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: 32 parts have a Board Side property, and Gerber export sends a bottom-side part's silkscreen to the bottom layer with its text mirrored. Parts can be selected, dragged and stretched through the Underside Display mirror. The canvas does not show the side yet, and the Flip to Other Side action does not exist; the next steps below remain.

1. Current state

Traces, pads, PCB text and ground fills carry a PCBLayer: Bottom and Top, plus the deprecated Inner 1..6, kept only so that old files load and hidden in the editor. It feeds PCBLayer.toGerberCopperLayer(), PCBLayerFilter, the trace-mask print and continuity (ILayeredComponent.getLayerId()).
IDrawingObserver.startDrawingSide(PCBLayer) / stopDrawingSide() already exist (copper-side-view.md): paint on the side facing away from the viewer is drawn faded, in both the main view and the Underside Display mirror.
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.
One method decides the layer for a simple component: GerberExporter.drawComponent. It is package-private so that GerberExporterTest can call it. It uses getGerberLayer(), flipped with GerberLayer.flip() when the component is an IBoardSideComponent on _1 (Bottom).
org.diylc.core.IBoardSideComponent: PCBLayer getBoardSide(), returning _1 or _2 only. It is deliberately separate from ILayeredComponent (see Decisions). Core knows a part's side only through this interface.
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
The side is a `PCBLayer` (`_2` Top, `_1` Bottom). No separate side enum	DIYLC only models two-sided boards. The inner layers are deprecated and hidden in the editor (`copper-side-view.md`), so only the two valid choices are offered.
A part's side goes through its own `IBoardSideComponent`, never through `ILayeredComponent`	`getLayerId()` drives continuity, and a through-hole part's leads must connect to copper on both sides. Reporting a resistor as layer `_1` there would cut it off from top copper.
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 `_2` (Top), with `null` meaning Top	Fields added to existing classes deserialize as `null` in old files.
The side is both an editable property and the Flip to Other Side action	The action is the normal way to move a part, because it also mirrors it. The property stays for a part that is already drawn mirrored and only needs its side set.
Bottom-side text is mirrored centrally in the Gerber output	Parts draw their labels readable from the top, which is backwards on the bottom silkscreen once the board is turned over. While `GerberExporter.drawComponent` draws a bottom-side part, `GerberG2DWrapper.setMirrorText` mirrors each string about the centre of its layout box, so no component changes and the canvas keeps readable labels.
A `SolderPad` opens the solder mask on its own side	It used to write `SolderMaskBot` whatever its layer, which was wrong for top-layer pads. It now writes `SolderMaskTop` for a pad on `_2`.

3. Design

diylc-core

Canvas rendering: bottom-side components use the far-side fade from copper-side-view.md, wrapping their painting in startDrawingSide(getBoardSide()). They are then faded in the main view and solid in the Underside Display mirror, the same as bottom copper.

diylc-library

Done. Every part that uses the SilkscreenTop default and whose transformer can mirror it implements IBoardSideComponent: a nullable PCBLayer boardSide field and a getter/setter pair with @EditableProperty(name = "Board Side") on the getter, which returns _2 (Top) when the field is null. The key is in every language file.
- On a shared base, covering every subclass: AbstractFilmCapacitor, AbstractPotentiometer, AbstractRadialComponent, PassiveSMDComponent.
- On the class itself: Resistor, HeatSinkResistor, AxialElectrolyticCapacitor, CrystalOscillator, ToroidalInductor, DiodePlastic, DiodeGlass, DIL_IC, SIL_IC, TransistorTO1, TransistorTO92, TransistorTO220, CliffJack1_4, CliffJack1_8, ClosedJack1_4, MiniToggleSwitch, PCBTerminalBlock. The transistors are not done on AbstractTransistorPackage because TransistorTO126 cannot be mirrored, and a property on the base cannot be hidden from one subclass.
- Left out because they cannot be mirrored: TubeSocket, RotarySwitchOpen, RotarySwitchSealed, RotarySelectorSwitch, SlideSwitch, PlasticDCJack, TactileMicroSwitch, MiniRelay, AudioTransformer, MultiSectionCapacitor, MultiwattIC, BridgeRectifier, TransistorTO126. Making one of them mirrorable is what it takes to give it a side.
- Label is left out too: it is text, not a mounted part, and PCBText already has a layer.
- BoardSideComponentsTest checks that every such part starts on Top, takes Bottom, and can be mirrored.
CopperTrace, CurvedTrace, GroundFill, SolderPad, PCBText and DrillHole do not implement it. Their PCBLayer already says where they are.

