A component can report which side of the board it is mounted on, and Gerber export puts the silkscreen of a part mounted underneath on the bottom layer

diylc/diylc-core/src/main/java/org/diylc/core/IBoardSideComponent.java

@@ -0,0 +1,42 @@
+/*
+
+    DIY Layout Creator (DIYLC).
+    Copyright (c) 2009-2025 held jointly by the individual authors.
+
+    This file is part of DIYLC.
+
+    DIYLC is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    DIYLC is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with DIYLC.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+package org.diylc.core;
+
+import org.diylc.common.PCBLayer;
+
+/**
+ * Interface for components mounted on one side of a board, such as a resistor or an IC. The side
+ * says where the part sits physically; it is separate from {@link ILayeredComponent}, which drives
+ * continuity, because the leads of a through-hole part connect to copper on both sides whichever
+ * side the part is mounted on. Components whose copper already has a layer (traces, pads, ground
+ * fills) do not implement it.
+ * 
+ * @author Branislav Stojkovic
+ */
+public interface IBoardSideComponent {
+
+  /**
+   * @return {@link PCBLayer#_2} for the top of the board or {@link PCBLayer#_1} for the bottom,
+   *         never an inner layer or null
+   */
+  PCBLayer getBoardSide();
+}

diylc/diylc-core/src/main/java/org/diylc/core/gerber/GerberExporter.java

@@ -16,8 +16,10 @@ import org.apache.log4j.Logger;
 
 import com.bancika.gerberwriter.DataLayer;
 
+import org.diylc.common.PCBLayer;
 import org.diylc.core.ComponentState;
 import org.diylc.core.IBoard;
+import org.diylc.core.IBoardSideComponent;
 import org.diylc.core.IDIYComponent;
 import org.diylc.core.IView;
 import org.diylc.core.Project;
@@ -158,8 +160,14 @@ public class GerberExporter {
       gerberComponentCustom.draw(g2d, ComponentState.NORMAL, outlineMode, currentProject, g2d, g2d);
     } else if (gerberComponent instanceof IGerberComponentSimple) {
       IGerberComponentSimple gerberComponentSimple = (IGerberComponentSimple) gerberComponent;
-      g2d.startGerberOutput(gerberComponentSimple.getGerberLayer(),
-              gerberComponentSimple.getGerberFunction(), gerberComponentSimple.isGerberNegative());
+      GerberLayer layer = gerberComponentSimple.getGerberLayer();
+      // components emit their top-side layer; a part mounted underneath goes to the bottom one
+      if (c instanceof IBoardSideComponent
+          && ((IBoardSideComponent) c).getBoardSide() == PCBLayer._1) {
+        layer = layer.flip();
+      }
+      g2d.startGerberOutput(layer, gerberComponentSimple.getGerberFunction(),
+              gerberComponentSimple.isGerberNegative());
       c.draw(g2d, ComponentState.NORMAL, outlineMode, currentProject, g2d);
       g2d.stopGerberOutput();
     }

diylc/diylc-core/src/test/java/org/diylc/core/gerber/GerberExporterTest.java

@@ -29,7 +29,9 @@ import java.awt.geom.Rectangle2D;
 import java.awt.image.BufferedImage;
 import java.util.EnumSet;
 
+import org.diylc.common.PCBLayer;
 import org.diylc.core.ComponentState;
+import org.diylc.core.IBoardSideComponent;
 import org.diylc.core.IDrawingObserver;
 import org.diylc.core.Project;
 import org.diylc.testcomponents.MockDIYComponent;
@@ -51,6 +53,27 @@ public class GerberExporterTest {
     assertEquals(EnumSet.of(GerberLayer.CopperBot), EnumSet.copyOf(g2d.getLayerMap().keySet()));
   }
 
+  @Test
+  public void testTopSideComponentKeepsItsLayer() {
+    GerberG2DWrapper g2d = draw(new SidedGerberComponent(PCBLayer._2));
+
+    assertEquals(EnumSet.of(GerberLayer.SilkscreenTop), EnumSet.copyOf(g2d.getLayerMap().keySet()));
+  }
+
+  @Test
+  public void testBottomSideComponentGoesToTheOtherSide() {
+    GerberG2DWrapper g2d = draw(new SidedGerberComponent(PCBLayer._1));
+
+    assertEquals(EnumSet.of(GerberLayer.SilkscreenBot), EnumSet.copyOf(g2d.getLayerMap().keySet()));
+  }
+
+  @Test
+  public void testComponentWithoutSideStaysOnTop() {
+    GerberG2DWrapper g2d = draw(new SidedGerberComponent(null));
+
+    assertEquals(EnumSet.of(GerberLayer.SilkscreenTop), EnumSet.copyOf(g2d.getLayerMap().keySet()));
+  }
+
   private static GerberG2DWrapper draw(SimpleGerberComponent component) {
     BufferedImage img = new BufferedImage(100, 100, BufferedImage.TYPE_INT_ARGB);
     Graphics2D graphics = img.createGraphics();
@@ -85,4 +108,22 @@ public class GerberExporterTest {
       g2d.fill(new Rectangle2D.Double(10, 10, 20, 20));
     }
   }
+
+  public static class SidedGerberComponent extends SimpleGerberComponent
+      implements IBoardSideComponent {
+
+    private static final long serialVersionUID = 1L;
+
+    private PCBLayer side;
+
+    public SidedGerberComponent(PCBLayer side) {
+      super(null);
+      this.side = side;
+    }
+
+    @Override
+    public PCBLayer getBoardSide() {
+      return side;
+    }
+  }
 }