DP44/enriched-qwen3-coder-next/DataPRO/FftSharp/Windows.md

---
source_files:
  - DataPRO/FftSharp/Windows/Bartlett.cs
  - DataPRO/FftSharp/Windows/Cosine.cs
  - DataPRO/FftSharp/Windows/Welch.cs
  - DataPRO/FftSharp/Windows/Rectangular.cs
  - DataPRO/FftSharp/Windows/Hanning.cs
  - DataPRO/FftSharp/Windows/Hamming.cs
  - DataPRO/FftSharp/Windows/FlatTop.cs
  - DataPRO/FftSharp/Windows/Blackman.cs
  - DataPRO/FftSharp/Windows/BlackmanHarris.cs
  - DataPRO/FftSharp/Windows/Tukey.cs
  - DataPRO/FftSharp/Windows/Kaiser.cs
generated_at: "2026-04-16T04:24:07.110679+00:00"
model: "Qwen/Qwen3-Coder-Next-FP8"
schema_version: 1
sha256: "330f92c33efc913c"
---

# Windows

## Documentation: FftSharp.Windows Module

### 1. Purpose  
This module provides a suite of window functions used in spectral analysis (e.g., before applying the FFT) to mitigate spectral leakage. Each window class implements a specific mathematical formulation to taper the signal at its edges, reducing discontinuities introduced by the finite-length FFT. The classes inherit from an abstract `Window` base class and implement the `IWindow` interface, offering a consistent API for generating window coefficients of a given size. These windows are intended for use in signal processing pipelines where accurate frequency-domain representation is required.

### 2. Public Interface  
All window classes are concrete, non-abstract subclasses of `Window` and implement `IWindow`. Each defines a public constructor (with optional parameters for tunable windows) and overrides `Name`, `Description`, and the protected `windowValue(int index, int size)` method. The base `Window` class (not shown in source) is assumed to expose a public method (e.g., `Generate(int size)`) that populates an array by calling `windowValue` for each index.

- **`Bartlett`**  
  - `Name`: `"Bartlett–Hann"`  
  - `Description`: Triangular (2nd-order B-spline), convolution of two half-sized rectangular windows.  
  - `windowValue(index, size)`: `1 - |(index - size/2) / (size/2)|`  

- **`Cosine`**  
  - `Name`: `"Cosine"`  
  - `Description`: Simple sine-shaped window (`sin(π·i/(N−1))`), reaches zero at edges; discouraged for practical use.  
  - `windowValue(index, size)`: `Math.Sin(index * Math.PI / (size - 1))`  

- **`Welch`**  
  - `Name`: `"Welch"`  
  - `Description`: Parabolic window, better frequency response than Bartlett below π, but with a distinct bump above π; used for antialiasing/resampling.  
  - `windowValue(index, size)`: `1 - ((index - (size-1)/2) / ((size-1)/2))²`  

- **`Rectangular`**  
  - `Name`: `"Rectangular"`  
  - `Description`: Boxcar/Dirichlet window (all ones); preserves transients but causes high spectral leakage.  
  - `windowValue(index, size)`: `1`  

- **`Hanning`**  
  - `Name`: `"Hanning"`  
  - `Description`: Sinusoidal window touching zero at edges; low leakage, good for general use.  
  - `windowValue(index, size)`: `0.5 - 0.5 * cos(2π·i / size)`  

- **`Hamming`**  
  - `Name`: `"Hamming"`  
  - `Description`: Sinusoidal window *not* zero at edges; designed to cancel the largest sidelobe.  
  - `windowValue(index, size)`: `0.54 - 0.46 * cos(2π·i / size)`  

- **`FlatTop`**  
  - `Name`: `"FlatTop"`  
  - `Description`: Partially negative-valued window; minimal scalloping loss, ideal for amplitude measurement. Inherits from `Blackman` with fixed coefficients.  
  - Constructor: `FlatTop()` calls `base(0.2810639, 0.5208972, 0.1980399)`  
  - Uses `Blackman.windowValue` with those coefficients:  
    `A - B·cos(2π·i/size) + C·cos(4π·i/size)`  

- **`Blackman`**  
  - `Name`: `"Blackman"`  
  - `Description`: Three-term cosine window; exact coefficients null 3rd/4th sidelobes.  
  - Constructors:  
    - `Blackman()` — uses default coefficients (`A=0.42659071`, `B=0.49656062`, `C=0.07684867`)  
    - `Blackman(double a, double b, double c)` — custom coefficients  
  - `windowValue(index, size)`: `A - B·cos(2π·i/size) + C·cos(4π·i/size)`  

