Code archives/Algorithms/Point in Triangle

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Point in Triangle by Rob Farley
(Posted 20 years ago)
Usage:

InTriangle(x0,y0,x1,y1,x2,y2,x3,y3)

(x0,y0) is the point you are checking

(x1,y1) (x2,y2) and (x3,y3) are the points of the triangle.

Returns True or False

You only need the function at the bottom, the stuff at the top is simply a demo. 
Graphics 640,480,32,2
SeedRnd MilliSecs()

Repeat
Cls
	x1 = Rand(100,540)
	y1 = Rand(100,380)
	x2 = Rand(100,540)
	y2 = Rand(100,380)
	x3 = Rand(100,540)
	y3 = Rand(100,380)
	
	
	Color 255,0,0
	
	For x=0 To 639
	For y=0 To 479
		If intriangle (x,y,x1,y1,x2,y2,x3,y3) Then Plot x,y
	Next
	Next
	
	Color 255,255,255
	Line x1,y1,x2,y2
	Line x2,y2,x3,y3
	Line x3,y3,x1,y1
	
	WaitKey

Until KeyHit(1)


Function InTriangle(x0,y0,x1,y1,x2,y2,x3,y3)

	b0# =  (x2 - x1) * (y3 - y1) - (x3 - x1) * (y2 - y1)
	b1# = ((x2 - x0) * (y3 - y0) - (x3 - x0) * (y2 - y0)) / b0 
	b2# = ((x3 - x0) * (y1 - y0) - (x1 - x0) * (y3 - y0)) / b0
	b3# = ((x1 - x0) * (y2 - y0) - (x2 - x0) * (y1 - y0)) / b0 
	
	If b1>0 And b2>0 And b3>0 Then Return True Else Return False

End Function