Tutorials · Physics and Mechanisms · Chapter 14

A gallery of physics

Two scenes beside the tower

1. Choose File › New scene, or press Ctrl+N. The Hierarchy is empty.
2. Click + Add node and choose Node. Rename it gallery.
3. Choose File › Save as…. In the dialog, open the project’s scenes folder, name the file gallery.ron, and save.
4. Select gallery, click + Add node and choose Camera Confiner. Rename it confiner, set its Position to 448, 128, the room’s middle, and its Bounds to 896, 256, the room’s size. Until the camera arrives with the knight, confiner carries a warning, and Problems reads “This Confiner works on the active camera and this scene has no active Camera”: leave it.
5. Select gallery and add a Sprite, backdrop. Drag art/old-dark-castle-interior-background.png onto Texture. Set Draw Mode to Tiled, Size to 896, 256, Tile Size to 960, 304, the picture’s own size, Anchor to 0, 0 and Sort Layer to -10, as the tower’s floors have it. Its Position stays 0, 0.
6. Select gallery and add a Tilemap, stone.

1. With stone selected, open the Tilemap tab beside the Inspector. Drag art/old-dark-castle-interior-tileset.png from Assets onto New tilemap…. The editor writes a new tilemap beside the sheet under the first name not already taken, and points stone at it. The tower’s tilemap has the sheet’s own name, so this one is art/old-dark-castle-interior-tileset-2.tilemap, and the status line at the foot of the window says so.
2. Before you change anything in it, rename it: in Assets, right-click the new file, choose Rename, and type gallery.tilemap. stone still shows it.
3. In the tab’s Tileset fold, set Margin y to 8.
4. In the Layers fold, the new tilemap has two layers, back and ground; the second is the solid one. Double-click ground: its name turns into a box, the caret after the name. Press Ctrl+A to select the name, type stone, and press Enter. Then click stone, so it is the layer you paint on.
5. Under Tools, click Rect. Choosing a tool turns on Paint mode at the top of the tab, as Brush would: a drag in the Viewport now paints, and selects nothing.
6. Paint the room as the table below sets out. For each tile it names, hover the tiles in the picker at the tab’s foot until the tooltip reads its number, #490 for the roof’s first, and click it: the Tile line over the picker shows it. If the tiles are too small to hit, raise Zoom beside it. Drag over the cells the tile covers, from inside the first to inside the last: both corner cells are painted.
7. A run of two tiles by turns, along a row or down a column, starts with its first two cells, painted one tile each. Then click Pick and drag over the two, click Stamp, and click the next cell along the run, and every second cell after it, to the run’s end; a cell left over at the end you paint on its own. Two rows by turns at once, as the roof’s cap and the stone under it, are a block two by two, picked and stamped the same way, each click on its top row.
8. Press the tab’s Save, which writes the tilemap, and Ctrl+S for the scene.

Margin

The Tilemap tab’s Margin, in its Tileset fold: where the grid of tiles starts on the sheet, in pixels from its top-left corner. New tilemap… starts it at the corner, 0, 0, and the castle’s tiles are 8 lower: the pillar’s cap begins at the sheet’s y 152, which with no margin is half way down a row. With Margin y 8, every row of the pillar is a whole tile you can pick.

New tilemap…

The Tilemap tab’s button that makes a tilemap from the sheet dropped on it, beside the sheet, and points the selected Tilemap at it. It never overwrites: a name already taken gets -2, then -3. Rename the file before you paint: the tab keeps unsaved work under the name it opened the file by. A file’s id goes with it when it is renamed, so the node still finds it.

1. Double-click scenes/tower.ron in Assets. Select knight and press Ctrl+C.
2. Double-click scenes/gallery.ron. Right-click gallery in the Hierarchy and choose Paste as child.
3. Set the pasted knight’s Position to 232, 224: on the floor, east of the pen. Press Ctrl+S.

A hopping gargoyle

1. Select gallery and add a Sprite, perch, at Position 112, 190. Drag chapter 1’s art/scaffold-plank.png onto Texture, set Draw Mode to Tiled, Size to 96, 8, Tile Size to 48, 8, and Sort Layer to 1. Its Anchor stays 0.5, 0.5.
2. Click Add component and choose ColliderComponent. Set its Size to 96, 8 and tick One Way, as chapter 1’s scaffolding has it.
3. Select gallery and add a Sprite, gargoyle, at Position 112, 224. Drag art/gargoyle.png onto Texture, and set Size to 62, 56, Anchor to 0.5, 1 and Sort Layer to 2.
4. Click Add component and choose CharacterBodyComponent. Set its Size to 40, 52, Driven By to Script, Gravity to 1200 and Jump Speed to 300. In its Jumping fold, set Jump Cut to 0.4 and Air Jumps to 1.

