DXF import and export

DXF is the drawing exchange format of AutoCAD, and almost every CAD program reads and writes it. MechPilot reads ASCII DXF files of any version and writes DXF R12 (AC1009, the most widely readable) and DXF 2000 (AC1015, which keeps ellipses, splines, lineweights and true colours).

Reference implementation: shared/dxf/ (read.ts, write.ts, codes.ts). Tests: tests/dxf.test.ts with hand-written files in tests/fixtures/dxf/. Group codes follow the DXF Reference published by Autodesk (sections HEADER, TABLES, BLOCKS, ENTITIES, OBJECTS).

Precision

Lengths are millimetres and are never rounded (product rule 1). Numbers are written with the shortest text that reads back to exactly the same double (0.30000000000000004, not 0.3), so a MechPilot → DXF → MechPilot round trip gives the same numbers, bit for bit. Two values go through a conversion and can change in the last digit (about 1e-15 relative): the angles of arcs and the rotation of texts, which DXF stores in degrees while MechPilot uses radians. Ellipse parameters are radians in both, so they are exact.

Units

On export MechPilot writes $INSUNITS = 4 (millimetres) and $EXTMIN / $EXTMAX (the drawing extents). R12 predates $INSUNITS; R12 readers ignore it and newer readers use it.

On import the unit comes from the HEADER variable $INSUNITS (DXF Reference, HEADER section, $INSUNITS, group code 70) and every length is converted to millimetres:

Drawings from R12-era programs often have no $INSUNITS. If such a part looks 25.4 times too small, it was drawn in inches: scale it by 25.4.

Export

Before exporting, the app can ask dxfExportNotes(doc, version) what the file will change and show it: ellipses and splines approximated in R12, layer names DXF cannot hold, texts with line breaks. For a DXF 2000 export of ordinary geometry the list is empty.

Every entity carries the name of its layer (group code 8). The file also has the tables other programs expect: line types (LTYPE), layers (LAYER) and the text style STANDARD; DXF 2000 files add handles, the BLOCK_RECORD table, the model and paper space blocks and the root dictionary in OBJECTS. The current layer is written as $CLAYER.

Layers

Line types

The dash lengths written in the LTYPE table (HIDDEN 6.35/−3.175, CENTER 31.75/−6.35/6.35/−6.35, PHANTOM 31.75/−6.35/6.35/−6.35/6.35/−6.35, DASHED 12.7/−6.35 mm) are the usual display patterns of DXF programs, not ISO 128-2 values; they only affect how other programs draw the dashes.

Import

Entities

Entities marked invisible (60 = 1) are skipped silently. Entities in paper space (67 = 1: title blocks, viewports) are skipped with one warning; only model space is imported. Everything else (HATCH, DIMENSION, LEADER, SOLID, 3DFACE, IMAGE, POINT, ATTRIB, …) is skipped with one warning per type that says how many there were.

Colours, line types or lineweights set on single entities are not kept (MechPilot draws each entity with its layer's settings); one warning says how many entities had them.

Z coordinates are dropped. If any entity had a z value (or a tilted extrusion direction) one warning says so. Arcs, circles, polylines and texts drawn with the extrusion direction (0, 0, −1), which some programs use for mirrored geometry, are converted to the equivalent plan geometry.

Blocks

Every INSERT is expanded into plain entities: the block's entities are moved by the insertion point minus the block base point, scaled by the x scale (41), rotated (50, degrees) and repeated for arrays (70/71 columns and rows, 44/45 spacing). Nested blocks are expanded up to 8 levels (a deeper or self-inserting block is cut there, with a warning). Entities on layer 0 inside a block take the layer of the INSERT, as in AutoCAD. A mirrored insert (scale −1) is kept as a mirror. When the x and y scales differ, the x scale is used for both and a warning says so (non-uniform scaling would turn circles into ellipses). Blocks that are missing or are external references are skipped with a warning listing their names.

Layers

Entities on a layer that is not in the layer table get that layer, created with default settings (one warning lists them). A file without a layer table starts from MechPilot's default layers. $CLAYER becomes the active layer.

AutoCAD Color Index

Robustness

Files with CRLF or LF line endings, spaces around group codes, 999 comments, a missing HEADER or TABLES section or a missing EOF are read. The CLASSES and OBJECTS sections are ignored. A file that has no SECTION or no ENTITIES section is refused with "This file is not a DXF drawing" and a hint; a binary DXF is refused with a hint to save it again as ASCII DXF.

Warnings

Import returns the document and a list of warnings { code, message, count }, one per kind (and one per entity type for skipped entities), in plain words:

Limitations

• Dimensions, hatches, leaders, points, attributes and images are not imported yet.
• MTEXT becomes single-line text; its width, line spacing and horizontal alignment are not kept.
• TEXT alignment other than baseline left is read from the first alignment point, which DXF writers set to the baseline-left point of the text.
• Text encoding: MechPilot writes only ASCII (other characters as \U+XXXX and %% codes). When reading, the caller decodes the file bytes (UTF-8 for AutoCAD 2007 and later, the code page in $DWGCODEPAGE before); \M+ multibyte escapes are dropped.
• R12 export approximates ellipses and splines with polylines within 0.01 mm, has no lineweights and stores colours as the nearest ACI colour.
• Line types 02.2 and 04.2 come back from DXF only when the file has lineweights (DXF 2000).
• MechPilot dimensions, notes, balloons, surface symbols, tolerance frames and datums are exported as the lines, arcs, circles, polylines and texts they are drawn with (arrowheads as closed polylines, not filled SOLIDs), so other programs show exactly what the sheet shows but cannot edit them as dimensions. DXF DIMENSION entities are not written or read yet.

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