Ignore path constructors that do not begin with m

[jsvine]

I have encountered a couple of PDFs (see jsvine/pdfplumber#636 (comment) and jsvine/pdfplumber#644 (comment)) that include a ("h", ) path — i.e., just the h operator and no others. This causes this second statement here to throw ValueError: not enough values to unpack (expected 2, got 1):

pdfminer.six/pdfminer/converter.py

Lines 126 to 129 in 1bf3c42

	raw_pts = [
	cast(Point, p[-2:] if p[0] != "h" else path[0][-2:]) for p in path
	]
	pts = [apply_matrix_pt(self.ctm, pt) for pt in raw_pts]

Pull request

Although there may be narrower solutions to this particular problem (e.g., handling the specific edge-case of ("h",)), this PR introduces what — at least to me — seems like the most generalized-yet-accurate solution, after reading the PDF Reference.

Per Section 4.4.1, "path construction operators may be invoked in any sequence, but the first one invoked must be m or re to begin a new subpath." Since pdfminer.six already converts all re (rectangle) operators to their equivelent mlllh representation, paths ingested by .paint_path(...) that do not begin with the m operator are invalid.

I am, however, open to other approaches 👍

How Has This Been Tested?

I have added a test to tests/test_converter.py. I have also examined all non-encrypted PDFs in the samples/ directory. Of the 31,116 paths and subpaths in those 33 PDFs, all start with the “m” operator (or have been translated from the “re” operator to take that form).

Checklist

• I have formatted my code with black.
• I have added tests that prove my fix is effective or that my feature
  works
• [not applicable] I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or verified that this
  is not necessary
• I have updated the readthedocs documentation or
  verified that this is not necessary
• I have added a concise human-readable description of the change to
  CHANGELOG.md

[Nebukadneza]

Not a review, but user-originating test: This solves the problem I initially encountered in #729!

[pietermarsman]

Thanks @jsvine and @Nebukadneza!

[pietermarsman]

Fixes #729