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### 5.1.2 Lines

`BSpline ( expression ) = { expression-list };`

Creates a B-spline curve. The expression inside the parentheses is the B-spline curve’s identification number; the expression-list on the right hand side should contain the identification numbers of all the B-spline’s control points. Repeating control points has the expected effect.

`Circle ( expression ) = { expression, expression, expression };`

Creates a circle arc (strictly) smaller than Pi. The expression inside the parentheses is the circle arc’s identification number; the first expression inside the braces on the right hand side gives the identification number of the start point of the arc; the second expression gives the identification number of the center of the circle; the last expression gives the identification number of the end point of the arc.

`CatmullRom ( expression ) = { expression-list };`

`CatmullRom` is a synonym for `Spline`.

`Ellipse ( expression ) = { expression, expression, expression, expression };`

Creates an ellipse arc. The expression inside the parentheses is the ellipse arc’s identification number; the first expression inside the braces on the right hand side gives the identification number of the start point of the arc; the second expression gives the identification number of the center of the ellipse; the third expression gives the identification number of any point located on the major axis of the ellipse; the last expression gives the identification number of the end point of the arc.

`Line ( expression ) = { expression, expression };`

Creates a straight line segment. The expression inside the parentheses is the line segment’s identification number; the two expressions inside the braces on the right hand side give identification numbers of the start and end points of the segment.

`Spline ( expression ) = { expression-list };`

Creates a spline curve. The expression inside the parentheses is the spline’s identification number; the expression-list on the right hand side should contain the identification numbers of all the spline’s control points.

`Line Loop ( expression ) = { expression-list };`

Creates an oriented line loop. The expression inside the parentheses is the line loop’s identification number; the expression-list on the right hand side should contain the identification numbers of all the elementary lines that constitute the line loop. A line loop must be a closed loop, and the elementary lines should be ordered and oriented (using negative identification numbers to specify reverse orientation). If the orientation is correct, but the ordering is wrong, Gmsh will actually reorder the list internally to create a consistent loop. Although Gmsh supports it, it is not recommended to specify multiple line loops (or subloops) in a single `Line Loop` command. (Line loops are used to create surfaces: see Surfaces.)

`Compound Line ( expression ) = { expression-list };`

Creates a compound line from several elementary lines. When meshed, a compound line will be reparametrized as a single line, whose mesh can thus cross internal boundaries. The expression inside the parentheses is the compound line’s identification number; the expression-list on the right hand side contains the identification number of the elementary lines that should be reparametrized as a single line. See `Compound Surface` for additional information on compound entities.

`Physical Line ( expression | char-expression ) = { expression-list };`

Creates a physical line. The expression inside the parentheses is the physical line’s identification number (if a char-expression is given instead, a unique identification number is automatically created); the expression-list on the right hand side should contain the identification numbers of all the elementary lines that need to be grouped inside the physical line. Specifying negative identification numbers in the expression-list will reverse the orientation of the mesh elements belonging to the corresponding elementary lines in the saved mesh.

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