[QGIS-Developer] Interrogation about algorithms to offset/translate features

DelazJ delazj at gmail.com
Mon Aug 20 06:17:14 PDT 2018

Good day devs,

Today I was looking for an algorithm to create parallel lines in another
layer and a search provides me with many candidates. I found

- "translate": creates one parallel feature moved at a specified x/y/z/m
offset distance
- "array of offset features": same thing than translate but generating the
parallel of the parallels up to the specified number of copies
- "offset lines": applies an offset to the features (from what I see, it's
an homothetic operation), generating one feature that is not a copy of the
previous (not the same length, if multiline eg)
- "create parallel lines": same as  the offset lines tool but generating
multiple offsets.

I have some concerns/questions:

-"translate" vs "array of offset features": why not just add the "number of
features to create" parameter to the translate algorithm instead of having
created a new one?
- "offset lines" vs "create parallel lines": same question as above, in
favor of the offset lines algorithm
- the "create parallel lines" algorithm creates more features than expected
(actually it adds the original feature to the output which imho should be a
bug (and not consistent with the array alg behavior))
- I'm pretty lost with the use of the word "offset" in those algs. I don't
know what it really implies in English but my understanding was more of a
translation than a homothety. It tooks me some time to sort out the above
explanation of the algorithms behavior:
  - Would that be possible to review the description and insert this
homothetic idea so that people understand clearly that one do copy the same
features and the other one may be different? with less algs, there will be
less translation to do and I hope less confusion around this concept.
  - and I still wonder if "create parallel lines" and "array of offset
features" are not inverted names: in my mind, "array of offset features"
sounds closer to "offset lines" than to "translate" which in turn is looks
closer to "create parallel lines".
- what about allowing polygons for the currently called "offset lines" and
"create parallel lines" algs?

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