From c0a6a139cf9e53bcf1ae3d1867b02c10ab2b5754 Mon Sep 17 00:00:00 2001
From: Seth Hillbrand <seth@kipro-pcb.com>
Date: Mon, 23 Feb 2026 08:53:11 -0800
Subject: [PATCH] Add $INSUNITS header variable to DXF export

The DXF exporter was missing the $INSUNITS header variable, which tells
CAD software what physical units the drawing coordinates represent. Without
it, other programs have no reliable way to determine the coordinate scale.

Add $INSUNITS to the HEADER section alongside the existing $MEASUREMENT
variable. When exporting in millimeters, $INSUNITS is set to 4; when
exporting in inches, it is set to 1.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/23181
---
 common/plotters/DXF_plotter.cpp |  8 +++++++-
 include/plotters/plotter_dxf.h  | 11 +++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/common/plotters/DXF_plotter.cpp b/common/plotters/DXF_plotter.cpp
index 34e2588360..0e3242ae73 100644
--- a/common/plotters/DXF_plotter.cpp
+++ b/common/plotters/DXF_plotter.cpp
@@ -701,12 +701,14 @@ void DXF_PLOTTER::SetUnits( DXF_UNITS aUnit )
     case DXF_UNITS::MM:
         m_unitScalingFactor = 0.00254;
         m_measurementDirective = 1;
+        m_insUnits = 4;
         break;
 
     case DXF_UNITS::INCH:
     default:
         m_unitScalingFactor = 0.0001;
         m_measurementDirective = 0;
+        m_insUnits = 1;
     }
 }
 
@@ -779,6 +781,10 @@ bool DXF_PLOTTER::StartPlot( const wxString& aPageNumber )
                 "$MEASUREMENT\n"
                 "  70\n"
                 "{}\n"
+                "  9\n"
+                "$INSUNITS\n"
+                "  70\n"
+                "{}\n"
                 "  0\n"
                 "ENDSEC\n"
                 "  0\n"
@@ -873,7 +879,7 @@ bool DXF_PLOTTER::StartPlot( const wxString& aPageNumber )
                 "-0.2\n"
                 "  0\n"
                 "ENDTAB\n",
-                GetMeasurementDirective() );
+                GetMeasurementDirective(), GetInsUnits() );
 
     // Text styles table
     // Defines 4 text styles, one for each bold/italic combination
diff --git a/include/plotters/plotter_dxf.h b/include/plotters/plotter_dxf.h
index e5f851f0c2..34017011ea 100644
--- a/include/plotters/plotter_dxf.h
+++ b/include/plotters/plotter_dxf.h
@@ -476,6 +476,16 @@ public:
         return m_measurementDirective;
     }
 
+    /**
+     * Get the correct value for the $INSUNITS header variable given the current units.
+     *
+     * @return the $INSUNITS value (1 = inches, 4 = millimeters)
+     */
+    unsigned int GetInsUnits() const
+    {
+        return m_insUnits;
+    }
+
     // Finds the nearest legacy color to the given RGB values (aR, aG, aB).
     int FindNearestLegacyColor( int aR, int aG, int aB );
 
@@ -493,4 +503,5 @@ protected:
     DXF_UNITS    m_plotUnits;
     double       m_unitScalingFactor;
     unsigned int m_measurementDirective;
+    unsigned int m_insUnits;
 };