Tutorials · Physics and Mechanisms · Chapter 4

Plates and doors

The storeroom

1. Select tower in the Hierarchy, click + Add node and choose Node. Rename it storeroom. Its Position stays 0, 0: it only holds the storeroom’s pieces.

The portcullis

1. Select storeroom, click + Add node and choose Sprite. Rename it portcullis and set its Position to 272, 1520: the middle of the doorway’s top edge.
2. In its Sprite, drag art/portcullis.png from Assets onto Texture. Set Size to 32, 48, Anchor to 0.5, 0 and Sort Layer to -1. In the Draw mode fold, set Draw Mode to Tiled and Tile Size to 16, 48: the picture twice, side by side, from wall face to wall face.
3. Click Add component and choose ColliderComponent. Set its Size to 32, 48 and its Offset to 0, 24: the drawn box, hung from the node as the picture is. Solid stays ticked, as a Collider comes.

Anchor

The Sprite’s Anchor: the point of the picture that sits on the node, as fractions of its width and height from its top left corner. 0.5, 0 is the middle of the top edge, so the portcullis hangs down from its node into the doorway. door.rgs raises the gate by changing this: at 0.5, 0.75 the node is three quarters of the way down the picture, so the picture is drawn 36 higher, behind the wall, all but its spiked foot. A new Sprite’s is 0.5, 0.5, its middle.

Draw Mode and Tile Size

The Sprite’s Draw Mode, in its Draw mode fold: Simple, as a Sprite comes, stretches the picture over Size; Sliced stretches a panel’s middle and keeps its border; Tiled repeats the picture every Tile Size, in world units, from its top left. The gate’s 16, 48 over a Size of 32, 48 is two whole gates side by side, not one stretched to double width.

The plate and its Area

1. Select storeroom, click + Add node and choose Sprite. Rename it plate and set its Position to 352, 1568: on the floor, the middle of the plate’s top edge.
2. In its Sprite, drag art/plate.png from Assets onto Texture. Set Size to 24, 6, Anchor to 0.5, 0 and Sort Layer to 1. Hung from its top edge, the picture lies in the floor’s moss caps, its top on the floor’s.
3. Click Add component and choose AreaComponent. Open its Shape, and set Size to 20, 4 and Offset to 0, -2.
4. Click the button beside Mask, which reads 0. A list of boxes opens, one a layer, numbered from 0. Untick 0, tick 1, and click away from the list. The button reads 1. Leave Layer at 0 and Track empty, as they come.
5. In the Viewport strip’s Gizmos menu, tick Triggers, which chapter 1 had you untick: it draws an Area’s Shape as a yellow box. Leave Collision and Bodies ticked.

Shape

The Area’s Shape: the outline it covers, in the node’s world units, a whole Collider’s rows under one header. The plate’s is the box 20 by 4 on its top edge; the knight’s, in the next section, is his body’s box.

Kind

The Area’s Shape’s Kind: Rectangle, Circle or Capsule, as on a Collider. Both Areas here are Rectangles, as a new one comes. The gallery’s bell, in chapter 14, is a Circle.

Size

The Area’s Shape’s Size: the box’s width and height, in world units, about its middle. The plate’s 20 by 4 is 2 inside its picture, 24 wide, at each end, and 4 tall: a body standing on the plate has its feet in it, and one jumping over clears it by far more than 4. A new Area’s is 64 by 64.

Outline

The Area’s Shape’s Outline: points that replace the box, as chapter 1’s ledge has on its Collider. Empty, as on both Areas here, the box is the shape. An Area takes its outline as it is drawn, curve or slant, and is never cut into steps.

Offset

The Area’s Shape’s Offset: where its middle sits from the node, in world units. The plate’s node is the middle of its top edge, on the floor, so 0, -2 puts the box, 4 tall, just above it, its bottom on the plate’s top. A new Area’s is 0, 0, the box centred on the node.

