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Extended Initialisation

Anton Gerdelan. Last Updated 8 Apr 2016

So, following on from the minimal start-up last time, it's useful to get some statistics on what the computer's video driver can handle, and specify some additional parameters for start-up. Our GL programme will dump a lot of information, which can be a little infuriating to pick through when there's a problem, so we'll start by setting up a log file for capturing GL-related output.

Starting a Log File

Debugging graphical programmes is a huge pain. They don't have functions to print text to the screen any more, and having a console open on the side printing things out can quickly get overwhelming. I strongly suggest starting a "GL log" straight away, so you can load it up to check out what specifications a user's system has, and also debug any problems after the programme has finished.

The actual structure of your log functions will depend on your preferences. I prefer C fprintf() to C++ output streams so I'm going to make something that takes a variable number of arguments like printf() does. You might prefer to stream variables out, cout style. To make functions that take a variable number of arguments; #include <stdarg.h>.

#include 
#include 
#define GL_LOG_FILE "gl.log"

bool restart_gl_log () {
  FILE* file = fopen (GL_LOG_FILE, "w");
  if (!file) {
    fprintf (
      stderr,
      "ERROR: could not open GL_LOG_FILE log file %s for writing\n",
      GL_LOG_FILE
    );
    return false;
  }
  time_t now = time (NULL);
  char* date = ctime (&now);
  fprintf (file, "GL_LOG_FILE log. local time %s\n", date);
  fclose (file);
  return true;
}

This first function just opens the log file and prints the date and time at the top - always handy. It might make sense to print the version number of your code here too. In GCC this would be the built-in strings __DATE__ and __TIME__. Note that after printing to the log file we close it again rather than keep it open.

bool gl_log (const char* message, ...) {
  va_list argptr;
  FILE* file = fopen (GL_LOG_FILE, "a");
  if (!file) {
    fprintf (
      stderr,
      "ERROR: could not open GL_LOG_FILE %s file for appending\n",
      GL_LOG_FILE
    );
    return false;
  }
  va_start (argptr, message);
  vfprintf (file, message, argptr);
  va_end (argptr);
  fclose (file);
  return true;
}

This function is the main log print-out. The "..." parameter is part of C's variable arguments format, and lets us give it any number of parameters, which will be mapped to corresponding string formatting in the message string, just like printf(). We open the file in "a[ppend]" mode, which means adding a line to the existing end of the file, which is what we want, because we just closed it again since we wrote the time at the top. C has a rather funny-looking start and end function for processing the variable arguments. After writing to the file, we close it again. Why? Because if the programme crashes we don't lose our log - the last appended message can be very enlightening.

bool gl_log_err (const char* message, ...) {
  va_list argptr;
  FILE* file = fopen (GL_LOG_FILE, "a");
  if (!file) {
    fprintf (
      stderr,
      "ERROR: could not open GL_LOG_FILE %s file for appending\n",
      GL_LOG_FILE
    );
    return false;
  }
  va_start (argptr, message);
  vfprintf (file, message, argptr);
  va_end (argptr);
  va_start (argptr, message);
  vfprintf (stderr, message, argptr);
  va_end (argptr);
  fclose (file);
  return true;
}

I wrote a slight variation of the log function, specifically for error messages. It's the same, but also prints to the stderr terminal. I usually run OpenGL with the terminal open. If I print my error message to stderr it should pop up as soon as it occurs, which can make it obvious when something has gone wrong.

Start GLFW Again

Error Checks

We can start GLFW in the same way as before, but add some extra checks. This will tell us if we've made a mistake such as calling a GLFW function with the wrong parameters. Before intialising GLFW, we can set up an error callback, which we can use to spit out some error information, then exit the programme. We create a little function for the callback:

void glfw_error_callback (int error, const char* description) {
  gl_log_err ("GLFW ERROR: code %i msg: %s\n", error, description);
}

This will tell is if there was a special problem intialising GLFW. I also put in an assert () to make sure that the log file could be opened. If you want to use this then you'll need to include assert.h.

int main () {
  assert (restart_gl_log ());
  // start GL context and O/S window using the GLFW helper library
  gl_log ("starting GLFW\n%s\n", glfwGetVersionString ());
  // register the error call-back function that we wrote, above
  glfwSetErrorCallback (glfw_error_callback);
  if (!glfwInit ()) {
    fprintf (stderr, "ERROR: could not start GLFW3\n");
    return 1;
  }
...

