Gerber export decides each component's layer in one place, and Gerber layers can be mapped to the other side of the board

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

@@ -145,8 +145,12 @@ public class GerberExporter {
     LOG.info("Completed export to gerber");
   }
 
-  private static void drawComponent(Project currentProject, GerberG2DWrapper g2d,
-                                    IDIYComponent<?> c, boolean outlineMode) {
+  /**
+   * Draws the specified component into the Gerber layers it belongs to. This is the only place that
+   * decides which layer a component's output goes to.
+   */
+  static void drawComponent(Project currentProject, GerberG2DWrapper g2d,
+                            IDIYComponent<?> c, boolean outlineMode) {
     IGerberComponent gerberComponent = (IGerberComponent) c;
     g2d.startedDrawingComponent();
     if (gerberComponent instanceof IGerberComponentCustom) {

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

@@ -54,6 +54,29 @@ public enum GerberLayer {
   public boolean isMirrored() {
     return mirrored;
   }
+
+  /**
+   * @return the same layer on the other side of the board; the outline and the drill layers go
+   *         through the whole board and are returned unchanged
+   */
+  public GerberLayer flip() {
+    switch (this) {
+      case CopperBot:
+        return CopperTop;
+      case CopperTop:
+        return CopperBot;
+      case SilkscreenBot:
+        return SilkscreenTop;
+      case SilkscreenTop:
+        return SilkscreenBot;
+      case SolderMaskBot:
+        return SolderMaskTop;
+      case SolderMaskTop:
+        return SolderMaskBot;
+      default:
+        return this;
+    }
+  }
   
   public String formatFileName(String fileNameBase, String boardName) {
     return fileNameBase + (fileNameBase.endsWith(".") ? "" : ".")

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

@@ -25,29 +25,10 @@ import com.bancika.gerberwriter.Point;
 import com.bancika.gerberwriter.path.MoveTo;
 import com.bancika.gerberwriter.path.Path;
 
-import org.diylc.core.ComponentState;
-import org.diylc.core.IDIYComponent;
-import org.diylc.core.Project;
 import org.diylc.core.measures.SizeUnit;
 
 public class GerberPathRenderer {
 
-  public static void drawComponent(Project currentProject, GerberG2DWrapper g2d, IDIYComponent<?> c,
-      boolean outlineMode) {
-    IGerberComponent gerberComponent = (IGerberComponent) c;
-    g2d.startedDrawingComponent();
-    if (gerberComponent instanceof IGerberComponentCustom) {
-      IGerberComponentCustom gerberComponentCustom = (IGerberComponentCustom) gerberComponent;
-      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());
-      c.draw(g2d, ComponentState.NORMAL, outlineMode, currentProject, g2d);
-      g2d.stopGerberOutput();
-    }
-  }
-
   public static void outputPathArea(PathIterator pathIterator, DataLayer dataLayer, double d,
       boolean isNegative, String function) {
     List<PathWithNegative> outputPath =

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

@@ -0,0 +1,88 @@
+/*
+
+    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.gerber;
+
+import static org.junit.Assert.assertEquals;
+
+import java.awt.Color;
+import java.awt.Graphics2D;
+import java.awt.geom.Rectangle2D;
+import java.awt.image.BufferedImage;
+import java.util.EnumSet;
+
+import org.diylc.core.ComponentState;
+import org.diylc.core.IDrawingObserver;
+import org.diylc.core.Project;
+import org.diylc.testcomponents.MockDIYComponent;
+import org.junit.Test;
+
+public class GerberExporterTest {
+
+  @Test
+  public void testSimpleComponentGoesToSilkscreenTopByDefault() {
+    GerberG2DWrapper g2d = draw(new SimpleGerberComponent(null));
+
+    assertEquals(EnumSet.of(GerberLayer.SilkscreenTop), EnumSet.copyOf(g2d.getLayerMap().keySet()));
+  }
+
+  @Test
+  public void testSimpleComponentGoesToItsOwnLayer() {
+    GerberG2DWrapper g2d = draw(new SimpleGerberComponent(GerberLayer.CopperBot));
+
+    assertEquals(EnumSet.of(GerberLayer.CopperBot), 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();
+    GerberG2DWrapper g2d =
+        new GerberG2DWrapper(graphics, "test", new Rectangle2D.Double(0, 0, 100, 100));
+    GerberExporter.drawComponent(new Project(), g2d, component, false);
+    graphics.dispose();
+    return g2d;
+  }
+
+  public static class SimpleGerberComponent extends MockDIYComponent
+      implements IGerberComponentSimple {
+
+    private static final long serialVersionUID = 1L;
+
+    // null uses the interface default
+    private GerberLayer layer;
+
+    public SimpleGerberComponent(GerberLayer layer) {
+      this.layer = layer;
+    }
+
+    @Override
+    public GerberLayer getGerberLayer() {
+      return layer == null ? IGerberComponentSimple.super.getGerberLayer() : layer;
+    }
+
+    @Override
+    public void draw(Graphics2D g2d, ComponentState componentState, boolean outlineMode,
+        Project project, IDrawingObserver drawingObserver) {
+      g2d.setColor(Color.black);
+      g2d.fill(new Rectangle2D.Double(10, 10, 20, 20));
+    }
+  }
+}

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

@@ -0,0 +1,53 @@
+/*
+
+    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.gerber;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+public class GerberLayerTest {
+
+  @Test
+  public void testFlipSwapsSides() {
+    assertEquals(GerberLayer.CopperBot, GerberLayer.CopperTop.flip());
+    assertEquals(GerberLayer.CopperTop, GerberLayer.CopperBot.flip());
+    assertEquals(GerberLayer.SilkscreenBot, GerberLayer.SilkscreenTop.flip());
+    assertEquals(GerberLayer.SilkscreenTop, GerberLayer.SilkscreenBot.flip());
+    assertEquals(GerberLayer.SolderMaskBot, GerberLayer.SolderMaskTop.flip());
+    assertEquals(GerberLayer.SolderMaskTop, GerberLayer.SolderMaskBot.flip());
+  }
+
+  @Test
+  public void testFlipKeepsLayersThroughTheBoard() {
+    assertEquals(GerberLayer.Outline, GerberLayer.Outline.flip());
+    assertEquals(GerberLayer.DrillPlated, GerberLayer.DrillPlated.flip());
+    assertEquals(GerberLayer.DrillNonPlated, GerberLayer.DrillNonPlated.flip());
+  }
+
+  @Test
+  public void testFlipTwiceReturnsLayer() {
+    for (GerberLayer layer : GerberLayer.values()) {
+      assertEquals(layer, layer.flip().flip());
+    }
+  }
+}