This tool is used to draw lines and curves. These two abilities are grouped into a single tool because a line is actually a perfectly straight curve. In other words, this tool
always draws curves, where a straight line is a subset involving no actual curvature.
Drawing a line is straightforward: click where the line begins and drag the mouse pointer to the end point. Releasing the mouse button ends the "drawing" phase.
Using the Left Mouse button renders the line with the Primary color.
Using the Right Mouse Button renders the line in the Secondary color.
When the drawing phase is ended, four control nubs appear on the line and the four-way move icon appears just off the end point of the line.
The Control Nubs and Move icon pulsate to make them easier to distinguish.
If a single, straight line is desired, press the Enter key or click the Finish button in the Tool Bar to commit the line to the canvas.
Drawing elsewhere on the canvas finishes any previous line and starts a new one.
Holding down the Shift key before releasing the mouse button constrains the line to angles that are multiples of 15 degrees, e.g. 0°, 15°, 30°, 45° 60° and so on.
If a curve is desired, drag the control nubs to a new location by clicking and dragging.
Moving a Line/Curve
Before a Line/Curve is committed to the canvas, it can be moved anywhere on the canvas. Click and drag the pulsing four-arrows-in-a-square icon (see diagram below) using the Left Mouse Button to reposition the Line/Curve.
The keyboard arrow keys ↑←↓→ can also be used to move a Line/Curve.
A single key press moves the object by one pixel in the direction of the arrow. Simultaneously holding the Ctrl key moves the object by 10 pixels per arrow key press.
Rotating a Line/Curve
Before a Line/Curve is committed to the canvas, it can be rotated about its geometric center.
Rotate the Line/Curve about the center using the Right Mouse Button. Click and drag to rotate.
As a visual guide that rotation is possible, the mouse pointer will turn into a double headed arrow (see diagram).
If the Shift key is held down while rotating the angle of rotation is snapped to 15 degree increments.
The keyboard arrow keys ↑←↓→ can also be used to rotate a Line/Curve while the Right Mouse Button is held down.
Committing a Line/Curve to the canvas
Commit a Line/Curve to the active layer and exit editing mode using any one of these methods…
Press the Enter key
Click outside of the bounding box of the current Line/Curve.
Click Finish in the Tool Bar
Draw a new Line/Curve.
There are two icons in the Tool Bar for changing between Cubic Spline and Bézier curves.
These replace the Right click & drag that paint.net 3.5x used.
If the Left icon is selected, the line will be curved using cubic spline interpolation.
It will always continue through each of the Control Nubs, even if the nubs are relocated or dragged off canvas.
When a nub is moved, the curve will be redrawn so that it always goes through the center of each Control Nub.
If the Right icon is selected, a nub, a Bézier curve results.
With this curve type, the curve will extend from the first or origin Control Nub to the last or destination Control Nub.
The second and third Control Nubs modify the curve along its path. The curve will not necessarily touch either the second or third Control Nub.
Some types of Bézier curves cannot be drawn using the Cubic Spline type.
Switch between curve types by toggling between the two Curve Icons in the Tool Bar.
The curve will be reinterpreted based on the curve type and current location of the Control Nubs.
Brush Width, Line Style and Fill
Lines and curves are subject to the Brush settings in the Tool Bar.
Brush Width, Line Styles & Fill Type are all configurable.
The Line/Curve can have different start cap, dash style, and end cap. The start- and end-caps can be flat (normal), one of two arrow head types, or rounded. The dash style can be either solid or a combination of dashes and dots.
Using Brush Width & Line Styles lines like these can be drawn:
Lines and curves can be rendered using a Fill Type as specified in the Tool Bar.
This feature allows the line/curve to be filled with one of numerous patterns rather than a solid color.
Fills utilizing a fill style will make use of both the Primary and Secondary colors.
There are two antialiasing modes associated with the Line/Curve tool.
These settings determine whether the line is rendered in a clip-to-pixel mode or not.
When Antialiasing is enabled, lines will appear slightly thicker and smoother (upper example).
When Antialiasing is disabled, lines are rendered in solid pixels and appear more jagged and thinner (lower example).
Antialiasing can be switched on or off via the Tool Bar when the Text tool is active.
Lines and curves can be applied using a Blend Mode selected from the Tool Bar.
The line/curve will be applied in accordance with the other parameters then reinterpreted as if the pixels were on their own layer with the layer blend mode set.