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PD

PolygonDetector_AI

Posted on August 4, 2025 • Advanced

🔍 Need help with polygon point detection algorithm

Hi everyone! I’m working on a more powerful state machine AI for one of my games and I need a way to detect if a point is inside a set of four other points. Does anyone have a good algorithm for this?

This is for advanced collision detection and AI pathfinding systems. Any help with efficient algorithms would be greatly appreciated! 🙏

GM

GeometryMaster_Pro

Replied 3 hours later • ⭐ Best Answer

Great question @PolygonDetector_AI! There are several excellent algorithms for point-in-polygon detection. Here are the most effective methods for Scratch:

🎯 Method 1: Ray Casting Algorithm

This is the most versatile method that works for any polygon shape:

    define point in polygon (px) (py)
set [intersections v] to [0]
set [i v] to [1]
repeat (length of [polygon x v])
set [next v] to ((i) mod (length of [polygon x v])) + [1]
if <((py) < (item (i) of [polygon y v])) ≠ ((py) < (item (next) of [polygon y v]))> then
set [x intersect v] to (((item (i) of [polygon x v]) - (item (next) of [polygon x v])) * ((py) - (item (next) of [polygon y v])) / ((item (i) of [polygon y v]) - (item (next) of [polygon y v])) + (item (next) of [polygon x v]))
if <(px) < (x intersect)> then
change [intersections v] by [1]
end
end
change [i v] by [1]
end
set [inside v] to ((intersections) mod [2])

⚡ Method 2: Cross Product Method (For Convex Polygons)

Faster for simple quadrilaterals and convex shapes:

    define point in quad (px) (py) (x1) (y1) (x2) (y2) (x3) (y3) (x4) (y4)
set [inside v] to [true]
set [cross1 v] to (((px) - (x1)) * ((y2) - (y1)) - ((py) - (y1)) * ((x2) - (x1)))
set [cross2 v] to (((px) - (x2)) * ((y3) - (y2)) - ((py) - (y2)) * ((x3) - (x2)))
if <((cross1) * (cross2)) < [0]> then
set [inside v] to [false]
end
set [cross3 v] to (((px) - (x3)) * ((y4) - (y3)) - ((py) - (y3)) * ((x4) - (x3)))
if <((cross2) * (cross3)) < [0]> then
set [inside v] to [false]
end
set [cross4 v] to (((px) - (x4)) * ((y1) - (y4)) - ((py) - (y4)) * ((x1) - (x4)))
if <((cross3) * (cross4)) < [0]> then
set [inside v] to [false]
end

🌟 Method 3: Winding Number Algorithm

Most accurate for complex polygons with holes:

    define winding number (px) (py)
set [angle sum v] to [0]
set [i v] to [1]
repeat (length of [polygon x v])
set [next v] to ((i) mod (length of [polygon x v])) + [1]
set [dx1 v] to ((item (i) of [polygon x v]) - (px))
set [dy1 v] to ((item (i) of [polygon y v]) - (py))
set [dx2 v] to ((item (next) of [polygon x v]) - (px))
set [dy2 v] to ((item (next) of [polygon y v]) - (py))
set [angle v] to ([atan v] of (((dx1) * (dy2)) - ((dy1) * (dx2))) / (((dx1) * (dx2)) + ((dy1) * (dy2))))
change [angle sum v] by (angle)
change [i v] by [1]
end
set [inside v] to <([abs v] of (angle sum)) > [180]>

🚀 Implementation Tips

  • For simple quadrilaterals: Use the cross product method (fastest)
  • For complex polygons: Use ray casting (most reliable)
  • For high precision: Use winding number algorithm
  • Store polygon vertices in lists: [polygon x] and [polygon y] for easy iteration

Hope this helps with your AI system! Let me know if you need clarification on any method! 😊

PD

PolygonDetector_AI

Replied 2 hours later

@GeometryMaster_Pro This is absolutely perfect! Thank you so much! 🎉

The cross product method works great for my quadrilateral collision zones. My AI pathfinding is now much more accurate!

VB

Vibelf_Community

Pinned Message • Moderator

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