Setting a Minimum OpenGL Version to Use

Before creating a window with GLFW, we can give it a number of "hints" to set specific window and GL settings. Our primary reason for doing this is to force OpenGL to use at least the minimum version of OpenGL that we are writing our code to support. For example; if we're using tessellation shaders, then we should probably stop the programme from running if the drivers can't support OpenGL 4. If you're using a Mac then this step is necessary - only a limited set of OpenGL implementations are available; 4.1 and 3.3 on Mavericks, and 3.2 on pre-Mavericks. These are also limited to a "forward-compatible, core profile" context - the most conservative set of features with no backwards-compatibility support for features that have been made obsolete. To request the newest of these three versions, we "hint" to the window-creation process that we want OpenGL 3.2 forward-compatible core profile. Yes we can put "3.2", even if we are on Mavericks:

glfwWindowHint (GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint (GLFW_CONTEXT_VERSION_MINOR, 2);
glfwWindowHint (GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
glfwWindowHint (GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

This should make us Mac-compatible. In the previous version of GLFW I had to manually hint the number of bits to use for depth and colour channels on Mac, but it looks like GLFW3 has sensible defaults instead. The "forward compatible" profile disables all of the functionality from previous versions of OpenGL that has been flagged for removal in the future. If you get the OpenGL 3.2 or 4.x Quick Reference Card, then this means that all of the functions in blue font are not available. This future-proofs our code, and there's no other option on Mac. The "compatibility" profile doesn't flag any functions as deprecated, so for this to work we also must enable the "core" profile, which does mark deprecation. Check your GL version printout. Mine now says:

OpenGL version supported 3.2.11903 Core Profile Forward-Compatible Context

You can also use the hinting system to enable things like stereoscopic rendering, if supported by your hardware.

Anti-Aliasing

Whilst we're adding window hints, it's good to know that we can put anti-aliasing hints here too. Even if our textures and colours are nicely filtered, the edges our our meshes and triangles are going to look harsh when drawn diagonally on the screen (we'll see pixels along the edges). OpenGL has a built-in "smoothing" ability that blurs over these parts called multi-sample anti-aliasing. The more "samples" or passes it does, the more smoothed it will look, but it gets more expensive. Set it to "16" before taking screen shots!

glfwWindowHint (GLFW_SAMPLES, 4);

Window Resolution and Full-Screen

To change the resolution, or start in a full-screen window, we can set the parameters of the glfwCreateWindow function. To use full-screen mode we need to tell it which monitor to use, which is a new feature of GLFW 3.0. You can get quite precise control over what renders on the different monitors, which you can read about here. We can just assume that we will use the primary monitor for full-screen mode.

You can ask GLFW to give you a list of supported resolutions and video modes with glfwGetVideoModes() which will be useful for supporting a range of machines. For full-screen we can just use the current resolution, and change our glfwCreateWindow call:

GLFWmonitor* mon = glfwGetPrimaryMonitor ();
const GLFWvidmode* vmode = glfwGetVideoMode (mon);
GLFWwindow* window = glfwCreateWindow (
  vmode->width, vmode->height, "Extended GL Init", mon, NULL
);

Now we can run in full-screen mode! It's a little bit tricky to close the window though - you might want to look at implementing GLFW's keyboard handling to allow an escape key to close the window. Put this at the end of the rendering loop:

if (GLFW_PRESS == glfwGetKey (window, GLFW_KEY_ESCAPE)) {
  glfwSetWindowShouldClose (window, 1);
}

Remember that our loop ends when the window is told to close. You'll find a list of all the key codes and other input handling commands at http://www.glfw.org/docs/latest/group__input.html.

You'll notice GLFW 3.0 has functions for getting and setting the gamma ramp of the monitor itself, which gives you much more control over the range of colours that are output, which was kind of a pain to do before. This is more of an advanced rendering topic so don't worry about this if you're just starting.

If you're running in a window then you'll want to know when the user resizes the window, or if the system does (for example if the window is too big and needs to be squished to fit the menu bars). You can then adjust all your variables to suit the new size.

// keep track of window size for things like the viewport and the mouse cursor
int g_gl_width = 640;
int g_gl_height = 480;

// a call-back function
void glfw_window_size_callback (GLFWwindow* window, int width, int height) {
  g_gl_width = width;
  g_gl_height = height;
  
  /* update any perspective matrices used here */
}

Then we can call: glfwSetWindowSizeCallback (window, glfw_window_size_callback);

You'll notice that if you resize your window that the OpenGL part doesn't scale to fit. We need to update the viewport size. Put this in the rendering loop, just after the glClear () function:

glViewport (0, 0, g_gl_width, g_gl_height);