- **`BlackmanHarris`**  
  - `Name`: `"Blackman-Harris"`  
  - `Description`: Four-term cosine window; wide main lobe but strong sidelobe suppression.  
  - Constructors:  
    - `BlackmanHarris()` — uses default coefficients (`A=0.35875`, `B=0.48829`, `C=0.14128`, `D=0.01168`)  
    - `BlackmanHarris(double a, double b, double c, double d)` — custom coefficients  
  - `windowValue(index, size)`: `A - B·cos(2π·i/size) + C·cos(4π·i/size) - D·cos(6π·i/size)`  

- **`Tukey`**  
  - `Name`: `"Tukey"`  
  - `Description`: Flat center with cosine taper at edges; controlled by `Alpha` (0–1). Ideal for transient analysis.  
  - Constructors:  
    - `Tukey()` — `Alpha = 0.5`  
    - `Tukey(double alpha = 0.5)`  
  - `windowValue(index, size)`:  
    - If `index < edgeSizePoints` or `index > size - edgeSizePoints`: `(1 - cos(index * 2π / (Alpha * size))) / 2`  
    - Else: `1`  
    - Where `edgeSizePoints = (int)(size * Alpha / 2)`  

- **`Kaiser`**  
  - `Name`: `"Kaiser-Bessel"`  
  - `Description`: Tunable via `Beta`; balances main lobe width, sidelobe height, and distance.  
  - Constructors:  
    - `Kaiser()` — `Beta = 15`  
    - `Kaiser(double beta)`  
  - `windowValue(index, size)`: `I0(Beta * sqrt(1 - ((index - (size-1)/2) / ((size-1)/2))²)) / I0(Beta)`  
  - `I0(double x)`: Modified Bessel function of the first kind (order 0), implemented via piecewise series.  

### 3. Invariants  
- All windows are **symmetric** about `index = (size - 1) / 2`. This is evident from the use of `(index - center)` in all formulas.  
- Window values are computed for indices `0 ≤ index < size`.  
- For all windows except `Rectangular`, `windowValue(0, size) == windowValue(size - 1, size)`.  
- For `Hanning`, `Hamming`, `Blackman`, `BlackmanHarris`, and `FlatTop`, the window is periodic with period `size` in the cosine argument (i.e., `cos(2π·i/size)`).  
- `Tukey` uses integer truncation for `edgeSizePoints`, which may cause asymmetry for odd `size` and non-integer `size * Alpha / 2`.  
- `Kaiser` window is defined only for `size > 1`; division by zero occurs if `size == 1` (since `alpha = (size-1)/2 = 0`).  

### 4. Dependencies  
- **Internal**: All classes depend on the abstract base class `Window` and interface `IWindow` (not shown), which define the contract for window generation (e.g., a public method like `double[] Generate(int size)`).  
- **External**:  
  - `System` namespace (for `Math`, `Abs`, `Cos`, `Sin`, `Exp`, `Sqrt`, `Pow`).  
  - `FlatTop` inherits from `Blackman`, reusing its `windowValue` implementation with fixed coefficients.  
- **Inferred usage**: These classes are likely consumed by an FFT processing pipeline (e.g., `FftSharp.Fft` or similar) that applies windowing before forward FFT and/or after inverse FFT.  

### 5. Gotchas  
- **`Cosine` window naming**: Despite the class name, `windowValue` computes `sin(π·i/(N−1))`, not a cosine. This is likely intentional (sine is a shifted cosine), but the name may mislead.  
- **`Bartlett` vs. standard Bartlett**: The description says "Bartlett–Hann", but the formula matches the *standard* Bartlett (triangular) window, not the Bartlett–Hann (which combines triangular and cosine terms). This may be a documentation or naming error.  
- **`Tukey` edge handling**: For non-integer `size * Alpha / 2`, `edgeSizePoints` truncates, causing asymmetric tapering when `size` is odd and `Alpha` is not tuned to avoid this.  
- **`Kaiser` division by zero**: If `size == 1`, `alpha = 0`, leading to division by zero in `((index - alpha) / alpha)²`. This case is not guarded.  
- **`FlatTop` inheritance**: `FlatTop` inherits from `Blackman` and hardcodes coefficients, but `Blackman`’s constructor is not `protected`, so `FlatTop` must call `base(...)` explicitly. This tight coupling makes `FlatTop` inflexible (e.g., no custom coefficients).  
- **Missing `IWindow` implementation**: The source does not show the `IWindow` interface or `Window` base class, so the exact public API (e.g., method names, return types) is inferred.  
- **No validation of `size`**: All `windowValue` methods assume `size > 1`. For `size == 1`, `Welch` and `Kaiser` involve division by zero; `Tukey`’s `edgeSizePoints` becomes 0, but the logic may still hold.  

None identified beyond the above.