Finding the closest point on a line to another point

I have a affine equation y = ax + b where a is the coefficient (coeff).
Let D be a line that goes through axis and described by the previous equation.

I'm trying with this piece of code to find the coordinates of the closest point on D to position (ignoring the y coordinate, because 2D in 3D)

    double a = coeff;

    double b = position.getZ();

    double c = axis.getZ() - axis.getX() * coeff;

    double x0 = position.getX();

    double y0 = position.getZ();



    return new Vector((b * (b * x0 - a * y0) - a * c) / (a * a + b * b), position.getY(),

            (a * (-b * x0 + a * y0) - b * c) / (a * a + b * b));

Using this as a refernece

However, this does not work and return weird results

asked Nov 20 '18 at 21:54

Vinz243

3,91153165

How are you not getting a compile error? afaik Vector has no constructor for three arguments: docs.oracle.com/javase/8/docs/api/java/util/Vector.html

– Jere
Nov 20 '18 at 22:11

What is "position" in your code? Is it an instance of a point in 3d? If so, then getZ is probably getting the 3rd dim and not what you'd want here.

– sofend
Nov 20 '18 at 22:24

I think this is a 3D solution. (Top tip: Write the simplest thing first. Then adapt it. Possibly keep the original solution in a separate method and call that.) @Jere Vector doesn't appear to be java.util.Vector but a 3D Vector. @sofend axis appears to refer to one of the X/Y/Z axises. Can't be bothered to look through this in detail given the state of the question, but straight away, if converting to the cited but not quoted Wikipedia article, it looks as if a should be -coeffs. Also confusing using the same letters for different things.

– Tom Hawtin - tackline
Nov 20 '18 at 22:43

@TomHawtin-tackline yes Vector is a homemade class. Position is another vector, but I only want the closest point on the (x,z) plane, so that's why I'm not using the y coordinate. But I don't see how -coeff instead of coeff for a should change anything, a can be negative or positive

– Vinz243
Nov 21 '18 at 8:07

add a comment |

I have a affine equation y = ax + b where a is the coefficient (coeff).
Let D be a line that goes through axis and described by the previous equation.

I'm trying with this piece of code to find the coordinates of the closest point on D to position (ignoring the y coordinate, because 2D in 3D)

    double a = coeff;

    double b = position.getZ();

    double c = axis.getZ() - axis.getX() * coeff;

    double x0 = position.getX();

    double y0 = position.getZ();



    return new Vector((b * (b * x0 - a * y0) - a * c) / (a * a + b * b), position.getY(),

            (a * (-b * x0 + a * y0) - b * c) / (a * a + b * b));

Using this as a refernece

However, this does not work and return weird results

asked Nov 20 '18 at 21:54

Vinz243

3,91153165

How are you not getting a compile error? afaik Vector has no constructor for three arguments: docs.oracle.com/javase/8/docs/api/java/util/Vector.html

– Jere
Nov 20 '18 at 22:11

What is "position" in your code? Is it an instance of a point in 3d? If so, then getZ is probably getting the 3rd dim and not what you'd want here.

– sofend
Nov 20 '18 at 22:24

I think this is a 3D solution. (Top tip: Write the simplest thing first. Then adapt it. Possibly keep the original solution in a separate method and call that.) @Jere Vector doesn't appear to be java.util.Vector but a 3D Vector. @sofend axis appears to refer to one of the X/Y/Z axises. Can't be bothered to look through this in detail given the state of the question, but straight away, if converting to the cited but not quoted Wikipedia article, it looks as if a should be -coeffs. Also confusing using the same letters for different things.

– Tom Hawtin - tackline
Nov 20 '18 at 22:43

@TomHawtin-tackline yes Vector is a homemade class. Position is another vector, but I only want the closest point on the (x,z) plane, so that's why I'm not using the y coordinate. But I don't see how -coeff instead of coeff for a should change anything, a can be negative or positive

– Vinz243
Nov 21 '18 at 8:07

add a comment |

I have a affine equation y = ax + b where a is the coefficient (coeff).
Let D be a line that goes through axis and described by the previous equation.

I'm trying with this piece of code to find the coordinates of the closest point on D to position (ignoring the y coordinate, because 2D in 3D)

    double a = coeff;

    double b = position.getZ();

    double c = axis.getZ() - axis.getX() * coeff;

    double x0 = position.getX();

    double y0 = position.getZ();



    return new Vector((b * (b * x0 - a * y0) - a * c) / (a * a + b * b), position.getY(),

            (a * (-b * x0 + a * y0) - b * c) / (a * a + b * b));

Using this as a refernece

However, this does not work and return weird results

asked Nov 20 '18 at 21:54

Vinz243

3,91153165

I have a affine equation y = ax + b where a is the coefficient (coeff).
Let D be a line that goes through axis and described by the previous equation.

