Why is -.1 0 used as the end point coordinate in the SVG Path path parameter?

In-depth SVG Path Parameters: Path Arc Command

In SVG drawing, the path command of path element is the core, and it is crucial to understand its parameters. This article will analyze a common elliptical arc command case in detail to clarify the meaning and function of its parameters.

Case Study: Clever Approximate Closure

We analyze the following path commands:

 <code>offset: path("M 100,0 a 100 100 0 1 1 -.1 0 z");</code>

Where, the command a draws an ellipse arc. The key lies in its parameter a 100 100 0 1 1 -.1 0 and the closing command z .

Detailed explanation of parameters

• M 100,0 : Move the brush to coordinates (100, 0).
• a 100 100 0 1 1 -.1 0 : Ellipse arc command parameters:
  • 100 100 : The horizontal and vertical radii of the ellipse are 100.
  • 0 : The x-axis rotation angle of the ellipse arc is 0 degrees.
  • 1 1 : large-arc-flag and sweep-flag are both 1, indicating that large arcs are drawn and clockwise.
  • -.1 0 : End point coordinate. Not (100, 0), but slightly offset to (-0.1, 0). This is to avoid the beginning and end points being completely overlapped, thus preventing some browser rendering issues. The slight offset ensures the path closure while avoiding potential rendering errors.
• z : Close the path command to connect the current point and the starting point.

The meaning of upper and lower case letters

• a (lowercase): represents relative coordinates. -.1 0 is the relative offset from the current point (100,0).
• A (caps) means absolute coordinates, and the end coordinates will be the absolute position relative to the canvas origin (0,0).
• z (both upper and lower case): Closed path, the function is the same.

Through the above analysis, we understand why -.1 0 is used as the end point coordinate: this is a clever trick to avoid rendering problems, and achieve approximate closure through tiny offsets to ensure the correctness of the path drawing. Understanding the difference between relative and absolute coordinates, as well as the various parameters of the a command, is crucial to mastering SVG path commands.

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