Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/8184#discussion_r37246855
--- Diff: docs/ml-features.md ---
@@ -649,6 +649,70 @@ for expanded in polyDF.select("polyFeatures").take(3):
</div>
</div>
+## Discrete Cosine Transform (DCT)
+
+The [Discrete Cosine
Transform](https://en.wikipedia.org/wiki/Discrete_cosine_transform) transforms
a length $N$ real-valued sequence in the time domain into another length $N$
real-valued sequence in the frequency domain. A
[DCT](api/scala/index.html#org.apache.spark.ml.feature.DCT) class provides this
functionality, implementing the
[DCT-II](https://en.wikipedia.org/wiki/Discrete_cosine_transform#DCT-II) and
scaling the result by $1/\sqrt{2}$ such that the representing matrix for the
transform is unitary. No shift is applied to the transformed sequence (e.g. the
$0$th element of the transformed sequence is the $0$th DCT coefficient and
_not_ the $N/2$th).
--- End diff --
minor: Please maintain the line width at 100 and end sentences with new
lines. This helps reduce the diff size of future edits.
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