I'm trying with this piece of code to find the coordinates of the closest point on D to position (ignoring the y coordinate, because 2D in 3D)

    double a = coeff;

    double b = position.getZ();

    double c = axis.getZ() - axis.getX() * coeff;

    double x0 = position.getX();

    double y0 = position.getZ();



    return new Vector((b * (b * x0 - a * y0) - a * c) / (a * a + b * b), position.getY(),

            (a * (-b * x0 + a * y0) - b * c) / (a * a + b * b));

Using this as a refernece

However, this does not work and return weird results

java

asked Nov 20 '18 at 21:54

Vinz243

3,91153165

asked Nov 20 '18 at 21:54

Vinz243

3,91153165

asked Nov 20 '18 at 21:54

Vinz243

3,91153165

asked Nov 20 '18 at 21:54

Vinz243

3,91153165

asked Nov 20 '18 at 21:54

Vinz243

3,91153165

How are you not getting a compile error? afaik Vector has no constructor for three arguments: docs.oracle.com/javase/8/docs/api/java/util/Vector.html

– Jere
Nov 20 '18 at 22:11

What is "position" in your code? Is it an instance of a point in 3d? If so, then getZ is probably getting the 3rd dim and not what you'd want here.

– sofend
Nov 20 '18 at 22:24

I think this is a 3D solution. (Top tip: Write the simplest thing first. Then adapt it. Possibly keep the original solution in a separate method and call that.) @Jere Vector doesn't appear to be java.util.Vector but a 3D Vector. @sofend axis appears to refer to one of the X/Y/Z axises. Can't be bothered to look through this in detail given the state of the question, but straight away, if converting to the cited but not quoted Wikipedia article, it looks as if a should be -coeffs. Also confusing using the same letters for different things.

– Tom Hawtin - tackline
Nov 20 '18 at 22:43

@TomHawtin-tackline yes Vector is a homemade class. Position is another vector, but I only want the closest point on the (x,z) plane, so that's why I'm not using the y coordinate. But I don't see how -coeff instead of coeff for a should change anything, a can be negative or positive

– Vinz243
Nov 21 '18 at 8:07

add a comment |

How are you not getting a compile error? afaik Vector has no constructor for three arguments: docs.oracle.com/javase/8/docs/api/java/util/Vector.html

– Jere
Nov 20 '18 at 22:11

What is "position" in your code? Is it an instance of a point in 3d? If so, then getZ is probably getting the 3rd dim and not what you'd want here.

– sofend
Nov 20 '18 at 22:24

I think this is a 3D solution. (Top tip: Write the simplest thing first. Then adapt it. Possibly keep the original solution in a separate method and call that.) @Jere Vector doesn't appear to be java.util.Vector but a 3D Vector. @sofend axis appears to refer to one of the X/Y/Z axises. Can't be bothered to look through this in detail given the state of the question, but straight away, if converting to the cited but not quoted Wikipedia article, it looks as if a should be -coeffs. Also confusing using the same letters for different things.

– Tom Hawtin - tackline
Nov 20 '18 at 22:43

@TomHawtin-tackline yes Vector is a homemade class. Position is another vector, but I only want the closest point on the (x,z) plane, so that's why I'm not using the y coordinate. But I don't see how -coeff instead of coeff for a should change anything, a can be negative or positive

– Vinz243
Nov 21 '18 at 8:07

How are you not getting a compile error? afaik Vector has no constructor for three arguments: docs.oracle.com/javase/8/docs/api/java/util/Vector.html

– Jere
Nov 20 '18 at 22:11

What is "position" in your code? Is it an instance of a point in 3d? If so, then getZ is probably getting the 3rd dim and not what you'd want here.

– sofend
Nov 20 '18 at 22:24

I think this is a 3D solution. (Top tip: Write the simplest thing first. Then adapt it. Possibly keep the original solution in a separate method and call that.) @Jere Vector doesn't appear to be java.util.Vector but a 3D Vector. @sofend axis appears to refer to one of the X/Y/Z axises. Can't be bothered to look through this in detail given the state of the question, but straight away, if converting to the cited but not quoted Wikipedia article, it looks as if a should be -coeffs. Also confusing using the same letters for different things.

