Spline Fit

The splinefit command draws a cubic spline that passes through every point you click — an interpolating curve. Unlike Spline CV, where the curve is only attracted toward control vertices, here the curve is forced to hit each clicked coordinate exactly. Internally the editor fits control vertices to achieve this, and those CVs are stored alongside the fit points in the DXF file.

Drawing a spline through fit points

1. Type splinefit in the terminal or click the Spline Fit toolbar button.
2. Click to place fit points — the curve will pass through each one. Or type X,Y and press Enter for an exact coordinate.
3. Press Enter to finish (at least 2 points required).

  ●──────●──────●──────●  ← curve passes exactly through each click
  p1     p2     p3     p4

The live preview shows the current interpolated curve as you move the cursor, including the prospective next point at the cursor position. Press Escape to discard all placed points and exit.

Coordinate entry

Instead of clicking, type an exact position for any fit point:

1. Type the X value.
2. Press , — the terminal shows [X], [Y{cursor}].
3. Type the Y value.
4. Press Enter to place the fit point.

Keyboard reference

Key	Action
0–9, ., -	Start X coordinate entry
,	Lock X and move to Y entry
Backspace	Delete last typed character
Enter	Confirm typed coordinate, or finish the spline if no input is in progress and ≥ 2 points exist
Escape	Discard all points and exit

Grip editing — reshaping via fit points

A selected fit spline exposes one grip per fit point:

Grip	Position	What it does
Fit point	At each clicked position	Drag to move that fit point — the entire curve re-interpolates to pass through the new position

Dragging one grip re-fits the whole curve, not just the adjacent segments. This differs from polyline grip editing, where moving a vertex only reshapes the two adjacent segments.

There is no "move whole spline" grip. To translate the entire spline, use the Move command.

Selecting fit splines

Method	Behaviour
Click	Selects if the click lands near any point on the curve
Drag right (strict)	All sample points along the curve must lie inside the selection box
Drag left (crossing)	Any part of the curve that crosses the selection box boundary selects it

Supported edit commands

Command	What happens to the spline
Move	Translates all fit points and recomputed CVs by the same displacement
Copy	Creates an identical spline at a new position
Rotate	Rotates all fit points around the chosen base point
Mirror	Reflects all fit points across the mirror axis
Scale	Scales all fit points uniformly from the base point
Delete	Removes the spline

Splines do not support Offset, Trim, or Extend.

Properties

General

Property	Default	Meaning
Color	256 (ByLayer)	ACI color index
Layer	0	Layer assignment
Linetype	ByLayer	Named linetype pattern
Linetype Scale	1	Scale factor on the linetype pattern
Thickness	0	Extrusion thickness

Geometry

Property	Meaning
Degree	Polynomial degree — always 3 (cubic)
Fit Points	Coordinates of all clicked pass-through points
Control Vertices	Internally computed CVs used to render the curve

Spline Fit vs Spline CV — which to use

	Spline Fit	Spline CV
Curve passes through points	Every clicked point exactly	First and last only (clamped)
Grip edit effect	Fit point moves → whole curve re-interpolated	CV moves → curve attracted toward new position
Shape predictability	High — curve follows clicks	Lower — curve lags behind CVs
Best for	Curves that must hit specific coordinates	Smooth aesthetic curves, free-form paths

DXF — SPLINE entity (fit-point form)

Fit splines are saved as SPLINE entities in the DXF file, storing both the fit point coordinates and the computed control vertices. The splineFlag is set to 8 (fit-point spline) so a reloading application knows which set of points to display as editable grips. All properties — color, layer, linetype, linetype scale, and thickness — round-trip without loss. DXF applications that support fit-point splines (AutoCAD, LibreCAD) will display the fit points as the primary editable data.