Days    0    1    2    3    4    5    6    7    8
                                                                        
Frame error in ms @ 15  FPS:   0    3    4    4    4    4    4    4    4
Frame error in ms @ 30  FPS:   0    2    2    2    2    2    2   29   29
Frame error in ms @ 60  FPS:   0    1    1    1   14   14   14   16   16
Frame error in ms @ 100 FPS:   0    2    5    5    9    9    9    9    9
                                                                        
Delta should be 66ms, but is: 66   70   62   62   62   62   62   62   62
Delta should be 33ms, but is: 33   31   31   31   31   31   31   62   62
Delta should be 16ms, but is: 16   15   15   15   31   31   31    0    0
Delta should be 9ms,  but is:  9    7   15   15    0    0    0    0    0

      for( int test = 0; test < 4; test++ )
      {
            float fps;
            switch( test )
            {
            case 0: fps = 15.0f; break;
            case 1: fps = 30.0f; break;
            case 2: fps = 60.0f; break;
            case 3: fps = 100.0f; break;
            };
            float frameTime = 1 / fps;
            int frameTimeMS = int(frameTime*1000);
            std::cout << "-= Testing "<<fps<<" FPS =-"<<std::endl;
            std::cout << " actual Delta = "<<frameTimeMS<<"ms"<<std::endl<<std::endl;
            for( int days = 0; days < 10; days++ )
            {
                  float time1 = 60*60*24*days + 0.5f;
                  float time2 = time1 + frameTime;
                  float delta = time2 - time1;
                  float error = fabsf(frameTime - delta);
                  int deltaMS = int(delta * 1000);
                  int errorMS = int(error * 1000);
                  std::cout << std::setprecision(32);
                  std::cout << "after "<<days<<" days:"<<std::endl;
                  std::cout << "mesured Delta = "<<deltaMS<<"ms"<<std::endl;
                  std::cout << "error = "<<errorMS<<"ms"<<std::endl;
                  std::cout << std::endl;
            }
      }

Quote:Original post by Nitage

Quote:If you need to store several days at high precision then a double might be appropriate.

In one week there are 7*24*60*60*1000 milliseconds = 604800000
The maxiumum value of an unsigned 64 bit integer = 18446744073709551616

In other words, a 64 bit integer can store time at millisecond resolution for 585 million years - unless you can think of a scenario where your uptime would be higher(!) a uint64_t would be a good choice.

Quote:Original post by Kylotan
Firstly, there's quite a difference between accumulating gradual errors on one hand, and seeing everything show up as zero or sixty on the other. They don't look like the same problem. Or if they are the same problem, there's an intermediate step which is making matters worse that is nothing really to do with the precision.

Quote:Secondly, the output you've shown is not the output from the test application you wrote! How can we know the values you show are reasonable?

Quote:Thirdly, if delta time is variable, then you have no baseline to compare against so can't make any assumption about error. 7 days or 70 days down the line wouldn't matter.

 /\     /\    /\         /  \ /\/ | /\/  \  /\        \|   |/      \/  \/\ 

/\   /--\       /-\  /\  ||   |  |       | |  ||  ||___|  |_______| |__||_ 

Quote:Original post by Adam_42
If you only need time differences between one frame and the next do the time calculations using integers, and convert the delta to a float at the end. That will always be accurate.

Quote:Original post by Victarus
A little late now, but a single "typedef" would save a lot of hassle here. While going through, it might be worth it to put it in, just in case you want to switch it back later. I find it also makes the code a little more clear -- "TickTime" or something similar is more descriptive than "float", even if they both operate the same.

Quote:Even if you don't have a function in your OS that returns a non-floating point, you could at least use it as the value passed into the subsystems for greater accuracy in those sections -- just convert the returned value to an int before passing it in. Then you'd only have to change to a double in the main loop, since the values being used by the rest of the system are still in the same general range.

Quote:Original post by Hodgman

Quote:Original post by Kylotan
Firstly, there's quite a difference between accumulating gradual errors on one hand, and seeing everything show up as zero or sixty on the other. They don't look like the same problem. Or if they are the same problem, there's an intermediate step which is making matters worse that is nothing really to do with the precision.

My test program demonstrates the problem -
1) store a large number in a float #1
2) store the #1 plus a small number in float #2
3) measure the difference between #2 and #1 - it does not match the original small number.