– Tom Hawtin - tackline
Nov 20 '18 at 22:43

@TomHawtin-tackline yes Vector is a homemade class. Position is another vector, but I only want the closest point on the (x,z) plane, so that's why I'm not using the y coordinate. But I don't see how -coeff instead of coeff for a should change anything, a can be negative or positive

– Vinz243
Nov 21 '18 at 8:07

add a comment |

1 Answer
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If you stay in vector representation it may be easier.
I have an example code but only in C++ (and Direct3D):

D3DXVECTOR3 ProjectOnLine (const D3DXVECTOR3 &point,

                           const D3DXVECTOR3 &linePoint,

                           const D3DXVECTOR3 &lineUnityDir)

{

  float t = D3DXVec3Dot(&(point-linePoint), &lineUnityDir);

  return linePoint + lineUnityDir*t;

}

If I understand you then your parameters can be used like this:

D3DXVECTOR3 point = position;

D3DXVECTOR3 linePoint = axis;

D3DXVECTOR3 lineUnityDir = D3DXVECTOR3(1, a, 0)/sqrt(1+a*a);

edited Nov 22 '18 at 15:55

answered Nov 21 '18 at 8:12

Yigal Eilam

193

add a comment |

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1 Answer
1

active

oldest

votes

1 Answer
1

active

oldest

votes

If you stay in vector representation it may be easier.
I have an example code but only in C++ (and Direct3D):

D3DXVECTOR3 ProjectOnLine (const D3DXVECTOR3 &point,

                           const D3DXVECTOR3 &linePoint,

                           const D3DXVECTOR3 &lineUnityDir)

{

  float t = D3DXVec3Dot(&(point-linePoint), &lineUnityDir);

  return linePoint + lineUnityDir*t;

}

If I understand you then your parameters can be used like this:

D3DXVECTOR3 point = position;

D3DXVECTOR3 linePoint = axis;

D3DXVECTOR3 lineUnityDir = D3DXVECTOR3(1, a, 0)/sqrt(1+a*a);

edited Nov 22 '18 at 15:55

answered Nov 21 '18 at 8:12

Yigal Eilam

193

add a comment |

If you stay in vector representation it may be easier.
I have an example code but only in C++ (and Direct3D):

D3DXVECTOR3 ProjectOnLine (const D3DXVECTOR3 &point,

                           const D3DXVECTOR3 &linePoint,

                           const D3DXVECTOR3 &lineUnityDir)

{

  float t = D3DXVec3Dot(&(point-linePoint), &lineUnityDir);

  return linePoint + lineUnityDir*t;

}

If I understand you then your parameters can be used like this:

D3DXVECTOR3 point = position;

D3DXVECTOR3 linePoint = axis;

D3DXVECTOR3 lineUnityDir = D3DXVECTOR3(1, a, 0)/sqrt(1+a*a);

edited Nov 22 '18 at 15:55

answered Nov 21 '18 at 8:12

Yigal Eilam

193

add a comment |

If you stay in vector representation it may be easier.
I have an example code but only in C++ (and Direct3D):

D3DXVECTOR3 ProjectOnLine (const D3DXVECTOR3 &point,

                           const D3DXVECTOR3 &linePoint,

                           const D3DXVECTOR3 &lineUnityDir)

{

  float t = D3DXVec3Dot(&(point-linePoint), &lineUnityDir);

  return linePoint + lineUnityDir*t;

}

If I understand you then your parameters can be used like this:

D3DXVECTOR3 point = position;

D3DXVECTOR3 linePoint = axis;

D3DXVECTOR3 lineUnityDir = D3DXVECTOR3(1, a, 0)/sqrt(1+a*a);

edited Nov 22 '18 at 15:55

answered Nov 21 '18 at 8:12

Yigal Eilam

193

If you stay in vector representation it may be easier.
I have an example code but only in C++ (and Direct3D):

D3DXVECTOR3 ProjectOnLine (const D3DXVECTOR3 &point,

                           const D3DXVECTOR3 &linePoint,

                           const D3DXVECTOR3 &lineUnityDir)

{

  float t = D3DXVec3Dot(&(point-linePoint), &lineUnityDir);

  return linePoint + lineUnityDir*t;

}

If I understand you then your parameters can be used like this:

D3DXVECTOR3 point = position;

D3DXVECTOR3 linePoint = axis;

D3DXVECTOR3 lineUnityDir = D3DXVECTOR3(1, a, 0)/sqrt(1+a*a);

edited Nov 22 '18 at 15:55

answered Nov 21 '18 at 8:12

Yigal Eilam

193

edited Nov 22 '18 at 15:55

answered Nov 21 '18 at 8:12

Yigal Eilam

193

answered Nov 21 '18 at 8:12

Yigal Eilam

193

answered Nov 21 '18 at 8:12

Yigal Eilam

193

add a comment |

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