Step

The Area’s Shape’s Step, dimmed: “An Area is measured as drawn; Step cuts only a solid Collider’s shape into boxes.” On a Collider it is the length of the boxes a round or slanted solid is cut into (chapter 1’s pillar). Leave it at 32 u, as it comes.

Solid

The Area’s Shape’s Solid, dimmed: “An Area never blocks: nothing stands on it or slides on it.” Ticked or not, the plate stops nothing: the knight walks onto it and through it, and chapter 5 shoves a crate across it. It comes unticked on an Area.

One Way

The Area’s Shape’s One Way, dimmed for the same reason as Solid: an Area never blocks, from above or from below. On a Collider it makes a plank you land on and jump up through, as chapter 1’s scaffolding. It comes unticked.

Layer

The Area’s Shape’s own Layer, dimmed: “An Area detects by its own Layer and Mask, below; a Shape’s layers are a Collider’s.” It reads 1, and nothing reads it. Leave it as it comes. The Layer that counts is the Area’s, under the Shape.

Include Layers

The Area’s Shape’s Include Layers, dimmed with the Shape’s Layer and for the same reason: on a Collider it names layers that meet it whatever else says otherwise, and an Area meets nothing. Chapter 5 sets one on a Collider, the grate’s rail. It reads 0; leave it.

Exclude Layers

The Area’s Shape’s Exclude Layers, dimmed with the Shape’s Layer: on a Collider it names layers that pass through it, and nothing stops at an Area to begin with. Chapter 5 sets one on a Collider, the chute’s. It reads 0; leave it.

Material

The Area’s Shape’s Material, dimmed: “An Area never blocks: nothing stands on it or slides on it.” On a Collider it says how slippery and how springy it is underfoot. It comes unticked; leave it so.

Friction

The Area’s Shape’s Friction, inside its Material: tick Material and it shows, dimmed with it, for the same reason. Nothing slides on an Area. Chapter 6 types a Friction on a Collider’s Material, the ice’s.

Bounce

The Area’s Shape’s Bounce, inside its Material beside Friction and dimmed with it: nothing lands on an Area to bounce. Chapter 6 types one on a Collider’s Material, the drum’s.

Layer

The Area’s own Layer, under its Shape: the layers the Area is on, the boxes another Area’s Mask can find it by. The row is a button that names its ticked layers; click it for its boxes. The plate’s stays at 0, as a new Area comes. An Area on no layer notices nothing either: it looks out only from a layer it is on, and the Problems tab says so with a row, “this area notices nothing”.

Mask

The Area’s Mask: the layers it looks for. Another Area overlapping it is noticed when that Area’s Layer has a box this Mask ticks. The plate’s ticks 1, the layer the knight’s Area goes on in the next section, so it notices him and nothing else. A new Area’s ticks 0.

Track

The Area’s Track: the name of an event track in the node’s animation that switches the Area on only on the frames the track marks; empty, as on both Areas here, it is always on. It belongs to a hitbox that lives on an attack’s frames, and the tower has none, so the gallery’s blade, in chapter 14, is where it is set.

An Area on the knight

1. Select knight. Click Add component and choose AreaComponent. Open its Shape, and set Size to 12, 30 and Offset to 0, -15: his Character Body’s box, standing on his feet.
2. Click the button beside the Area’s Layer, untick 0 and tick 1. The button reads 1.
3. Click the button beside Mask, and untick 0. The button reads Nothing.

Two short scripts and a Signal

1. In Assets, click Create, choose New script, type plate and press Enter. In the Code tab, replace everything in plate.rgs with the script below, and press Ctrl+S.
2. Select plate, click Add component and choose Plate.