Printing Parameters from the GL Context

After intialising GLEW we can start to use the GL interface - glGet to print out some more parameters. Most of this information is from previous incarnations of OpenGL, and is no longer useful. Some information is going to be really useful to determine the capabilities of the graphics hardware - how big textures can be, how many textures each shader can use, etc. We can log that here. I called this function right after where I log the GL version being used.

void log_gl_params () {
  GLenum params[] = {
    GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS,
    GL_MAX_CUBE_MAP_TEXTURE_SIZE,
    GL_MAX_DRAW_BUFFERS,
    GL_MAX_FRAGMENT_UNIFORM_COMPONENTS,
    GL_MAX_TEXTURE_IMAGE_UNITS,
    GL_MAX_TEXTURE_SIZE,
    GL_MAX_VARYING_FLOATS,
    GL_MAX_VERTEX_ATTRIBS,
    GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS,
    GL_MAX_VERTEX_UNIFORM_COMPONENTS,
    GL_MAX_VIEWPORT_DIMS,
    GL_STEREO,
  };
  const char* names[] = {
    "GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS",
    "GL_MAX_CUBE_MAP_TEXTURE_SIZE",
    "GL_MAX_DRAW_BUFFERS",
    "GL_MAX_FRAGMENT_UNIFORM_COMPONENTS",
    "GL_MAX_TEXTURE_IMAGE_UNITS",
    "GL_MAX_TEXTURE_SIZE",
    "GL_MAX_VARYING_FLOATS",
    "GL_MAX_VERTEX_ATTRIBS",
    "GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS",
    "GL_MAX_VERTEX_UNIFORM_COMPONENTS",
    "GL_MAX_VIEWPORT_DIMS",
    "GL_STEREO",
  };
  gl_log ("GL Context Params:\n");
  char msg[256];
  // integers - only works if the order is 0-10 integer return types
  for (int i = 0; i < 10; i++) {
    int v = 0;
    glGetIntegerv (params[i], &v);
    gl_log ("%s %i\n", names[i], v);
  }
  // others
  int v[2];
  v[0] = v[1] = 0;
  glGetIntegerv (params[10], v);
  gl_log ("%s %i %i\n", names[10], v[0], v[1]);
  unsigned char s = 0;
  glGetBooleanv (params[11], &s);
  gl_log (m"%s %u\n", names[11], (unsigned int)s);
  gl_log ("-----------------------------\n");
}

Now, if we have a look at the log file after running this (call it after the window creation). My log says:

GL Context Params:
GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS 32
GL_MAX_CUBE_MAP_TEXTURE_SIZE 16384
GL_MAX_DRAW_BUFFERS 8
GL_MAX_FRAGMENT_UNIFORM_COMPONENTS 16384
GL_MAX_TEXTURE_IMAGE_UNITS 16
GL_MAX_TEXTURE_SIZE 16384
GL_MAX_VARYING_FLOATS 128
GL_MAX_VERTEX_ATTRIBS 29
GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS 16
GL_MAX_VERTEX_UNIFORM_COMPONENTS 16384
GL_MAX_VIEWPORT_DIMS 16384 16384
GL_STEREO 0

This tells me that my shader programmes can use 32 different textures each - lots of multi-texturing options with my graphics card here. I can access 16 different textures in the vertex shader, and 16 more in the fragment shader. My laptop can support only 8 textures, so if I want to write programmes that run nicely on both you can see that I would make sure that they don't use more than 8 textures at once. In theory my texture resolution can be up to 16384x16384, but multiply this by 4 bytes, and we see that 16k textures will use up my memory pretty quickly. I might get away with 8224x8224x4 bytes.

The "max uniform components" means that I can send tonnes and tonnes of floats to each shader. Each matrix is going to use 16 floats, and if we're doing hardware skinning we might want to send a complex skeleton of 256 joints - 4096 floats to the vertex shader. So we can say that we have plenty of space there.

"Varying" floats are those sent from the vertex shader to the fragment shaders. Usually these are vectors, so we can say that we can send around 30 4d vectors between shaders. Varyings are computationally expensive, and some devices still have a limit of 16 floats (4 vectors), so it's best to keep these to a minimum.

Vertex attributes are variables loaded from a mesh e.g. vertex points, texture coordinates, normals, per-vertex colours, etc. OpenGL means 4d vectors here. I would struggle to come up with more than about 6 useful per-vertex attributes, so no problem here. Draw buffers is useful for more advanced effects where we want to split the output from our rendering into different images - we can split this into 8 parts. And, sadly, my video card doesn't support stereo rendering.

