Flutter canvas. How to get a ray through two points, and calculate the position through the x or y c-CodePudding

My English is not good, I'll try to explain my question as well as I can. Thank you.

I use canvas to draw.

A, B, C is on the same line. There's two points and a x/y axis value of pointC:

Offset pointA = Offset(100, 200);
Offset pointB = Offset(200, 400);
double pointCx = 300;

Offset pointC = calculateOffsetByX(pointA, pointB, pointCx);

https://i.bmp.ovh/imgs/2021/12/0a7ee73b12749799.png

I need calculate the Offset of c if I have x/y value.

And encapsulated as a function like this:

/// @param [pointStart] start point of line
/// @param [point2] second point of line
/// @param [x] the position x of point need to calculate
/// @return Offset of the point on line
Offset calculateOffsetByX(Offset pointStart, Offset point2, double x){
  // TODO
}
/// @param [pointStart] start point of line
/// @param [point2] second point of line
/// @param [y] the position y of point need to calculate
/// @return Offset of the point on line
Offset calculateOffsetByY(Offset pointStart, Offset point2, double y){
  // TODO
}

And sometime the Offset is not positive number, need to consider those conditions: https://i.bmp.ovh/imgs/2021/12/c6352f50f5dfb00d.png

My language is not English, so the description may be confusing, if I don’t describe clearly, I will continue to explain, thank you very much!

CodePudding user response：

You can try this funcion. (I am not test this code, pls try this code and show me result if it not working):

  /// @param [pointStart] start point of line
  /// @param [point2] second point of line
  /// @param [x] the position x of point need to calculate
  /// @return Offset of the point on line
  Offset calculateOffsetByX(Offset pointStart, Offset point2, double x){
    //y =  ax   b
    final a = (pointStart.dy - point2.dy)/(pointStart.dx - point2.dx);
    final b = pointStart.dy - a*pointStart.dx;
    return Offset(x, a*x  b);
  }
  /// @param [pointStart] start point of line
  /// @param [point2] second point of line
  /// @param [y] the position y of point need to calculate
  /// @return Offset of the point on line
  Offset calculateOffsetByY(Offset pointStart, Offset point2, double y){
    //y =  ax   b
    final a = (pointStart.dy - point2.dy)/(pointStart.dx - point2.dx);
    final b = pointStart.dy - a*pointStart.dx;
    return Offset((y - b)/a, y);
  }