[SOLVED] Math: Shortest distance from point to line (EDITED)

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XHH
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[SOLVED] Math: Shortest distance from point to line (EDITED)

Post by XHH »

I have two line segments. All I have are the coordinates for these line segments. How can I find out if these two line segments intersect?

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Last edited by XHH on Mon Jul 15, 2013 12:38 am, edited 2 times in total.
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raidho36
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Re: [Help] Math-related: Do two line segments intersect?

Post by raidho36 »

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XHH
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Re: [Help] Math-related: Do two line segments intersect?

Post by XHH »

Thank you sir! One more new question though. How do I find the shortest distance from a point to a line?
I like to draw and program :)
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micha
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Re: [Help] Math: Shortest distance from point to line (EDITE

Post by micha »

Here is some code that determines the distance of a points from a (infinite) line. If you need the distance of a points from a line segment, it has to be modiefied.
The point is given as (px,py) the lines is given by two points (x1,y1) and (x2,y2)

Code: Select all

function distPointToLine(px,py,x1,y1,x2,y2)
  local dx,dy = x1-x2,y1-y2
  local length = math.sqrt(dx*dx+dy*dy)
  dx,dy = dx/length,dy/length
  return math.abs( dy*(px-x1) - dx*(py-y1))
end
Disclaimer: Untested.
This snippet calculates the unit vector pointing along the line (vector of length one). Then this vector is rotated by 90°, which is achieved by swapping x- and y-component and multiplying one with -1, that is (dx,dy) -> (dy,-dx). This is the normal vector for the line. To determine the distance, then the scalar product with any vector from the line to the point p is calculated.
If you leave away the math.abs, then you get a distance with sign. This is useful, if you want to determine if a point crossed the line in one frame (compare the sign of the distance before and after the frame, if the sign is different, the line has been crossed).
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XHH
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Re: [Help] Math: Shortest distance from point to line (EDITE

Post by XHH »

micha wrote:If you need the distance of a points from a line segment, it has to be modiefied.
Can you post that too then?
(thanks)
I like to draw and program :)
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micha
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Re: [Help] Math: Shortest distance from point to line (EDITE

Post by micha »

XHH wrote:Can you post that too then?
(thanks)
Sure. Disclaimer again: This is from the top of my head. Please test.

Code: Select all

function distPointToLine(px,py,x1,y1,x2,y2)
  local dx,dy = x2-x1,y2-y1
  local length = math.sqrt(dx*dx+dy*dy)
  dx,dy = dx/length,dy/length
  local posOnLine = dx*(px-x1) + dy*(px-y1)
  if posOnline < 0 then
    -- first end point is closest
    dx,dy = px-x1,py-y1
    return math.sqrt(dx*dx+dy*dy)
  elseif posOnLine > length then
    -- second end point is closest
    dx,dy = px-x2,py-y2
    return math.sqrt(dx*dx+dy*dy)    
  else
    -- point is closest to some part in the middle of the line
    return math.abs( dy*(px-x1) - dx*(py-y1))
  end
end
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XHH
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Re: [Help] Math: Shortest distance from point to line (EDITE

Post by XHH »

Didn't completely work. Sometimes it gives me 1.0 as the distance.
I like to draw and program :)
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raidho36
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Re: [Help] Math: Shortest distance from point to line (EDITE

Post by raidho36 »

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micha
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Re: [Help] Math: Shortest distance from point to line (EDITE

Post by micha »

I found a typo. This one seems to work:

Code: Select all

function distPointToLine(px,py,x1,y1,x2,y2)
  local dx,dy = x2-x1,y2-y1
  local length = math.sqrt(dx*dx+dy*dy)
  dx,dy = dx/length,dy/length
  local posOnLine = dx*(px-x1) + dy*(py-y1)
  if posOnLine < 0 then
    -- first end point is closest
    dx,dy = px-x1,py-y1
    return math.sqrt(dx*dx+dy*dy)
  elseif posOnLine > length then
    -- second end point is closest
    dx,dy = px-x2,py-y2
    return math.sqrt(dx*dx+dy*dy)   
  else
    -- point is closest to some part in the middle of the line
    return math.abs( dy*(px-x1) - dx*(py-y1))
  end
end
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Ref
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Re: [Help] Math: Shortest distance from point to line (EDITE

Post by Ref »

Saw the typo and after correction seems to work for me.
Question:
Easy to find closest-point-on-line but pretty mess to get closest-point-on-line-segment.
Do you have a 'neat' way of doing this?
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