wickra/docs/wiki/indicators/trend/Indicator-Dema.md

DEMA

Double Exponential Moving Average — Patrick Mulloy's 2·EMA − EMA(EMA), a single-line trend filter that removes the first-order lag of a plain EMA.

Quick reference

Field	Value
Family	Trend
Sub-category	Exponential family
Input type	f64 (single close)
Output type	f64
Output range	unbounded; tracks the input price scale
Default parameters	period is required (no default in either binding)
Warmup period	2·period − 1
Interpretation	EMA-style smoothing with less lag; sits ahead of Ema on a sustained trend.

Formula

Let EMA1 = EMA(price, period) and EMA2 = EMA(EMA1, period). Then:

DEMA_t = 2 * EMA1_t - EMA2_t

Both inner EMAs use the same period, hence the same α = 2 / (period + 1). The subtraction is a finite-difference approximation of "remove the lag introduced by single EMA smoothing": if EMA lags the true series by L, then EMA(EMA) lags by roughly 2L, so 2·EMA − EMA(EMA) cancels most of the first-order error.

Parameters

Name	Type	Default	Valid range	Description
period	usize	none	>= 1	Period shared by both internal EMAs. period = 0 errors with Error::PeriodZero.

(Python class wickra.DEMA(period) has no #[pyo3(signature)] default; pass period explicitly.)

Inputs / Outputs

From crates/wickra-core/src/indicators/dema.rs:

impl Indicator for Dema {
    type Input = f64;
    type Output = f64;
    // update(&mut self, input: f64) -> Option<f64>
}

Python update returns float | None, batch returns a 1-D numpy.ndarray (float64, NaN for warmup). Node update returns number | null, batch returns Array<number> with NaN placeholders.

Warmup

Dema::new(period).warmup_period() == 2 * period - 1. The comment in the source explains it cleanly:

EMA1 seeds at period, then EMA2 needs another period − 1 values to seed.

Ema::new(period) only starts producing output once it has seen period inputs. So ema1 emits its first value at input period. From that point on, ema2 starts receiving inputs (the outputs of ema1) and itself needs period of them to seed — first emission at "input period of ema1" = input 2·period − 1 of Dema. For Dema::new(14) this gives 27, matching the table in Warmup Periods.

The implementation uses the ? operator to short-circuit: let e1 = self.ema1.update(input)?; let e2 = self.ema2.update(e1)?;, so ema2 is only fed once ema1 actually emits — which is exactly what the warmup arithmetic above models.

Edge cases

• Constant series. Feeding [100.0; n] eventually produces Some(100.0): once both EMAs converge to 100.0, the output is 2 · 100 − 100 = 100. The unit test constant_series_yields_constant_dema pins this with Dema::new(5) over 60 constants.
• NaN / infinity inputs. Inherited from the inner Ema: non-finite inputs are silently dropped and the previously emitted value (if any) is preserved. Inputs that fail to pass is_finite() never reach the 2·EMA1 − EMA2 arithmetic.
• Reset. dema.reset() resets both internal EMAs. The next update starts a full 2·period − 1 warmup countdown.

Examples

Rust

use wickra::{BatchExt, Dema, Indicator};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut dema = Dema::new(5)?;
    let prices: Vec<f64> = (1..=20).map(f64::from).collect();
    let out: Vec<Option<f64>> = dema.batch(&prices);
    println!("warmup_period = {}", dema.warmup_period());
    println!("{:?}", out);
    Ok(())
}

Output:

warmup_period = 9
[None, None, None, None, None, None, None, None, Some(9.0), Some(10.0), Some(11.0), Some(12.0), Some(13.000000000000002), Some(14.000000000000002), Some(15.000000000000002), Some(16.000000000000004), Some(17.0), Some(18.0), Some(19.0), Some(20.0)]

The first Some arrives at index 8 (the 9th input), exactly as predicted by 2·5 − 1 = 9. On a linear ramp 1, 2, …, 20, DEMA tracks the input ramp almost perfectly because the lag has been cancelled to first order — the floating-point tail of 13.000000000000002 is ordinary IEEE-754 drift. The unit test linear_uptrend_dema_above_ema_eventually pins the property that Dema exceeds Ema of the same period on a sustained uptrend.

Python

import numpy as np
import wickra as ta

dema = ta.DEMA(5)
out = dema.batch(np.arange(1.0, 21.0))
print("warmup_period =", dema.warmup_period())
print(out)

Output:

warmup_period = 9
[nan nan nan nan nan nan nan nan  9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
 19. 20.]

Node

const ta = require('wickra');
const dema = new ta.DEMA(5);
const prices = Array.from({ length: 20 }, (_, i) => i + 1);
console.log(dema.batch(prices));
console.log('warmupPeriod:', dema.warmupPeriod());

Output:

[
                 NaN,                NaN,
                 NaN,                NaN,
                 NaN,                NaN,
                 NaN,                NaN,
                   9,                 10,
                  11,                 12,
  13.000000000000002, 14.000000000000002,
  15.000000000000002, 16.000000000000004,
                  17,                 18,
                  19,                 20
]
warmupPeriod: 9

Interpretation

Dema is the canonical "I want EMA, but with less lag" answer. On a sustained directional trend the DEMA line sits ahead of an Ema of the same period (the unit test pins this). The same signals you use for Ema — price-vs-MA crossover, fast-vs-slow MA crossover — apply, and they fire earlier. In return for the lower lag you accept more sensitivity to noise: on choppy data DEMA will whipsaw earlier than EMA of the same period.

Prefer Dema over Ema when you want a faster trend filter without moving to a smaller period (which would also amplify noise). Prefer Tema for even less lag at the cost of further noise sensitivity, or Hma if you want lag reduction plus an inherent smoothing step.

Common pitfalls

• Picking a period that's too short for a noisy market. Because Dema removes lag rather than adding smoothing, on choppy series it amplifies high-frequency oscillations. If you reach for Dema(5) on a tick-by-tick feed and get a jittery line, the fix is to raise period — Dema(20) is often a better compromise than Dema(5).
• Assuming the first Dema value lines up with the first Ema value at the same period. Ema(14) first emits at input 14; Dema(14) first emits at input 27. If you align a DEMA series to an EMA series in a backtest, account for the offset or use the ~np.isnan(...) mask (Python) / is_some() filter (Rust) to drop the warmup rows.

References

Patrick G. Mulloy, "Smoothing Data with Faster Moving Averages", Technical Analysis of Stocks & Commodities, January 1994 (DEMA), and "Smoothing Data with Less Lag", Technical Analysis of Stocks & Commodities, February 1994 (TEMA).

See also

• Indicator-Ema.md — the building block.
• Indicator-Tema.md — three-EMA version, less lag still.
• Indicator-Hma.md — same lag-reduction goal, built on WMAs.
• Indicators-Overview.md — the full taxonomy.