DXF: Cannot import hatches with boundaries that contain elliptical arcs

[Alan Thomas]

The importation of hatch entities with elliptical arcs in their boundaries is completely broken:

• The ratio of minor axis to major axis is incorrectly interpreted by OGR as a percentage between 0 and 100, where it is actually a ratio between 0 and 1.

• In AutoCAD, "start angle" and "end angle" refer to circular angles. OGR is incorrectly interpreting these values as elliptical angles, or parameters.

Given the common code, it's possible that the DWG format also has these issues (although I haven't tested that).

The specification (​http://images.autodesk.com/adsk/files/autocad_2014_pdf_dxf_reference_enu.pdf, PDF page 111, numbered 103 in document) is admittedly vague, but the problems are obvious when you export a DXF from AutoCAD and try reading it into OGR.

[Alan Thomas]

Screenshot of the example DXF file

[Alan Thomas]

Sample DXF file with elliptical arc on hatch boundary

[Alan Thomas]

Patch

[Even Rouault]

@atlight What is the rationale for the "180.0 * floor ( ( dfStartAngle + 90 ) / 180 )" term in the formula that converts from circular to elliptical angles ?

[Alan Thomas]

@Even: This part of the formula:

atan( ( 1.0 / dfRatio ) * tan( dfEndAngle * M_PI / 180 ) )

always returns an angle between -90 and 90 degrees. The term

180.0 * floor ( ( dfStartAngle + 90 ) / 180 )

returns a multiple of 180, which is added on in order to give the correct result.

If you know a clearer formula to convert circular to elliptical angles, please feel free to substitute that :)

[Even Rouault]

In 39826:

DXF: fix reading of hatches with boundaries that contain elliptical arcs (patch by atlight, fixes #6971)