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How to Determine Whether a Point Is Within a Sector in JavaScript?


To determine if a point lies on an arc when moving counterclockwise (CCW) from one point to another on a circle, follow these steps:

  1. Understand Angle Measurement: Angles increase in the CCW direction on a unit circle. Starting at 0 radians (east), moving CCW increases angles towards π/2 (north), π (west), 3π/2 (south), and then back to 0.

  2. Identify Start and End Points: Let’s denote α as the starting angle and β as the ending angle of the arc.

  3. Check Angle Relationships:

    • If α < β: The arc spans from α to β without wrapping around, so any θ between α and β (α ≤ θ ≤ β) lies on the arc.
    • If α > β: The arc wraps around the circle, meaning it goes from α to 2π and then continues from 0 to β. Therefore, θ lies on the arc if it is either ≥ α or ≤ β.
  4. Apply Conditions:

    • For α < β: θ ∈ [α, β]
    • For α > β: θ ∈ [α, 2π] ∪ [0, β]
  5. Example Application:

    • Let α = 3π/2 (~4.712) and β = π/2 (~1.5708).
    • Since α > β, the arc spans from 3π/2 to 2π and then from 0 to π/2.
    • To check if θ = π (~3.14) is on this arc:
      • Is π ≥ 3π/2? No (3.14 < 4.712).
      • Is π ≤ π/2? No (3.14 > 1.5708).
    • Therefore, θ = π does not lie on the CCW arc from α to β.

Final Answer: The point θ = π is not on the CCW arc from 3π/2 to π/2 because it does not fall within either [3π/2, 2π] or [0, π/2].

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5 responses to “How to Determine Whether a Point Is Within a Sector in JavaScript?”

  1. A great resource for anyone working with geometry and JavaScript! The examples make it easy to grasp the concept of sector inclusion.

  2. This guide is perfect for developers who need to handle sector calculations. It explains the conditions clearly and includes a practical example.

  3. I appreciate how the article breaks down the problem into manageable steps, especially when dealing with angles that wrap around the circle.

  4. The step-by-step approach is very helpful for understanding the logic behind angle comparisons, which is essential for working with circular movements in programming.

  5. This article provides a clear and concise method for determining whether a point lies within a sector, making it easier to implement in JavaScript.

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