Driven By › Script

The Character Body’s Driven By, set to Script: the keys leave the body alone, and a script walks and jumps it. Its walk is its Velocity’s x, which the script writes every step with set_number; its jumps are asked for with jump(), and gravity, walls and floors still act as on the knight. Chapter 5’s crates are driven by a script that says nothing; the gargoyle’s says a lot.

Air Jumps

The Character Body’s Air Jumps, in its Jumping fold: how many jumps it may take in the air before it lands again, each a whole jump at its Jump Speed from wherever it is. A landing gives them back. 0, as it comes and as the knight keeps it, is no jump in the air; the gargoyle’s 1 is one, at the top of its high hop. A few suits a double or triple jump.

Jump Cut

The Character Body’s Jump Cut, as chapter 3 set the knight’s: the share of a rising jump’s speed kept when the jump is cut, from 0 to 1. Letting go of Space cuts the knight’s; cut_jump() cuts the gargoyle’s. At 0.4, a hop cut the step after it leaves the ground rises 9 where a whole jump rises 35.

Gravity and Jump Speed

The Character Body’s Gravity, in units a second each second, and Jump Speed, the speed a jump leaves with, in units a second. The gargoyle’s 1200 and 300 make a heavy, low hop: a whole jump rises 35, under the pen’s roof. The knight’s are 2000 and 680.

Size

The Character Body’s Size, on the gargoyle: 40 by 52, its stone body inside its 62 by 56 picture, without the wings’ tips.

One Way

The Collider’s One Way, as chapter 1’s scaffolding has it: a body rises up through it from below and lands on it from above. The perch’s top is 38 over the pen’s floor, 3 over the top of a whole jump.

1. In Assets, click Create, choose New script, type gargoyle and press Enter. In the Code tab, replace everything in gargoyle.rgs with the script below, and press Ctrl+S.
2. Select gargoyle, click Add component and choose Gargoyle. Leave Speed and Rest as the script gives them. Click the picker button beside Perch and click perch in the Hierarchy. Press Ctrl+S.

/// The gargoyle. It hops along its pen by itself: small hops, each cut short
/// as it leaves the ground, and every third a high one, an air jump at its
/// top that takes it up to the pen's roof, past its perch, a one-way plank
/// across the pen: it comes down on the perch, sits there a while, and drops
/// through it. A wall turns it round. Its body's hooks keep count, and the
/// Inspector shows the counts as it plays.
component Gargoyle : Body {
    /// How fast it hops along.
    #[suffix("u/s")]
    speed: f32 = 60.0,
    /// How many steps it sits on its perch before it drops through it.
    #[suffix("steps")]
    rest: i32 = 30,
    /// Its perch: the one-way plank it sits on.
    #[node_path]
    perch: i32 = 0,
    /// Which way it hops: 1 east, -1 west.
    #[readonly]
    way: f32 = 1.0,
    /// Its hops so far.
    #[readonly]
    hops: i32 = 0,
    /// Whether this hop is a high one.
    #[readonly]
    high: bool = false,
    /// Whether this hop's air jump is spent.
    #[readonly]
    spent: bool = false,
    /// Whether it sits on its perch.
    #[readonly]
    perched: bool = false,
    /// How many steps it has sat there.
    #[readonly]
    sat: i32 = 0,
    /// How many times it has dropped through its perch.
    #[readonly]
    drops: i32 = 0,
    /// How many times it has landed.
    #[readonly]
    landings: i32 = 0,
    /// How many walls it has met.
    #[readonly]
    walls: i32 = 0,
    /// How many times it has met the roof.
    #[readonly]
    bumps: i32 = 0,

    fn loop() {
        set_number("CharacterBodyComponent", "velocity.x", way * speed);
        if perched {
            sat += 1;
            if sat >= rest {
                drop_down();
            }
        } else if grounded() {
            jump();
        } else if high && !spent && velocity().y >= 0.0 {
            jump();
            spent = true;
        }
    }

