ID2D1PathGeometry1::ComputePointAndSegmentAtLength method
Computes the point that exists at a given distance along the path geometry along with the index of the segment the point is on and the directional vector at that point.
Syntax
HRESULT ComputePointAndSegmentAtLength( FLOAT length, UINT startSegment, [in, optional] const D2D1_MATRIX_3X2_F *worldTransform, FLOAT flatteningTolerance, [out] D2D1_POINT_DESCRIPTION *pointDescription ) const;
Parameters
 length

Type: FLOAT
The distance to walk along the path.
 startSegment

Type: UINT
The index of the segment at which to begin walking. Note: This index is global to the entire path, not just a particular figure.
 worldTransform [in, optional]

Type: const D2D1_MATRIX_3X2_F*
The transform to apply to the path prior to walking.
 flatteningTolerance

Type: FLOAT
The flattening tolerance to use when walking along an arc or Bezier segment. The flattening tolerance is the maximum error allowed when constructing a polygonal approximation of the geometry. No point in the polygonal representation will diverge from the original geometry by more than the flattening tolerance. Smaller values produce more accurate results but cause slower execution.
 pointDescription [out]

Type: D2D1_POINT_DESCRIPTION*
When this method returns, contains a description of the point that can be found at the given location.
Return value
Type: HRESULT
The method returns an HRESULT. Possible values include, but are not limited to, those in the following table.
HRESULT  Description 

S_OK  No error occurred. 
E_OUTOFMEMORY  Direct2D could not allocate sufficient memory to complete the call. 
E_INVALIDARG  One of the inputs was in an invalid range. 
Remarks
A length that is less than 0 or is not a number is treated as if it were 0.
If length is greater than the total length of the path, then the end point of the path is returned.
Example Illustration
Consider this example that explains the value of different parameters returned for the given path geometry.
Here are two different scenarios.
You want to retrieve the segment at a length L2
You call ComputePointAndSegmentAtLength(Length = L2, startSegment =0). The API returns the following:
 D2D1_POINT_DESCRIPTION::point = p2.
 D2D1_POINT_DESCRIPTION::endSegment = 3 (segment DE). This is the value you want.
 D2D1_POINT_DESCRIPTION::lengthToEndSegment = length (AD).
You wants to improve the performance of calculating a point at a given length for animating along a path
Normally, the time intervals would be small and regular, resulting in many animation points per segment. For the purposes of demonstration, however, we will assume the you query ComputePointAndSegmentAtLength three times, with irregularlyspaced lengths L1, L2, L3:
You call ComputePointAndSegmentAtLength(Length = L1, startSegment = 0). The API returns the following:
 D2D1_POINT_DESCRIPTION::point = P1.
 D2D1_POINT_DESCRIPTION::endSegment = 1 (segment BC).
 D2D1_POINT_DESCRIPTION::lengthToEndSegment = length (AB).
You call ComputePointAndSegmentAtLength(Length = L2  Length(AB), startSegment = 1). The API returns the following:
 D2D1_POINT_DESCRIPTION::point = P2.
 D2D1_POINT_DESCRIPTION::endSegment = 3 (segment DE).
 D2D1_POINT_DESCRIPTION::lengthToEndSegment = length (AD).
You call ComputePointAndSegmentAtLength(= L3length(AB)length(BD), startSegment = 3). The API returns the following:
 D2D1_POINT_DESCRIPTION::point = P3.
 D2D1_POINT_DESCRIPTION::endSegment = 3 (segment DE).
 D2D1_POINT_DESCRIPTION::lengthToEndSegment =0.
Requirements
Minimum supported client 
Windows 8 and Platform Update for Windows 7 [desktop apps  Windows Store apps] 

Minimum supported server 
Windows Server 2012 and Platform Update for Windows Server 2008 R2 [desktop apps  Windows Store apps] 
Minimum supported phone 
Windows Phone 8.1 [Windows Phone Silverlight 8.1 and Windows Runtime apps] 
Header 

DLL 

See also