AvaloniaUI/Avalonia · error · ArgumentException

defaultFlatness must be greater than 1.0e-10

Error message

defaultFlatness must be greater than 1.0e-10

What it means

Thrown by SetDefaultFlatness when the requested flatness (the acceptable approximation error tolerance) is below 1.0e-10. The error-estimation math relies on flatness being a positive, non-degenerate magnitude; an extremely small value would demand unbounded subdivision and destabilize the rational-function error bounds.

Source

Thrown at src/Avalonia.Base/Media/PreciseEllipticArcHelper.cs:436

            public void SetMaxDegree(int maxDegree)
            {
                if (maxDegree < 1 || maxDegree > 3)
                {
                    throw new ArgumentException(@"maxDegree must be between 1 and 3", nameof(maxDegree));
                }
                _maxDegree = maxDegree;
            }

            /// <summary>
            /// Sets the default flatness for Bezier curve approximation
            /// </summary>
            /// <param name="defaultFlatness">default flatness (must be greater than 1e-10)</param>
            /// <exception cref="ArgumentException">Thrown if defaultFlatness is lower than 1e-10</exception>
            public void SetDefaultFlatness(double defaultFlatness)
            {
                if (defaultFlatness < 1.0E-10)
                {
                    throw new ArgumentException(@"defaultFlatness must be greater than 1.0e-10", nameof(defaultFlatness));
                }
                _defaultFlatness = defaultFlatness;
            }

            /// <summary>
            /// Computes the locations of the focii
            /// </summary>
            private void ComputeFocii()
            {
                double d = Math.Sqrt(A * A - B * B);
                double dx = d * _cosTheta;
                double dy = d * _sinTheta;
                FirstFocusX = Cx - dx;
                FirstFocusY = Cy - dy;
                SecondFocusX = Cx + dx;
                SecondFocusY = Cy + dy;
            }

View on GitHub (pinned to 11c5427268)

Solutions

  1. Use a flatness at or above 1e-10 (typical rendering values are ~0.25 to 1.0 in device units).
  2. Floor the input: var f = Math.Max(userFlatness, 1e-10); arc.SetDefaultFlatness(f);
  3. If 'maximum precision' is requested, pick a small-but-valid tolerance like 1e-9 rather than 0.

Example fix

// before
arc.SetDefaultFlatness(0); // intended 'max precision'

// after
arc.SetDefaultFlatness(1e-9); // small but valid tolerance
Defensive patterns

Strategy: validation

Validate before calling

var f = Math.Max(defaultFlatness, 1e-10);
arc.SetDefaultFlatness(f);

Type guard

static bool IsValidFlatness(double f) => f >= 1.0e-10;

Prevention

When it happens

Trigger: arc.SetDefaultFlatness(0) or arc.SetDefaultFlatness(1e-12) — any value strictly less than 1e-10, including zero and negatives.

Common situations: Exposing flatness as a user/config value without a floor; passing a 'max precision' sentinel of 0 expecting 'as precise as possible'; unit confusion (passing pixels-vs-device units).

Related errors

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/1de17511c31d2817. Report an issue: GitHub.