    fn on_leave_ground() {
        if perched {
            perched = false;
            high = false;
            drops += 1;
        } else {
            hops += 1;
            high = hops % 3 == 0;
            if !high {
                cut_jump();
            }
        }
    }

    fn on_land() {
        landings += 1;
        spent = false;
        sat = 0;
        perched = position().y < position_of(node(perch)).y;
    }

    fn on_hit_wall() {
        walls += 1;
        way = -way;
    }

    fn on_hit_ceiling() {
        bumps += 1;
    }
}

component Gargoyle : Body

A script with a Body base: it goes on a node with a Character Body, and it hears the body’s four hooks below, besides loop. Its fields are drawn in the Inspector; the #[readonly] ones (attributes) are counts you watch, not boxes you type into.

#[node_path]

Gives a field a picker in the Inspector, beside it, that you point at a node: Perch keeps the node picked. In the script, node(perch) is that node.

on_leave_ground

Runs the step after its body leaves the ground, whether it jumped or dropped. The gargoyle counts a hop there and cuts it, or counts a drop when it was sitting on its perch.

on_land

Runs the step its body lands. The gargoyle counts the landing, gives its air jump back to its own count, and asks where it landed.

on_hit_wall

Runs when its body’s walk is stopped by a wall. The gargoyle turns its Way round, so it hops back the other way the next step.

on_hit_ceiling

Runs when its body’s rise is stopped by a ceiling: the gargoyle’s head on the pen’s roof. It only counts it.

jump

Asks its body to jump, as Space asks the knight’s, by its Coyote Time, Jump Buffer and Air Jumps: standing, it jumps; in the air, it jumps only while it has an air jump left.

cut_jump

Cuts a rising jump to its Jump Cut’s share, as letting go of Space does. Called in on_leave_ground, a step after the jump, it makes a small hop.

velocity and grounded

Read its body as the step began: its Velocity, and whether it stands. velocity().y turned from rising (under 0, since y grows downward) to 0 or more is the top of a jump.

drop_down

Drops its body through the one-way plank it stands on, as S with Space drops the knight. On solid ground or in the air it does nothing. A drop leaves the ground as a jump does, so on_leave_ground hears it too, falling rather than rising.

set_number

Writes a number onto another component of this node. "velocity.x" is one half of the Character Body’s Velocity: the gargoyle’s walk, 60 units a second either way.

position and position_of

position() is this node’s place in the world: the gargoyle’s feet. position_of(node(perch)) is the perch’s, the middle of the plank. Landed with its feet above that, it is on the perch.

1. Select gargoyle, so the Inspector shows its counts while the game runs, and press F5. The pen is just west of the knight, in the game camera’s view.
2. Watch ten seconds: small hops, and every third the high one to the roof, down onto the perch, a moment’s sit and a drop through. Hops, Drops, Landings, Walls and Bumps go up as it plays. Press F5 to stop.

1. Select gargoyle and set its Air Jumps to 0. Press F5: the high hop’s second jump() is nothing, and it never reaches the roof, nor its perch, 3 over a whole jump. Press F5 to stop and set Air Jumps back to 1.
2. Set its Jump Cut to 1. Press F5: the cut keeps all of the rise, and its small hops go up as high as a whole one. Stop, and set Jump Cut back to 0.4.
3. In gargoyle.rgs, delete the line drop_down(); and press Ctrl+S. Press F5: the first time it comes down on its perch it stays there for good, walking the plank from wall to wall. Stop, put the line back, and press Ctrl+S.

A guillotine blade

1. Select gallery, click + Add node and choose Animated Sprite. Rename it blade and set its Position to 304, 160: the foot of its pillar.
2. In its Sprite, drag art/blade.png onto Texture, and set Size to 28, 42, Anchor to 0.5, 1 and Sort Layer to -1: behind the stone, so the blade is hidden while it is up in its pillar.

