SHX fonts: where they come from, and why TrueType is a better choice

Open an old DWG drawing: chances are its text uses romans.shx, simplex.shx, txt.shx or isocp.shx. These thin, angular fonts have been part of AutoCAD from the very beginning. They have real strengths, but they also cause trouble as soon as the drawing leaves AutoCAD.

Text written with txt.shx, AutoCAD's historical default font.

Born when nothing else existed

AutoCAD was released in December 1982. At the time, there was no standard vector font format on microcomputers. PostScript and its Type 1 fonts only arrived in 1984-1985, first for laser printers. TrueType, designed by Apple, only came to Windows with Windows 3.1, in 1992.

So Autodesk created its own format. An SHX font is the compiled version of a .shp shape file. Each character is described as a sequence of pen movements: draw so many units in a given direction, lift the pen, move, put it down again.

The sixteen directions a vector byte can encode, numbered in hexadecimal from 0 (to the right) to F, counterclockwise. The intermediate directions are not at 22.5°: direction 1, for example, moves two units across for one unit up.

That is exactly the language of a pen plotter, the output device of engineering offices at the time.

Most of these fonts are single-stroke: a letter has no outline to fill, it is made of plain strokes. The pen draws it in a single pass, quickly, with the width of the selected pen. An outline font, on the other hand, would have forced the plotter to draw the contour of each letter and then fill it.

Why drafters like them

They look like drawing-board lettering

Before CAD, the text on a drawing was written by hand, or with a lettering guide: a plastic template with letter-shaped cut-outs, traced with a technical pen. The result was regular lettering, with a constant stroke width and no serifs. The ISO 3098 standard still defines its proportions today.

Standardgraph lettering guides for ISO 3098 lettering — photo by Lucasbosch, CC BY-SA 3.0 license.

SHX fonts reproduce this lettering: isocp.shx follows ISO 3098, and romans.shx and simplex.shx are close to it. A drawing annotated with them looks like a drafter's drawing. This is not just nostalgia: it is the lettering everyone in a workshop or on a construction site is used to reading on a drawing.

They stay legible at small sizes, and take little space

A thin, constant stroke stays sharp at a height of 2 mm, where an outline font clogs up or blurs. The letters are narrow, which helps fit a dimension, a label or a note into a tight space. Finally, the text takes the width of the pen or of the assigned lineweight: it matches the rest of the drawing.

Where it goes wrong: exchanges and PDF

Other CAD software

A DWG file does not embed the fonts it uses: it only stores the name of the font file. To display the text as intended, the computer that opens the drawing needs the same .shx file.

This is already a problem between two AutoCAD workstations, as soon as a custom or third-party SHX font is missing. AutoCAD then replaces it with the font set by the FONTALT system variable, often simplex.shx. Text lengths change, and text overflows its title block or frame.

With another CAD application, it is worse. The SHX format is specific to Autodesk: every application that imports a DWG has to interpret it on its own, provide its own equivalents of AutoCAD's fonts, or fall back on a substitute font. The text gets through, but rarely identical.

PDF export

PDF can embed Type 1, TrueType and OpenType fonts. It does not know the SHX format. On export, AutoCAD therefore has no choice but to convert each SHX character into strokes. Visually, the result is faithful. But as far as the PDF is concerned, this text is no longer text:

• it cannot be searched or selected in the PDF viewer;
• it cannot be copied and pasted elsewhere;
• it escapes indexing and accessibility tools;
• a PDF imported back into a CAD application gives segments, not editable text.

AutoCAD offers a workaround: the PDFSHX system variable adds the SHX text to the PDF as invisible comments, on top of the strokes, which makes it searchable again. Likewise, PDFSHXTEXT tries to rebuild text from the strokes of an imported PDF. These are patches that do not work in every viewer or every application.

What to use instead

For a drawing that will be exchanged or published as PDF, prefer TrueType (.ttf) or OpenType fonts. The font file is still external to the DWG, but every CAD application can read a TrueType font, and PDF can embed it: the text stays text.

The safest choice is a font that is available everywhere:

• fonts close to the PDF base fonts, such as Arial (equivalent to Helvetica), which a PDF viewer can display even when they are not embedded;
• fonts shipped with Windows, present on every computer;
• to keep standard drawing lettering, isocpeur.ttf, which the AutoCAD installation places in the Windows fonts folder and which is the TrueType equivalent of isocp.shx — making sure the recipient has the font.

Before embedding a font in a PDF you distribute, also check that its license allows it.

To convert existing drawings, it is usually enough to change the text styles (STYLE) so they point to a TrueType font: all text and dimensions using that style follow. Across a large number of files, this can be automated.

SHX fonts have served well. They are still relevant for a drawing that never leaves AutoCAD and goes to a pen plotter. But a drawing that circulates today almost always ends up as a PDF or in another application: you might as well give it a font everyone can read.