shared_ptr<Texture>
ImageLoader::loadFromCharArray(const unsigned char *image, const unsigned int width, const unsigned int height) {
    std::shared_ptr<Texture> texture = std::make_shared<Texture>();

    texture->width = width;
    texture->height = height;

    glGenTextures(1, &texture->id);

    glBindTexture(GL_TEXTURE_2D, texture->id);

    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, (GLsizei) width, (GLsizei) height, 0, GL_RGBA, GL_UNSIGNED_BYTE, image);

    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_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);

    glGenerateMipmap(GL_TEXTURE_2D);

    glBindTexture(GL_TEXTURE_2D, 0);

    return texture;
}

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If the supplied code snippets should not suffice, I will gladly add more. This project is open source and available here on GitHub .

Answer 1

You assume that FreeType always generates 8-bits-per-channel RGBA images, but it does not.

You need to check bitmap.pixel_mode to see what image format you got.

Usually it will be either FT_PIXEL_MODE_GRAY , meaning 8-bits-per-pixel greyscale, or FT_PIXEL_MODE_MONO , meaning 1-bit-per-pixel monochrome.

See the manual for more details.