diy-layout-creator/diylc/docs/plans/bottom-side-components.md

Bottom-Side Components

Let a component be mounted on the bottom of a PCB, so that its silkscreen goes to the bottom Gerber layers and the canvas shows it as a part on the far side of the board.

Status: not started.

1. Current state

• Traces, pads, PCB text and ground fills carry a PCBLayer (Bottom, Top, Inner 1..6). It feeds PCBLayer.toGerberCopperLayer(), PCBLayerFilter, the trace-mask print and continuity (ILayeredComponent.getLayerId()).
• Every other Gerber-aware component implements IGerberComponentSimple, whose getGerberLayer() defaults to SilkscreenTop. About 35 classes in diylc-library rely on that default (resistors, film and radial capacitors, diodes, LEDs, transistor packages, DIL/SIL ICs, SMD passives, switches, jacks, relays, transformers, tube sockets, ...). None of them emit copper. Their pads are separate SolderPad components, which already have a layer.
• Two copies of the same dispatch decide the layer for a simple component: GerberExporter.drawComponent and GerberPathRenderer.drawComponent. Both call g2d.startGerberOutput(getGerberLayer(), ...).
• Z-order is paint order only. A trace on the bottom layer already paints above the board (zOrder = TRACE), so the codebase already treats the physical side as separate from z-order.

2. Decisions

Decision	Rationale
A new two-value BoardSide (Top, Bottom), not a reuse of PCBLayer	Parts mount on the outer faces only. Inner layers would be invalid choices in the editor.
The side is the physical fact. Z-order stays paint order and is not tied to it	Boards are drawn opaque, so painting bottom parts beneath the board would hide them. Bottom-layer traces already follow this rule.
The canvas shows the side visually and does not rely on z-order for it	That is what removes the ambiguity of a "bottom" part drawn above the board.
Flipping a part is an action that sets the side, mirrors the part and sends it back	A part seen from above through the board is a mirror image. Changing the side without mirroring gives a footprint that is wrong for fabrication.
The Gerber layer is swapped centrally, not inside each component	Components keep emitting their *Top layer. One place maps it to the other side.
Default Top, with null meaning Top	Fields added to existing classes deserialize as null in old files.

3. Design

diylc-core

• org.diylc.common.BoardSide: enum Top, Bottom, with a toString() label like PCBLayer.
• org.diylc.core.IBoardSideComponent: BoardSide getBoardSide(); with Javadoc and @author, in the same shape as ILayeredComponent. Core knows the side only through this interface.
• GerberLayer.flip(): maps CopperTop/CopperBot, SilkscreenTop/SilkscreenBot and SolderMaskTop/SolderMaskBot to each other. Outline and the drill layers map to themselves.
• Gerber dispatch: in the simple-component branch, use getGerberLayer().flip() when the component is an IBoardSideComponent on Bottom. Merge GerberExporter.drawComponent and GerberPathRenderer.drawComponent into one method first, so the rule is written once.
• Canvas rendering: DrawingManager.drawProject draws bottom-side components with a composite that sets them apart. Locked components already use a 0.5 alpha composite (lockedComposite), so the bottom-side treatment has to look different from that, for example a lower alpha or a tint. Pick one and check it against a locked bottom-side part.

diylc-library

• The ~35 classes that use the SilkscreenTop default implement IBoardSideComponent. Each adds a nullable BoardSide boardSide field and a getter/setter pair, with @EditableProperty(name = "Board Side") on the getter. The getter returns Top when the field is null. This duplicates the field across classes in the same way the PCBLayer field is duplicated today. Where several of these classes share an abstract base (AbstractRadialComponent, AbstractFilmCapacitor, AbstractTransistorPackage, ...), put it on the base.
• CopperTrace, CurvedTrace, GroundFill, SolderPad, PCBText and DrillHole do not implement it. Their PCBLayer already says where they are.
• Before allowing Bottom on a class, check that its transformer can mirror it (canMirror). A class that cannot be mirrored must not offer Bottom, or the footprint would be wrong.

diylc-swing

• Flip to Other Side action in the Edit menu and the canvas popup menu, following the align-distribute actions. For each selected IBoardSideComponent it:

  1. toggles the side,
  2. mirrors horizontally about the selection centre with the component's IComponentTransformer,
  3. sends the component to the back of its z-order band.

  The whole action is one undo step, and it is enabled only when the selection holds at least one IBoardSideComponent. The work is done in Presenter behind IPlugInPort, with the plugin only wiring the menu items.

• PCB print-outs: SilkScreenDrawingProvider prints one page of everything that has a silkscreen outline. Split it into top and bottom pages, filtered by side, the same way TraceMaskDrawingProvider gives one page per PCBLayer.

4. Open questions

• Bottom silkscreen text. Component labels drawn through IGerberComponentSimple have to read correctly once the board is turned over, which means they must look mirrored from the top. Check whether mirroring through the transformer does that for every class, or whether labels are drawn unmirrored regardless. GerberLayer.mirrored is currently false for every layer, and the exporter output must stay in top view.
• SolderPad solder mask. SolderPad always writes its mask opening to SolderMaskBot, whatever its own PCBLayer. Confirm whether this is intended for top-layer SMD pads before GerberLayer.flip() is introduced next to it.
• Plain property or action only. A plain editable property lets a user change the side without mirroring. Either keep both (the property for already-mirrored parts) or show the side as read-only and change it only through the action.

5. Verification

• Unit tests in diylc-core for GerberLayer.flip() and for the merged dispatch method, using a stub IBoardSideComponent (top, bottom and null).
• Load an old .diy with PCB parts and confirm every component reports Top and that the Gerber output is byte-identical to before.
• Export a board with one part on each side and open the files in a Gerber viewer: the bottom part appears only in .gbo, mirrored, and in the right position.
• Run the regression suite, since rendering and serialization both change.

6. Next steps

1. Merge the two drawComponent copies in diylc-core and add GerberLayer.flip() with tests.
2. Add BoardSide and IBoardSideComponent, and swap the Gerber layer in the dispatch.
3. Add the field to the library components, putting it on shared bases where they exist. Settle the silkscreen-text question while doing so.
4. Add the bottom-side composite in DrawingManager.
5. Add the Flip to Other Side action.
6. Split the silkscreen print-out by side.
7. Run the regression suite.