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.

Editing through the Underside Display mirror

The mirror is a second drawing of the same components, not a separate space: every component lives in main-view coordinates. Editing in the mirror therefore means mapping the mouse back through the mirror's reflection before the existing logic runs. A reflection is its own inverse, so drawing and hit-testing use the same transform. Each stage is useful by itself and ships in order.

Shared mirror geometry (done). org.diylc.presenter.UndersideMirror holds the reflection for a board, the region its mirror occupies (the board rectangle reflected about the pivot), and which components a mirror shows: the board and every component with a control point on it. DrawingManager draws the mirror with it, so drawing and hit-testing cannot drift apart.
Stage 1: selecting in the mirror (done). DrawingManager.findMirroredComponentsAt reflects a point that falls in a mirror region and tests it again, limited to the components that board mirrors. Presenter.findComponentsAtScaled falls back to it when nothing selectable is hit in the main view; locked and hidden parts are filtered out first, so a locked part under the mirror does not block it. Clicking a part in the mirror selects the original, so the property editor, delete, copy and the popup menu all work unchanged. Selection highlighting and the selected control points are drawn on both copies.
Stage 2: dragging in the mirror (done). A drag that starts on a part in a mirror, or on a control point of a selected part there, keeps that mirror's reflection in Presenter.dragMirrorTx and reflects every mouse point of the drag before it reaches the existing drag code. Movement along the mirrored axis is therefore reversed, which is what the user sees, and snapping and the shift axis lock apply to the reflected point. Control-point hover (Presenter.findControlPointsAt) and the hand cursor look through the mirror when the main view has nothing. A drag that starts on empty mirror space still draws a selection rectangle in screen space. Moving the board, or changing its Underside Offset, cancels nothing; the mirror simply follows.
Stage 3: placing in the mirror. A component instantiated with its first point in a mirror region is created at the reflected point. If it is an IBoardSideComponent, it is created with side _1 (Bottom) and mirrored, as Flip to Other Side would do. A PCBLayer component is created on _1. Anything else is placed at the reflected point unchanged.

4. Open questions

Components that belong to no side. In mirror stage 3 a jumper, eyelet or wire dropped in the mirror has no side to set. Confirm that placing it at the reflected point unchanged is what users expect, or whether such parts should be refused in the mirror.
Components inside two boards. A component whose control points fall inside two boards with Underside Display on appears in both mirrors. Selecting through either is fine. Check that dragging through one mirror while the other redraws does not confuse the user.

5. Verification

GerberLayerTest covers flip(). GerberExporterTest covers the dispatch for simple components, including a stub IBoardSideComponent on top, bottom and null.
Load an old .diy with PCB parts and confirm every component reports _2 (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.
UndersideMirrorTest covers the mirror region for each BoardUndersideDisplay direction and offset, that reflecting a point twice returns it, and which components a mirror shows. DrawingManagerTest.testComponentFoundThroughUndersideMirror covers the hit test, and PresenterTests.testStretchThroughUndersideMirror and testDragThroughUndersideMirror check that a drag through a mirror below the board moves the part up on the board.
In the app, with Underside Display set to each direction: select, drag and stretch a trace in the mirror and confirm that the original follows correctly. Place a part in the mirror and confirm that it is bottom-side, mirrored, faded on the main view and solid in the mirror.
Run the regression suite, since rendering and serialization both change.

6. Next steps

Wrap bottom-side painting in startDrawingSide(getBoardSide()).
Add the Flip to Other Side action.
Split the silkscreen print-out by side.
Mirror stage 3: placing in the mirror, after Flip to Other Side exists.
Run the regression suite.

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