[meshcat] More robust parsing of .mtl texture map filenames

[RussTedrake]

In RobotLocomotion/models#18
(homecart_basecart.mtl) I ran into a texture map specification that
specified a scale option in the map_Kd line. This defeated the simple
regex expression that I had implemented in meschat.cc.

meshcat.cc doesn't need to parse these options properly; it only needs
to successfully extract the filename so that the file can be shipped
over the websocket. I've updated the parsing line according to the
mtl specification to ignore options.

To reproduce the problem, run meshcat_manual_test with the change to
box.obj.mtl but without the fix to meshcat.cc. You will see that the
box appears in meshcat without a texture and a warning from meshcat.cc
is printed to the console. The change to meshcat.cc resolves the
problem.

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This change is 

[RussTedrake]

+@SeanCurtis-TRI for feature review, please.

Reviewable status: LGTM missing from assignee SeanCurtis-TRI(platform), needs at least two assigned reviewers (waiting on @SeanCurtis-TRI)

[SeanCurtis-TRI]

My single greatest regret is that this is untested. :( This is meaningful logic for parsing the file and it would be good if it were supported by test. Right now the test passes, regardless of what the regular expression looks like. However, I'm too rusty on Meshcat to recommend any specific solution to this. The correctness of this parsing wasn't tested before, so it's no worse now.

+@rpoyner-tri for platform review.

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: 2 unresolved discussions, LGTM missing from assignee rpoyner-tri(platform) (waiting on @rpoyner-tri and @RussTedrake)

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geometry/meshcat.cc line 413 at r1 (raw file):

        // Here we ignore the options and only extract the filename (by
        // extracting the last word before the end of line/string).
        //  - "map_.+"" matches the map_ plus any options,

nit: Extra ". ("map_.+"" <--)

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geometry/meshcat.cc line 414 at r1 (raw file):

        // extracting the last word before the end of line/string).
        //  - "map_.+"" matches the map_ plus any options,
        //  - "\s" matches one whitespace (before the filename),

BTW Here in this comment, sometimes you use proper reg ex tokens (\s and sometimes you use escaped tokens (\\s). I'd recommend the following:

1. Use proper regex tokens in the comment.
2. Use a raw string literal below (so the tokens match).

std::regex map_regex(R"""(map_.+\s([^\s]+)[$\r\n])""");

[rpoyner-tri]

concur with Sean's comments.

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: 2 unresolved discussions (waiting on @RussTedrake)

[RussTedrake]

Reviewable status: complete! all discussions resolved, LGTM from assignees rpoyner-tri(platform),SeanCurtis-TRI(platform) (waiting on @rpoyner-tri and @SeanCurtis-TRI)

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geometry/meshcat.cc line 414 at r1 (raw file):

Previously, SeanCurtis-TRI (Sean Curtis) wrote…

BTW Here in this comment, sometimes you use proper reg ex tokens (\s and sometimes you use escaped tokens (\\s). I'd recommend the following:

1. Use proper regex tokens in the comment.
2. Use a raw string literal below (so the tokens match).

std::regex map_regex(R"""(map_.+\s([^\s]+)[$\r\n])""");

Done.