Anton's Research Ramblings

How Textures are Managed in Crongdor the Barbarian

In this post I'm going to show you how I managed textures in the "Crongdor" video game, which I made with OpenGL. Crongdor the Barbarian has 152 textures.

The Limitation of Tutorials

Readers of my OpenGL tutorials e-book often ask me how to manage large collections of assets; shaders, textures, 3d meshes, in larger projects. This is a very valid problem that tutorials usually don't talk about. It's pretty common for students to master loading one asset, but get into trouble when trying to load multiple assets and switch them in and out for drawing different things.

Because OpenGL uses a global state machine with a series of overlapping, and very confusing, handles this makes it actually quite a difficult problem. The other side of the problem is that students tend to make far too much structure with their experimental software, and paint themselves into a corner, so to speak, when they find out that their assumptions about relationships between data weren't convenient for later use.

A Disclaimer on Structure

Most game engines will have some complex structured convention, so my code in general is not really industry standard, but because I just have loose functions and arrays it should be pretty easy to follow and apply to your own structure ideas. The main things that motivate my code structure are:

  1. Writing speed.
  2. Clarity from simplicity.

I dropped the conventional OOP structural stuff quite early in this "deep learning" project because I was tracking my hours in a notebook. After the first thousand hours I had to do some deep thinking about time efficiency - this was a hobby project that I really wanted to get done in my wee small spare hours. It quickly became clear that it was taking me far too long to write structure. My 2~8 hours a night, on nights that I can get them, are sacrosanct. I need to get a feedback buzz that I've made something significant, that works within that window, or I would never have kept motivated for 4+ years on one project. You have to manage your own psychology - I planned all my TODO list tasks around "How can I get this done in 2 hours?". I got really fast after applying that thinking. Far too many programmers get foaming-at-the-mouth furious if I suggest that I don't use all that complex OOP stuff, which I don't really understand. That's a silly attitude to have - I finished a game. I think you should be able to do what works for you, and I encourage you to try keeping an hours diary when you finish a day, with some sort of task estimates.

I Separated Textures from Other Data

Most modern game engines tend to bundle textures with shaders and other shading properties into a higher-level concept called a material. You can do this if you prefer - for me it didn't really appeal. I was quite happy to specify separate shaders and textures for each mesh. I therefore have completely separate shader and texture data arrays and functions. It reduces the complexity of the code a little bit, and makes it easier to render all of the items using a particular shader consecutively, then all of the items within that using the same texture in a row - mimimising state switching. I don't think these things are going to be a major game-changer in a small project - most state switches seem to take microseconds - so personal preference territory.

The Interface

I wrote the game in C++, but write in C-style code (I'll explain why in a future post about build systems). So, I have a header file of texture functions, and one matching implementation file. My header looks like this:

// Crongdor the Barbarian
// Texture loader and manager
// First version Anton Gerdelan, 2013?
// Latest code review 14 Sep 2015


#include "game_utils.h"
#include "gl_utils.h"

// A texture that has already been loaded from file and into GL
struct Loaded_Texture {
  char file_name[256];
  GLuint tex_id;
  bool has_mipmaps;

extern float g_anisotropy_factor;

// wraps activetexture and bindtexture2d to remove redundant calls
void bind_texture (int slot, GLuint tex);

void bind_cube_texture (int slot, GLuint tex);

// reserve memory for texture meta-data and grab GL caps
bool init_texture_manager ();

// free memory, but most importantly print stats on any reallocs/actual usage
// call at end of programme or if textures are unlikely to be used between
// levels
bool free_texture_manager ();

// use stb_image to load a texture 
// NOTE: does dynamic malloc/free inside
bool load_image_to_texture (const char* file_name, GLuint& tex, bool gen_mips,
  bool use_srgb);

const char* get_file_name_of_texture_id (GLuint t_i);

// called when user changes texture filtering quality
void refilter_textures ();


If you're a Java or C++ programmer proper this is the exact same thing as having a "TextureManager" class with some interface methods.

I have a struct which I use to remember some meta-data about each loaded OpenGL texture. I like to keep track of the name of the file each texture was loaded from. This way if I try to re-use the same texture again for something else I can check if it's already loaded and skip wasting time making a duplicate. I remember the OpenGL handle, and also if the texture has mipmaps, which is important to know if the user wants to globally change the texture filtering levels later - mipmapped textures have separate function arguments for this.

