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Member Since 07 Mar 2002
Online Last Active Today, 05:20 AM

Topics I've Started

SFML 2.1 Sound delay

30 October 2013 - 09:26 AM



[If you want to skip the background, jump to (*)] A friend of mine wants some help making a simple 2D scroller, and I offered myself to help with the programming. I haven't done any game programming in a long time, so I looked around a bit and it looks like C++ with SFML 2.1 is a good place to start: I am very familiar with C++ and the library's design is clean and uses C++ in an idiomatic way.


I have been very happy with what I have tried so far (setting up a render window in full-screen mode, measuring time, handling events, drawing a bunch of sprites either with sf::Sprite or by setting up a sf::VertexArray that draws a bunch of quads in a single call...) but then (*) I tried to play some sounds using sf::Sound and I find that there is a significant lag of perhaps 100 ms (estimated by ear). It's possible I can get away with that, but it sounds like too much of a delay. I am using sf::Sound in a very straight-forward manner, so I don't see the point of posting code.


Is this consistent with what other people experience? Are there any workarounds? Or perhaps I have to use a different library to play sound?


One possibly relevant piece of data: My programming environment is Ubuntu Linux running within VirtualBox, with a recent MacBook Air as the host.





Packing a 3D rotation into 32 bits

05 July 2012 - 07:02 AM

After seeing this thread, I was trying to think of how I would pack a 3D rotation into a fixed number of bits, for instance to store animation data. I wrote a little piece of code with the result and I think it's neat enough to share.

The idea is to encode a unit-length quaternion in a manner analogous to cube mapping, but with one extra dimension and taking advantage of the property that a sign flip doesn't change which rotation is being represented.

One can think of cube mapping as consisting of an encoding of points in a sphere by first indicating which coordinate has largest absolute value and what sign it has (i.e., which of the 6 faces of the axis-aligned cube the point projects to) and then the remaining coordinates divided by the largest one (i.e., what point of the face the point projects to).

In our case, we don't need to encode the sign of the largest component, so we only need to use 2 bits to encode what the largest component is, and we can use the remaining bits to encode the other three components.

I think 32 bits is probably good enough for animation data in a game, and it's convenient that 30 is a multiple of 3, so it's easy to encode the other components. Actually, even if we didn't have that convenience, it wouldn't be a big deal to use a resolution that is not a power of 2, but some integer divisions would be involved in the unpacking code.

Here's the code, together with a main program that generates random rotations and measures how bad the dot product between the original and the packed and unpacked gets (the dot product seems to be > 0.999993, although I haven't made a theorem out of it):
#include <iostream>
#include <cstdlib>
#include <boost/math/quaternion.hpp>

typedef boost::math::quaternion<double> quaternion;

int double_to_int(double x) {
  return static_cast<int>(std::floor(0.5 * (x + 1.0) * 1023.0 + 0.5));

double int_to_double(int x) {
  return (x - 512) * (1.0 / 1023.0) * 2.0;

struct PackedQuaternion {
  // 2 bits to indicate which component was largest
  // 10 bits for each of the other components
  unsigned u;

  PackedQuaternion(quaternion q) {
	int largest_index = 0;
	double largest_component = q.R_component_1();
	if (std::abs(q.R_component_2()) > std::abs(largest_component)) {
	  largest_index = 1;
	  largest_component = q.R_component_2();
	if (std::abs(q.R_component_3()) > std::abs(largest_component)) {
	  largest_index = 2;
	  largest_component = q.R_component_3();
	if (std::abs(q.R_component_4()) > std::abs(largest_component)) {
	  largest_index = 3;
	  largest_component = q.R_component_4();

	q *= 1.0 / largest_component;

	int a = double_to_int(q.R_component_1());
	int b = double_to_int(q.R_component_2());
	int c = double_to_int(q.R_component_3());
	int d = double_to_int(q.R_component_4());

	u = largest_index;
	if (largest_index != 0)
	  u = (u << 10) + a;
	if (largest_index != 1)
	  u = (u << 10) + b;
	if (largest_index != 2)
	  u = (u << 10) + c;
	if (largest_index != 3)
	  u = (u << 10) + d;

  quaternion get() const {
	int largest_index = u >> 30;
	double x = int_to_double((u >> 20) & 1023);
	double y = int_to_double((u >> 10) & 1023);
	double z = int_to_double(u & 1023);

	quaternion result;
	switch (largest_index) {
	case 0:
	  result = quaternion(1.0, x, y, z);
	case 1:
	  result = quaternion(x, 1.0, y, z);
	case 2:
	  result = quaternion(x, y, 1.0, z);
	case 3:
	  result = quaternion(x, y, z, 1.0);

	return result * (1.0 / abs(result));

double rand_U_0_1() {
  return std::rand() / (RAND_MAX + 1.0);

quaternion random_rotation() {
  quaternion result;
  do {
	result = quaternion(rand_U_0_1()*2.0-1.0, rand_U_0_1()*2.0-1.0, rand_U_0_1()*2.0-1.0, rand_U_0_1()*2.0-1.0);
  } while (norm(result) > 1.0);
  return result*(1.0/abs(result));

double dot_product(quaternion q, quaternion p) {
  return q.R_component_1() * p.R_component_1() +
	q.R_component_2() * p.R_component_2() +
	q.R_component_3() * p.R_component_3() +
	q.R_component_4() * p.R_component_4();

int main() {
  double worst_dot_product = 1.0;
  for (int i=0; i<1000000000; ++i) {
	quaternion q = random_rotation();
	PackedQuaternion pq(q);
	quaternion p = pq.get();
	if (dot_product(p,q) < 0)
	  p *= -1.0;
	if (dot_product(p,q) < worst_dot_product) {
	  worst_dot_product = dot_product(p,q);
	  std::cout << i << ' ' << q << ' ' << p << ' ' << worst_dot_product << '\n';

Any comments are welcome, and feel free to use the idea or the code if you find them useful.

Reputation++ for logging in? Really?

10 May 2012 - 07:21 AM


I just saw that I get one reputation point for logging in. To the extent that reputation is worth anything, it is an indicator of how helpful a member is to the community. Whether I check the option to stay logged in or I enter my password every time is not interesting at all and is just polluting the score, making it less informative.

Can we please get rid of this unfortunate feature?

Windows development setup

26 April 2012 - 05:12 PM

I just bought a new laptop and instead of just installing ubuntu on it right away as I usually do, I think I may try to use it with the Windows OS it came with. I haven't used Windows in a long long time and I would like to know if you guys have any advice on how to set it up for C++ development.

Since the laptop comes with an OK graphics card, I would love to try my hand at programming it with OpenCL, and some advice on how to set that up --or just what online guide to use-- would be great.


Python synth library?

15 February 2011 - 08:33 PM

I want to try my hand at procedural music generation. I have plenty of ideas about how to decide which notes to play, so now I just need a library to actually play the notes. I recently learned a bit of Python, so I wouldn't mind playing around with it for this project.

After some minimal looking around online I decided to give pyFluidSynth a try, but I didn't get it to work. Perhaps I could just try harder, but then I figured some people on these forums may already have some experience and might be able to provide some guidance.

So, do you have any advice on which library to use?