How to do OpenGL live text-rendering for a GUI?
I'm implementing a GUI built on top of OpenGL. I came to the problem that each GUI will have -- text rendering. I know of several methods of rendering text in OpenGL, however, I'm wonderin which of them would be best suited for a GUI.
Generally in a GUI we have two types of text -- static and live. Static is easy enough -- we can render a TTF to a texture and forget about it. It's the "live" text that is more bothering me -- imagine console, or a live chat in a multi-player game.
I thought of several options:
- no special cases -- render and load a texture each time the text changes, keeping in mind only to rerender it when actually new text appears, and trying to split the larger text into small parts (like per chat line). However this would still leave us hanging in cases like a score line that changes all the time, or a intro text that renders "per character" (typewriter style seen in some sci-fi games)
- quad-per character -- this also seems a popular solution, you prepare a texture with the ASCII table and render a textured quad character. However, I have serious doubts about the efficiency of such a solution. Tips how to make that faster would also be welcome.
- hybrid solutions -- however I have no idea how to implement that cleanly
The question hence is -- how to render text in OpenGL efficiently?
If this helps, I'm coding in STL/Boost-heavy C++ and aiming at GForce 6 and later graphics cards.
Answer
EDIT2: Sean Barrett just released Bitmap fonts for C/C++ 3D programmers.
EDIT: another code gem that's worth a look is Font Stash which leverages Sean Barrett's stb_truetype.h.
You can create a texture in which you render all the characters of your font. Then you just draw textured quads with orthographic projection and proper texture coordinates: this approach works when your text is in a language that doesn't contain much symbols: like english. The font texture is created once at the beginning of the application and then rendering quads is really fast.
That's what I'm using and I believe it's the fastest way to render text in OpenGL. At first, I used Angelcode's Bitmap Font Generator tool and then I integrated FreeType directly and built a big texture containing all the glyphs at application launch. As for tips to improve the quads rendering speed, I just used VBO as for the rest of the geometry in my application.
I'm surprised you have doubts about quad rendering while you don't seem to worry about the performance of generating a texture on the cpu, then uploading it, then binding it to finally render a rectangle with it, that for each frame. Changing OpenGL states is the bottleneck, not the 500 - 1000 quads you'll use for text.
Also, have a look at libraries like the FTGL library who converts the whole font into polygons (geometric fonts).