The init_texture_manager() and free_texture_manager() functions are called at the start and end of the program. Managing dynamic memory is a pain, and one of my "deep learning" lessons from this, longer, project is that it's much safer to predictably allocate all of your required memory at the start of the program. It's better to ring-fence your memory pool right at the start, and only change later if really, really necessary. There are a few advantages to this:

Initialisation and Shutdown

So the initialisation function does allocation of space, with the freeing function responsible for "All resources present and accounted for, sir!" checking, which comes in handy during the debugging process. Initialisation also asks for some of the relevant OpenGL capabilities of the user's machine. It looks like this:

bool init_texture_manager () {
  game_log ("------------------------------INIT TEXTURE MANAGER---------------"
  if (!g_loaded_textures) {
    game_log ("allocating space for %i textures\n", MAX_TEXTURES);
    g_loaded_textures = (Loaded_Texture*)malloc (MAX_TEXTURES * sizeof (
    g_loaded_texture_count_allocd = MAX_TEXTURES;
    memset (g_loaded_textures, 0, 256 * sizeof (Loaded_Texture));
    g_loaded_texture_count = 0;
  } else {
    game_log ("WARNING: texture space already allocated\n");
  if (!g_loaded_textures) {
    game_log_err ("ERROR: out of memory - can not alloc texture space\n");
    return false;
  glGetFloatv (GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT, &g_max_aniso);
  game_log ("anisotropy max %f\n", g_max_aniso);
  game_log ("anisotropy user %i\n", g_sett.aniso);
  if ((float)g_sett.aniso < g_max_aniso) {
    g_anisotropy_factor = (float)g_sett.aniso;
  } else {
    g_anisotropy_factor = g_max_aniso;
  game_log ("anisotropy set to %f\n", g_anisotropy_factor);
  // create a default texture
  if (!_create_default_texture ()) {
    game_log_err ("ERROR: init_texture_manager failed\n");
    return false;
  return true;

Rather than use printf, I intermittently write messages to a log file. This is mostly so I can get a user to send me a file after the game crashes to see at which stage it blew up, and if there was anything special about that machine's capabilities - "Are all the crashes on one particular GPU or CPU?"

The most interesting thing about the above function is that it calls the curious-looking _create_default_texture().

Bullet-Proofing: The Default Texture

The game with the entire textures folder removed - a pink paradise!

I actually picked up this little tip watching John Romero tweet about coding his Dangerous Dave remake. It's possible that a file fails to load into the game. During development this happens a lot - someone will modify or rename something. You don't want this to stop the game running completely whilst you test something else that you're working on, but you do want an indication that something didn't load properly.

After shipping software this can also happen - usually when a user is trying to run a program off of a networked drive and there's some networking problem. It might also be a file that got corrupted during download, some disk issue, the folder structure got flattened during an unzip, or simply that a user is trying to modify something for fun, and it didn't quite work. The user's perception of the quality of the software is going to be much higher if it will keep running even if some of the parts are missing.

If a texture fails to load from a file it is replaced with this fall-back. If you delete the entire texture folder from Crongdor the game will still run. Some groups use a silly picture for their default texture. I went for a garish hot pink checkerboard that would stand out. I don't load this image from a file - I hard-code the pink-and-black pattern into memory.

bool _create_default_texture () {
  game_log ("creating default texture\n");
  int dt_pixel_c = 16 * 16;
  char* dt_data = (char*)malloc (4 * dt_pixel_c);
  if (!dt_data) {
    game_log_err ("ERROR: out of memory. malloc default texture\n");
    return false;
  // gen RGBA pixels
  for (int i = 0; i < dt_pixel_c * 4; i += 4) {
    int sq_ac = i / 16;
    if ((sq_ac / 2) * 2 == sq_ac) {
      dt_data[i] = 0;
      dt_data[i + 1] = 0;
      dt_data[i + 2] = 0;
      dt_data[i + 3] = (char)255;
    } else {
      dt_data[i] = (char)255;
      dt_data[i + 1] = 0;
      dt_data[i + 2] = (char)255;
      dt_data[i + 3] = (char)255;
    int sq_dn = i / (16 * 16);
    if ((sq_dn / 2) * 2 == sq_dn) {
      dt_data[i] = (char)255 - dt_data[i];
      dt_data[i + 2] = (char)255 - dt_data[i + 2];
  { // GL
    GLuint tex = 0;
    glGenTextures (1, &tex);
    glActiveTexture (GL_TEXTURE0);
    glBindTexture (GL_TEXTURE_2D, tex);
    g_bound_textures[0] = tex;
    glTexImage2D (GL_TEXTURE_2D, 0, GL_RGBA, 16, 16, 0, GL_RGBA,
      GL_UNSIGNED_BYTE, dt_data);
    strcpy (g_loaded_textures[g_loaded_texture_count].file_name, "default");
    g_loaded_textures[g_loaded_texture_count].tex_id = tex;
    g_loaded_textures[g_loaded_texture_count].has_mipmaps = false;
    g_default_texture_index = g_loaded_texture_count;
    game_log ("texture loaded\n");
  free (dt_data);
  dt_data = NULL;
  return true;

