# D3DXVec4CatmullRom function

Performs a Catmull-Rom interpolation, using the specified 4D vectors.

## Syntax

```
D3DXVECTOR4* D3DXVec4CatmullRom(
_Inout_  D3DXVECTOR4 *pOut,
_In_     const D3DXVECTOR4 *pV0,
_In_     const D3DXVECTOR4 *pV1,
_In_     const D3DXVECTOR4 *pV2,
_In_     const D3DXVECTOR4 *pV3,
_In_     FLOAT s
);

```

## Parameters

pOut [in, out]

Type: D3DXVECTOR4*

Pointer to the D3DXVECTOR4 structure that is the result of the operation.

pV0 [in]

Type: const D3DXVECTOR4*

Pointer to a source D3DXVECTOR4 structure, a position vector.

pV1 [in]

Type: const D3DXVECTOR4*

Pointer to a source D3DXVECTOR4 structure, a position vector.

pV2 [in]

Type: const D3DXVECTOR4*

Pointer to a source D3DXVECTOR4 structure, a position vector.

pV3 [in]

Type: const D3DXVECTOR4*

Pointer to a source D3DXVECTOR4 structure, a position vector.

s [in]

Type: FLOAT

Weighting factor. See Remarks.

## Return value

Type: D3DXVECTOR4*

Pointer to a D3DXVECTOR4 structure that is the result of the Catmull-Rom interpolation.

## Remarks

Given four points (p1, p2, p3, p4), find a function Q(s) such that:

```

Q(s) is a cubic function.
Q(s) interpolates between p2 and p3 as s ranges from 0 to 1.
Q(s) is parallel to the line joining p1 to p3 when s is 0.
Q(s) is parallel to the line joining p2 to p4 when s is 1.

```

The Catmull-Rom spline can be derived from the Hermite spline by setting:

```

v1 = p2
v2 = p3
t1 = (p3 - p1) / 2
t2 = (p4 - p2) / 2

```

where:

v1 is the contents of pV0.

v2 in the contents of pV1.

p3 is the contents of pV2.

p4 is the contents of pV3.

Using the Hermite spline equation:

```

Q(s) = (2s3 - 3s2 + 1)v1 + (-2s3 + 3s2)v2 + (s3 - 2s2 + s)t1 + (s3 - s2)t2

```

and substituting for v1, v2, t1, t2 yields:

```

Q(s) = (2s3 - 3s2 + 1)p2 + (-2s3 + 3s2)p3 + (s3 - 2s2 + s)(p3 - p1) / 2 + (s3 - s2)(p4 - p2) / 2

```

This can be rearranged as:

```

Q(s) = [(-s3 + 2s2 - s)p1 + (3s3 - 5s2 + 2)p2 + (-3s3 + 4s2 + s)p3 + (s3 - s2)p4] / 2

```

## Requirements

D3DXVec2CatmullRom
D3DXVec3CatmullRom