---
title: vDSP.DCTTransformType.III
framework: accelerate
role: symbol
role_heading: Case
path: accelerate/vdsp/dcttransformtype/iii
---

# vDSP.DCTTransformType.III

A constant that represents a type-III discrete cosine transform.

## Declaration

```swift
case III
```

## Discussion

Discussion The type-III DCT uses the following operation for a discrete cosine transform: // `N` is the length given in the setup. // `h` is the input array that contains real numbers. // `H` is the output array that contains real numbers.

For 0 <= k < N     Or[k] = Ir[0]/2         + sum(Ir[j] * cos((k+1/2) * j * pi / N), 1 <= j < N)

## See Also

### Discrete Cosine Transform Type Constants

- [vDSP.DCTTransformType.II](accelerate/vdsp/dcttransformtype/ii.md)
- [vDSP.DCTTransformType.IV](accelerate/vdsp/dcttransformtype/iv.md)