DOT implementation

Question

I have some DOT (damage over time) implementation problems. My game runs on 30 FPS speed. Current implementation is: let's say the hero casts a spell which deals 1 damage per second. On every frame I do (pseudo code):

damage_done = getRandomDamage() * delta_time;

I accumulate damage and when it becomes more than 0, I subtract rounded damage from current health and so on. With 30 FPS and 1 DPS it will be 1/33 = 0.05... We know that floats are not precise enough to sum 30 circulating decimals and have exactly 1 in the end. But HP is a discrete value and that's why 1 DPS will not result in 1 damage after 1 second, because the value will be 0.9999... instead. It's not such a big deal when you have 100000 DPS, ±1 damage will not be noticeable. But what if I have 1,5 DPS? How do modern RPGs implement DOTs?

Answer 1

In my experience, i prefer to use a single "big" hit every so often than to accumulate a lot of very small differences.

I would recomend you to use a frequency (for the sake of simplicity i'd go with 1 second) and on your update loop check how much time has passed since the last time the dot has applied damage. Once you get to a time where you know damage has to be applied, you apply it, reset the counter and wait for the next tick of the dot.

float dot_frequency = DOT_FREQUENCY;
bool apply_first_tick_immediately = true; //<--With this you control the behavior as you asked

void on_dot_applied()
{
    if (apply_first_tick_immediately)
        time_since_last_tick = dot_frequency;
    else
        time_since_last_tick = 0;
}

void update(float elapsed_time)
{
    time_since_last_tick += elapsed_time;
    time_since_start += elapsed_time;
    if (time_since_last_tick >= dot_frequency)
    {
        apply_dot_damage();  // <-- here get your formula for dot damage
        time_since_last_tick -= dot_frequency;
    }
    if (time_since_start >= DOT_DURATION)
    {
        remove_dot();
    }
 }

I hope this helps.

EDIT: Added an example of how you can work with the "deal damage immediately" idea. Probably the on_dot_applied is an event that is called immediately as a dot is applied to setup certain effects of the specific dot, such as a fire particle effect, maybe a shader change.

Answer 2

If you're worried about imprecisions, you could simply apply the damage every second:

time_last_damage += delta_time;
if (time_last_damage >= 1.0f)
{
    time_elapsed = floor(time_last_damage);

    damage_done = getRandomDamage() * time_elapsed;

    time_last_damage -= time_elapsed; // keep fractional time
}