Additional State Accounting

Rather than bind textures directly with OpenGL commands I call my own functions. This isn't necessary with small projects, but when you're debugging it's nice to be able to quickly dig into which textures are bound into which active texture units. I added these wrappers in late development when I started getting paranoid about random errors on other machines. I generally dislike function wrappers because they're usually done foolishly and hide important code that should not be hidden.

I keep a GLuint g_bound_textures[32]; variable to track the bindings. You can use glGet...() to check these things too - just as good. I wouldn't track GL states manually in a new project. The downside is that if you use a library or piece of code that doesn't use your wrapping functions then your accounting is wrong, and probably all your texture bindings will break.

If I find I'm trying to make a redundant binding call I increment a counter - I like to know if I'm making lots of pointless bindings per frame. This way I know some restructuring is probably warranted. I would have gotten away without doing this.

Loading Textures

I used stb_image to load images because it's fairly small and easy to use, and can load lossless PNGs. Cartoon-ish graphics can look terrible with lossy compression. The official libpng is not such a nice library to link against. There's no justification whatsoever for using one of the OpenGL-specific image loaders because images aren't OpenGL-specific, and most of them hide important OpenGL commands from you - don't let a library make any OpenGL calls in your software! You need to track them all. You'll see a lot of OpenGL helper libraries using antique OpenGL calls or generating their own mipmaps, which is really slow and usually not what you want anyway. The only downside with stb is that decompressing PNGs with it is pretty slow. It's the slowest part of the program. It might have been worth using a different or even uncompressed image format as disk space is not a big deal these days anyway, and I'm not using that much.

I load textures the same way as in my tutorial code, except that I check if the file has already been loaded first against my meta-data array. If not I'll load it and store its file name in said array.

Re-Filtering Textures

In Crongdor you can fiddle with all the graphics settings!

Users like to be able to fiddle with all the advanced graphics options. It's quite interesting to see, live, how much quality and performance difference different graphics settings make. I have quite an involved graphics sub-menu, and I let the user change the texture filtering level. I think it's worth spending time coding in as much active visualisation as you can during development so you're not stabbing in the dark with performance/quality analysis.

To make this work I have a refilter_textures() function that gets called if menu changes are made. This attempts to set the new anisotropy factor, after checking against the machine's maximum level. It then loops over all of the textures, which I'm tracking in my little meta-data array. I simplify different levels of texture filtering in the menu. What this actually means is defined in my function. Mipmaps have complicated filtering options, so I choose a sensible-ish match if we recorded the texture as having mipmaps.

// called when user changes texture filtering quality
void refilter_textures () {
  if ((float)g_sett.aniso < g_max_aniso) {
    g_anisotropy_factor = (float)g_sett.aniso;
  } else {
    g_anisotropy_factor = g_max_aniso;
  if (!g_gl_started) { return; }
  glActiveTexture (GL_TEXTURE0);
  for (long int i = 0; i < g_loaded_texture_count; i++) {
    glBindTexture (GL_TEXTURE_2D, g_loaded_textures[i].tex_id);
    g_bound_textures[0] = g_loaded_textures[i].tex_id;
    if (g_loaded_textures[i].has_mipmaps) {
      if (g_sett.texf == 2) {
        glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,
      } else if (g_sett.texf == 1) {
        glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,
      } else {
        glTexParameteri (GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER,
    } else {
      if (g_sett.texf > 0) {
      } else {
    if (g_sett.texf > 0) {
    } else {
    if (g_sett.aniso > 0) {

That's it - quite simple really, but with some additional management and accounting functionality.

If you enjoyed this let me know over on Crongdor's Steam Greenlight page!