# globalToLocal method

- Offset point,
- {RenderObject? ancestor}

Convert the given point from the global coordinate system in logical pixels to the local coordinate system for this box.

This method will un-project the point from the screen onto the widget, which makes it different from MatrixUtils.transformPoint.

If the transform from global coordinates to local coordinates is degenerate, this function returns Offset.zero.

If `ancestor`

is non-null, this function converts the given point from the
coordinate system of `ancestor`

(which must be an ancestor of this render
object) instead of from the global coordinate system.

This method is implemented in terms of getTransformTo.

## Implementation

```
Offset globalToLocal(Offset point, { RenderObject? ancestor }) {
// We want to find point (p) that corresponds to a given point on the
// screen (s), but that also physically resides on the local render plane,
// so that it is useful for visually accurate gesture processing in the
// local space. For that, we can't simply transform 2D screen point to
// the 3D local space since the screen space lacks the depth component |z|,
// and so there are many 3D points that correspond to the screen point.
// We must first unproject the screen point onto the render plane to find
// the true 3D point that corresponds to the screen point.
// We do orthogonal unprojection after undoing perspective, in local space.
// The render plane is specified by renderBox offset (o) and Z axis (n).
// Unprojection is done by finding the intersection of the view vector (d)
// with the local X-Y plane: (o-s).dot(n) == (p-s).dot(n), (p-s) == |z|*d.
final Matrix4 transform = getTransformTo(ancestor);
final double det = transform.invert();
if (det == 0.0) {
return Offset.zero;
}
final Vector3 n = Vector3(0.0, 0.0, 1.0);
final Vector3 i = transform.perspectiveTransform(Vector3(0.0, 0.0, 0.0));
final Vector3 d = transform.perspectiveTransform(Vector3(0.0, 0.0, 1.0)) - i;
final Vector3 s = transform.perspectiveTransform(Vector3(point.dx, point.dy, 0.0));
final Vector3 p = s - d * (n.dot(s) / n.dot(d));
return Offset(p.x, p.y);
}
```