Monitoring the GL State Machine

If you look at the list for glGet you will see plenty of state queries; "the currently enabled buffer", the "currently active texture slot" etc. OpenGL works on the principle of a state machine. This means that once we set a state (like transparency, for example), it is then globally enabled for all future drawing operations, until we change it again. In GL parlance, setting a state is referred to as "binding" (for buffers of data), "enabling" (for rendering modes), or "using" for shader programmes.

The state machine can be very confusing. Lots of errors in OpenGL programmes come from setting a state by accident, forgetting to unset a state, or mixing up the numbering of different OpenGL indices. Some of the most useful state machine variables can be fetched during run-time. You probably don't need to write a function to log all of these states, but keep in mind that, if it all gets a bit confusing, you can check individual states.

Frame Rate Counter

We can add our familiar drawing (clearing) loop, but at the top call a _update_fps_counter() function which will update the title bar of our window with the number of times this loop draws per second.

while (!glfwWindowShouldClose (window)) {
  _update_fps_counter (window);
  glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glViewport (0, 0, g_gl_width, g_gl_height);
  
  /* DRAW STUFF HERE */
  
  glfwSwapBuffers (window);
  glfwPollEvents ();
  if (GLFW_PRESS == glfwGetKey (window, GLFW_KEY_ESCAPE)) {
    glfwSetWindowShouldClose (window, 1);
  }
}

So, GLFW has a function glfwGetTime which gives us a double-precision floating point number, containing the number of seconds since GLFW started. A lot of OpenGL tutorials use the (notoriously inaccurate) GLUT microseconds timer, but this one seems to do the trick. I didn't want to have floating global variables keeping track of the previous time, so I made it a static double. Working out the frame rate every frame is too accurate, and will incur a small cost as well. So I average the rate accrued over 0.25 seconds (or thereabouts - I assume it never gets slower than 4 frames per second). This gives me a readable result that is still responsive to changes in the scene. I use the glfwSetWindowTitle function to put the rate in the title bar. You may prefer to render this as text on the screen...but we don't have the functionality to do that just yet.

void _update_fps_counter (GLFWwindow* window) {
  static double previous_seconds = glfwGetTime ();
  static int frame_count;
  double current_seconds = glfwGetTime ();
  double elapsed_seconds = current_seconds - previous_seconds;
  if (elapsed_seconds > 0.25) {
    previous_seconds = current_seconds;
    double fps = (double)frame_count / elapsed_seconds;
    char tmp[128];
    sprintf (tmp, "opengl @ fps: %.2f", fps);
    glfwSetWindowTitle (window, tmp);
    frame_count = 0;
  }
  frame_count++;
}

Graphics programmers tend to measure time in milliseconds for comparisons, so you might like to put the frame time in milliseconds next to, or instead of, the frames-per-second count. Keep in mind that the frame rate is not an objective measure of how fast your code is. You can't display faster than the refresh rate of the monitor (around 60Hz or 60FPS). Rendering at 100Hz is therefore not beneficial in the same way as it would be to game logic, which can then compute more time steps per second (more detailed movment paths and stuff). Fast GPU clocks will give you huge numbers when drawing nothing. This doesn't really mean anything that you can compare until you start drawing a more involved scene. Measuring frame rate is useful when optimising more complex scenes. If you are drawing at 30fps in a game with fast-moving animations it will be noticeably bad, but it might be okay in a slightly slower-paced game. You can use the FPS counter to improve rendering techniques to get it back to whatever your programme's reasonable level is. Remember that frame rate is dependent on your particular hardware configuration - you want to look at frame rate on your "minimum spec" machine. That said, on any machine, it can give you a good idea of which techniques are relatively more GPU hungry than others.

The maxim never optimise early has some truth behind it. You can waste a huge amount of your programming time on optimisations that don't end up making much practical improvement - or are for experimental features that end up getting cut from the project. I still catch myself doing this all the time. I think it takes some experience to know how to write graphics (and general) code that is efficient to begin with for whatever platform you are using, and how to be very selective about which areas would genuinely benefit from some hours spent on optimisation work. It would be wise to wait until you know your programme is running too slowly, then use all the profiling and timing tools at your disposal to find how inefficient different conventions and blocks of code are in reality - perhaps we can modify the maxim to optimise selectively.

Extending Further

When we look at shaders next, we will log a lot more information. Lots of bugs will come from mixing up uniforms and attributes sent to shaders, so we will dump all of those identifiers to a log as well.

Problem?

Remember to link GL, glfw, and GLEW. My link path looks like this:

g++ -o demo main.cpp -lglfw -lGLEW -lGL

Remember to initialise GLFW first, then do any parameter setting, then create the window, then start GLEW, then start the drawing loop. Parameter fetching code can go just about anywhere.

Include the GLEW header file before GLFW.