/// A pressure plate. Anything its Area notices on it presses it, and the
/// last thing off lets it go: it says `pressed` and `released` to whoever
/// its Signal wires them to.
component Plate : Area {
    /// How many things are on it.
    #[readonly]
    on_it: i32 = 0,

    fn on_enter() {
        on_it += 1;
        if on_it == 1 {
            emit("pressed");
        }
    }

    fn on_exit() {
        on_it -= 1;
        if on_it == 0 {
            emit("released");
        }
    }
}

component Plate : Area

A script with an Area base: it goes on a node that has an Area, and hears what that Area notices. Its name, Plate, is what Add component lists.

On It

The Plate’s one field, on_it, drawn in the Inspector as On It: how many things are on the plate. #[readonly] draws it and does not let you type into it, so in Play you can watch it count; Field attributes lists the others. It starts at 0.

on_enter

Runs once, on the step another Area starts overlapping this one. The reference writes it fn on_enter(other: NodeId), with the node that came in; the plate counts, and does not ask who, so it leaves other out. A hook may leave out a value it does not use.

on_exit

Runs once, on the step an Area that was overlapping stops. Here too other is left out.

emit

Tells whoever the node’s Signal wires the name to. They hear it on the next step, never the same one. pressed is emitted only when the count goes from none to one, and released only when it goes back to none, so a second thing stepping on says nothing.

1. With plate still selected, click Add component and choose SignalComponent. Click the + beside Connections twice: two rows, [0] and [1].
2. Open [0]. Type pressed into Signal. Click To and choose portcullis from the list, or drag portcullis from the Hierarchy onto the slot.
3. Open [1]. Type released into Signal, and pick portcullis for its To the same way.

Connections

The Signal’s Connections: its rows, each one name this node emits and the node that hears it. Several rows may share a name, and several may go to one node, as these two do.

Signal

A row’s Signal: the name, spelled as the script emits it: pressed, released.

To

A row’s To: the node that hears it. The row keeps the node itself, so renaming the portcullis later does not break it.

1. In Assets, click Create, choose New script, type door and press Enter. In the Code tab, replace everything in door.rgs with the script below, and press Ctrl+S.
2. Select portcullis, click Add component and choose Door. Press Ctrl+S.

/// The portcullis. The plate's `pressed` raises it into the wall, where only
/// its spikes show, and its `released` drops it again. Raised, it stops
/// nothing. A step that hears both, one thing off the plate as another steps
/// on, ends raised.
component Door {
    fn on_signal() {
        if heard("released") {
            set_flag("ColliderComponent", "solid", true);
            set_point("SpriteComponent", "anchor", vec2(0.5, 0.0));
        }
        if heard("pressed") {
            set_flag("ColliderComponent", "solid", false);
            set_point("SpriteComponent", "anchor", vec2(0.5, 0.75));
        }
    }
}

component Door

A script with no base: it goes on any node. The portcullis has no Area, and hears only signals.

on_signal and heard

on_signal runs on a step when a signal has reached this node, and heard("released") is true on that step if that one did. A step can bring several, so the script asks about each.

set_flag

Writes true or false onto another component on this node: set_flag("ColliderComponent", "solid", false) unticks the Collider’s Solid, and the gate stops nothing.

set_point and vec2

set_point writes a pair of numbers onto another component on this node, and vec2(0.5, 0.75) makes the pair: the Sprite’s Anchor, which hangs the picture higher, into the wall.

The plate in Play

1. If you take the skip: with the game stopped, select knight and set his Position to 560, 1568, on the scaffolding’s top plank, where chapter 3’s climb leaves him.
2. Select plate, so that the Inspector shows its On It while the game runs, and press F5 to play in the editor.
3. Walk the knight up the scaffolding, as in chapter 3, or from the skip’s plank, west onto the storeroom’s floor. Stop short of the plate.
4. Walk onto the plate. The portcullis rises into the wall, only its spikes showing under the arch, and On It reads 1. Stand there: it stays up.
5. Walk off it, east. The portcullis drops into the doorway, and On It reads 0. Press F5 again to stop.
6. If you took the skip, select knight and set his Position back to 48, 1792, his start.

The plate and the gate

Check it works