The Correct Way to Draw Text in OpenGL ES 2
Introduction
OpenGL ES 2, a graphics API widely used in mobile and embedded systems, doesn’t directly support drawing text. It primarily focuses on rendering 2D and 3D primitives. To display text, we need to leverage external libraries or create custom solutions.
Methods for Drawing Text in OpenGL ES 2
Here are some popular methods for displaying text in OpenGL ES 2:
1. Using Text Rendering Libraries
Several libraries offer text rendering functionalities for OpenGL ES 2, including:
– **FreeType**: A widely used library for high-quality font rendering. It provides advanced features like kerning and glyph positioning.
– **stb_truetype**: A lightweight header-only library for loading and rendering TrueType fonts.
– **FTGL**: A wrapper library that simplifies using FreeType within OpenGL ES 2.
– **TextKit**: (iOS specific) A powerful framework provided by Apple for text layout and rendering.
2. Creating Custom Text Rendering
An alternative to using libraries is to build a custom text rendering system. This involves:
– **Loading Fonts:** Reading font data from a file and storing it in memory.
– **Glyph Generation:** Creating textures for each character and mapping them to their respective coordinates.
– **Vertex Buffer Creation:** Constructing vertex buffers that hold the character positions and texture coordinates.
– **Rendering:** Drawing the vertex buffers with the corresponding textures.
Example: Custom Text Rendering using FreeType
This example demonstrates drawing text using the FreeType library:
1. Set up FreeType
– Include the FreeType library header file:
“`cpp
#include
#include FT_FREETYPE_H
“`
– Initialize FreeType:
“`cpp
FT_Library library;
FT_Error error = FT_Init_FreeType(&library);
if (error) {
// Handle FreeType initialization error
}
“`
2. Load Font
– Load the font file:
“`cpp
FT_Face face;
error = FT_New_Face(library, “font.ttf”, 0, &face);
if (error) {
// Handle font loading error
}
“`
3. Generate Glyphs
– Set the font size:
“`cpp
FT_Set_Pixel_Sizes(face, 0, 48); // Set font size to 48 pixels
“`
– Generate textures for each character:
“`cpp
for (int i = 0; i < 256; i++) {
// Load character glyph
error = FT_Load_Char(face, i, FT_LOAD_RENDER);
if (error) {
continue; // Skip if character cannot be loaded
}
// Create a texture for the glyph
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);
// … set texture filtering and wrapping modes …
}
“`
4. Create Vertex Buffer
– Create a vertex buffer for character positions and texture coordinates:
“`cpp
GLfloat vertices[] = {
// … character positions and texture coordinates …
};
“`
5. Rendering
– Bind the vertex buffer and texture:
“`cpp
glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer);
glBindTexture(GL_TEXTURE_2D, texture);
“`
– Draw the vertex buffer:
“`cpp
glDrawArrays(GL_TRIANGLES, 0, 6); // Draw 6 vertices for each character
“`
Comparison of Methods
| Method | Pros | Cons |
|—|—|—|
| Text Rendering Libraries | Easy to use, high quality, feature-rich | Requires external library dependencies, might add overhead |
| Custom Text Rendering | Full control over rendering, minimal dependencies | More complex to implement, requires expertise |
Conclusion
Rendering text in OpenGL ES 2 can be achieved through the use of external libraries or by building a custom system. Choose the approach that best fits your project’s requirements and resources.