SVG import: Add normalize_coordinates option

[chrysn]

This adds an option as described in #3460 (comment), closing #3460:

• literal_coordinate
  Boolean. If set to true, the numeric values of x and y coordinates are used without format specific transformations.

  This argument only applies to SVG (or similar future formats): SVG's coordinate system applies the y value toward the bottom of the screen on the drawing plane, whereas in OpenSCAD's top view, y points toward the top of the screen. By default (i.e., when literal_coordinate=false), the import command flips an SVG file's y coordinates, and shifts the whole drawing into the positive area.

  Using literal_coordinate=true is particularly useful when the OpenSCAD script has additional information about positions inside the SVG file's coordinate system.

  This is mutually exclusive with the "center" argument.

There is open point from #3460 (comment): Should literal_coordinate also disregard scaling and force the import units to be SVG units, or is it fine as it is (and the documentation just needs slight adjustment)?

[t-paul]

The option name feels a bit awkward, but I suppose the feature itself is useful. I don't have a good suggestion yet though.

This is mutually exclusive with the "center" argument.

Why? Is there a technical reason? It does not bring any additional feature as it should be identical to normal centered + y axis flipped but it might be helpful to not impose the exclusivity restriction.

[chrysn]

I'm open for any other name.

As for being mutually exclusive with centering, this overrides all the coordinate processing -- scaling, flipping, viewbox adjustment and center offset. It's probably possible to apply center offsets still, but it kind of defeats the purpose of having the raw coordinates applied. All use cases where I've run into this were along the lines of "some tool generates me an SVG with a sidecar file of coordinates, and I want to put them together in 3D". (About 10 years ago it was visualizing PCBs, now it's assembling laser cut boxes in 3D). If you think it's useful to have, I think I can make it work, but I think it reduces maintainability without a practical benefit.

[t-paul]

Oh, it skips transformations even on the objects? I don't think that will work, e.g. Inkscape has the tendency to modify the object transforms instead of the actual coordinates (create circle, make it a bit smaller, move slightly). Skipping those will just import random data.

[MichaelAtOz]

I'm open for any other name.

flip=<true|*false>??

By default (i.e., when literal_coordinate=false), the import command FLIPS an SVG file's y coordinates, and shifts the whole drawing into the positive area.

[gsohler]

default is more but only flip. maybe name it "normalize" ?

…

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[chrysn]

Oh, it skips transformations even on the objects?

No, it skips only the transformations that OpenSCAD would apply (the handedness correction / flip, the shift, and any centering). Anything internal to the SVG is preserved.

normalize

Sounds good, so it'd be normalize_coordinates=true/false. This also has the upside that (unlike the "literal" term) it'd be justifiable to stick with the current convention of OpenSCAD units being millimeters (as applying 'literal' coordinates to SVG's complex units system would be troublesome anyway).

[chrysn]

With the PR updated, the documentation to be added would now be:

• normalize_coordinates
  Boolean. If true (which is the default), OpenSCAD transforms coordinates from the file to be suitable in its coordinate system.

  This argument only applies to SVG (or similar future formats): SVG's coordinate system applies the y value toward the bottom of the screen on the drawing plane, whereas in OpenSCAD's top view, y points toward the top of the screen. During normalization of coordinates, the import command flips an SVG file's y coordinates, and shifts the whole drawing into the positive area.

  Using normalize_coordinates=false is particularly useful when the OpenSCAD script has additional information about positions inside the SVG file's coordinate system. The resulting object will appear to be mirrored upside down in the default view, but correct when viewed from the bottom.

  When this is set to false, the "center" argument is ignored, and a warning is shown if center=true.

[chrysn]

Is there anything more I should adjust for this? From my point of view it is ready for final review.

(Picture related: Assembling parts from SVG files requires precise control over the coordinate space.)

[chrysn]

I've slightly extended the help text (around appearing mirrored), but that's kind of outside the PR. Is there anything I can do (add or remove) to advance this issue?