How to draw text using only OpenGL methods?

Theory

Why it is hard

Popular font formats like TrueType and OpenType are vector outline formats: they use Bezier curves to define the boundary of the letter.

Image source.

Transforming those formats into arrays of pixels (rasterization) is too specific and out of OpenGL's scope, specially because OpenGl does not have non-straight primitives (e.g. see Why is there no circle or ellipse primitive in OpenGL?)

The easiest approach is to first raster fonts ourselves on the CPU, and then give the array of pixels to OpenGL as a texture.

OpenGL then knows how to deal with arrays of pixels through textures very well.

Texture atlas

We could raster characters for every frame and re-create the textures, but that is not very efficient, specially if characters have a fixed size.

The more efficient approach is to raster all characters you plan on using and cram them on a single texture.

And then transfer that to the GPU once, and use it texture with custom uv coordinates to choose the right character.

This approach is called a texture atlas and it can be used not only for textures but also other repeatedly used textures, like tiles in a 2D game or web UI icons.

The Wikipedia picture of the full texture, which is itself taken from freetype-gl, illustrates this well:

I suspect that optimizing character placement to the smallest texture problem is an NP-hard problem, see: What algorithm can be used for packing rectangles of different sizes into the smallest rectangle possible in a fairly optimal way?

The same technique is used in web development to transmit several small images (like icons) at once, but there it is called "CSS Sprites": https://css-tricks.com/css-sprites/ and are used to hide the latency of the network instead of that of the CPU / GPU communication.

Non-CPU raster methods

There also exist methods which don't use the CPU raster to textures.

CPU rastering is simple because it uses the GPU as little as possible, but we also start thinking if it would be possible to use the GPU efficiency further.

This FOSDEM 2014 video explains other existing techniques:

• tesselation: convert the font to tiny triangles. The GPU is then really good at drawing triangles. Downsides:
  • generates a bunch of triangles
  • O(n log n) CPU calculation of the triangles
• calculate curves on shaders. A 2005 paper by Blinn-Loop put this method on the map. Downside: complex. See: Resolution independent cubic bezier drawing on GPU (Blinn/Loop)
• direct hardware implementations like OpenVG. Downside: not very widely implemented for some reason. See:
  • OpenGL, OpenVG. Draw text?
  • OpenVG implementation?

Fonts inside of the 3D geometry with perspective

Rendering fonts inside of the 3D geometry with perspective (compared to an orthogonal HUD) is much more complicated, because perspective could make one part of the character much closer to the screen and larger than the other, making an uniform CPU discretization (e.g. raster, tesselation) look bad on the close part. This is actually an active research topic:

• What is state-of-the-art for text rendering in OpenGL as of version 4.1?
• http://www.valvesoftware.com/publications/2007/SIGGRAPH2007_AlphaTestedMagnification.pdf

Distance fields are one of the popular techniques now.

Implementations

The examples that follow were all tested on Ubuntu 15.10.

Because this is a complex problem as discussed previously, most examples are large, and would blow up the 30k char limit of this answer, so just clone the respective Git repositories to compile.

They are all fully open source however, so you can just RTFS.

FreeType solutions

FreeType looks like the dominant open source font rasterization library, so it would allow us to use TrueType and OpenType fonts, making it the most elegant solution.

• https://github.com/rougier/freetype-gl

  Was a set of examples OpenGL and freetype, but is more or less evolving into a library that does it and exposes a decent API.

  In any case, it should already be possible to integrate it on your project by copy pasting some source code.

  It provides both texture atlas and distance field techniques out of the box.

  Demos under: https://github.com/rougier/freetype-gl/tree/master/demos

  Does not have a Debian package, and it a pain to compile on Ubuntu 15.10: https://github.com/rougier/freetype-gl/issues/82#issuecomment-216025527 (packaging issues, some upstream), but it got better as of 16.10.

  Does not have a nice installation method: https://github.com/rougier/freetype-gl/issues/115

  Generates beautiful outputs like this demo:

  

• libdgx https://github.com/libgdx/libgdx/tree/1.9.2/extensions/gdx-freetype

Examples / tutorials:

• a NEHE tutorial: http://nehe.gamedev.net/tutorial/freetype_fonts_in_opengl/24001/
• http://learnopengl.com/#!In-Practice/Text-Rendering mentions it, but I could not find runnable source code
• SO questions:
  • OpenGL font rendering using Freetype2
  • Text not rendering correctly - OpenGL using FreeType2

Other font rasterizers

Those seem less good than FreeType, but may be more lightweight:

• https://github.com/nothings/stb/blob/master/stb_truetype.h
• http://www.angelcode.com/products/bmfont/

Anton's OpenGL 4 Tutorials example 26 "Bitmap fonts"

• tutorial: http://antongerdelan.net/opengl/
• source: https://github.com/capnramses/antons_opengl_tutorials_book/blob/9a117a649ae4d21d68d2b75af5232021f5957aac/26_bitmap_fonts/main.cpp

The font was created by the author manually and stored in a single .png file. Letters are stored in an array form inside the image.

This method is of course not very general, and you would have difficulties with internationalization.

Build with:

make -f Makefile.linux64

Output preview:

opengl-tutorial chapter 11 "2D fonts"

• tutorial: http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-11-2d-text/
• source: https://github.com/opengl-tutorials/ogl/blob/71cad106cefef671907ba7791b28b19fa2cc034d/tutorial11_2d_fonts/tutorial11.cpp

Textures are generated from DDS files.

The tutorial explains how the DDS files were created, using CBFG and Paint.Net.

Output preview:

For some reason Suzanne is missing for me, but the time counter works fine: https://github.com/opengl-tutorials/ogl/issues/15

FreeGLUT

GLUT has glutStrokeCharacter and FreeGLUT is open source... https://github.com/dcnieho/FreeGLUT/blob/FG_3_0_0/src/fg_font.c#L255

OpenGLText

https://github.com/tlorach/OpenGLText

TrueType raster. By NVIDIA employee. Aims for reusability. Haven't tried it yet.

ARM Mali GLES SDK Sample

http://malideveloper.arm.com/resources/sample-code/simple-text-rendering/ seems to encode all characters on a PNG, and cut them from there.

SDL_ttf

Source: https://github.com/cirosantilli/cpp-cheat/blob/d36527fe4977bb9ef4b885b1ec92bd0cd3444a98/sdl/ttf.c

Lives in a separate tree to SDL, and integrates easily.

Does not provide a texture atlas implementation however, so performance will be limited: How to render fonts and text with SDL2 efficiently?

Related threads

• How to do OpenGL live text-rendering for a GUI?
• Text/font rendering in OpenGLES 2 (iOS - CoreText?) - options and best practice?

Drawing text in plain OpenGL isn't a straigth-forward task. You should probably have a look at libraries for doing this (either by using a library or as an example implementation).

Some good starting points could be GLFont, OpenGL Font Survey and NeHe Tutorial for Bitmap Fonts (Windows).

Note that bitmaps are not the only way of achieving text in OpenGL as mentioned in the font survey.