1. Double-click art/blade.png in Assets. The Sprite sheet tab opens on it. Under Grid, set Cell to 28 and 42, the picture’s own size, and click cell 0, 0: Selected 1 x 1.
2. Under Clips, press From selection. Name the clip chop, choose Loop, and press Generate 1. A file, art/blade.anim, appears beside the picture.
3. Select blade. In its Animated Sprite, drag art/blade.anim onto Animations, type chop into Clip, and tick Frame Size. Speed stays 1 and Playing ticked.
4. Open the Animation tab in the dock under the Viewport, and drag the border above the dock up to give it room. In the clip’s ruler, right-click its one frame and choose Hold this frame. Do it three more times: each makes a copy, so five frames now each show the whole picture.
5. Under Frames, give each frame its seconds in dur and its pivot in pivot x and y, as the table below says, moving from one frame to the next with the transport row’s next-frame button.
6. Under the frame strip, type edge into Track name, choose Active, and press Add track. In the edge row of the grid, click the third cell: frame 2, counting from 0, the frame the blade is down. Press Save, and Ctrl+S.

1. Select blade, click + Add node and choose Node. Rename it blade_edge and set its Position to 0, 28, under the blade’s node.
2. Click Add component and choose AreaComponent. Open its Shape, set Kind to Capsule and Size to 28, 16: the crescent’s width, round at its tips. Click the button beside Mask, untick World and tick Knight. Type edge into Track.
3. In Assets, click Create, choose New script, type edge and press Enter. Replace everything in edge.rgs with the script below, and press Ctrl+S.
4. Select blade_edge, click Add component and choose Edge. Press Ctrl+S.

/// The blade's edge. Its Area names the blade's `edge` track: it notices
/// only on the frames of the blade's chop that the track covers, when the
/// blade is down. It counts each time it catches the knight.
component Edge : Area {
    /// How many times it has caught him.
    #[readonly]
    hits: i32 = 0,

    fn on_enter() {
        hits += 1;
    }
}

Track

The Area’s Track: the name of an event track of the clip its node’s Animated Sprite plays, on its own node or one above it. Named, the Area is in the world only on the frames that track covers, and its node is hidden on the rest. Empty, as it comes, the Area is always in the world. Chapter 4 left it empty; the edge names edge.

Kind › Capsule

The Area’s Shape › Kind set to Capsule: a box rounded at its two ends by half its height, along its longer side. A Rectangle, as it comes, notices anything that reaches its box; a capsule leaves out the box’s corners, so the edge’s tips are round, as a crescent’s are.

Size

The Area’s Shape › Size: its box, in world units. For a capsule, its length and its thickness: the edge’s 28 by 16 is round at each end with a radius of 8.

Mask

The Area’s Mask: the layers it notices, as chapter 4 set the plate’s. The edge notices the knight’s layer alone.

Active

An event track’s kind in the Animation tab: on while the clip shows a frame the track covers, a state held across those frames. A Trigger fires once as its frame begins. An Area’s Track reads an Active one.

component Edge : Area

A script with an Area base, like chapter 4’s plate: on_enter runs when something its Area notices comes into it. The edge counts each time it catches him in Hits.

1. Select blade_edge, so the Inspector shows its Hits, and press F5. Hold D until the knight is under the blade, and let go.
2. Stand there four seconds. Each time the blade comes down, its edge catches him once, the step it comes down, and Hits goes up by one; while it is up, nothing, though he stands under it the whole time. Press F5 to stop.

1. Select blade_edge and empty its Track. Press F5 and hold D as before: the edge is always in the world, and it catches him once, as he walks under it, and never again however often the blade comes down. Stop, and type edge back into Track.
2. Select knight and set his Position to 322.7, 224: under the edge’s east tip. Select blade_edge, press F5, and press nothing: the blade comes down past him, and Hits stays 0. Stop.
3. Set the edge’s Shape › Kind to Rectangle. Press F5: each time the blade comes down, the square edge catches him at the same stand, and Hits goes up by one. Stop, set Kind back to Capsule, and set the knight’s Position back to 232, 224.

A crooked sign

1. Select gallery and add a Sprite, sign_post, at Position 352, 224. Drag art/sign-post.png onto Texture, and set Size to 8, 40, Anchor to 0.5, 1 and Sort Layer to 1.
2. Select gallery and add a Sprite, crooked_sign, at Position 366, 194. Drag art/sign-crooked.png onto Texture, and set Size to 32, 16 and Sort Layer to 2. Leave its Rotation at 0: it is drawn straight.
3. Click Add component and choose CharacterBodyComponent: Size 4, 4, Driven By Script.
4. Click Add component and choose JointComponent. Set Kind to Fixed, type sign_post into Target, set Target Anchor to 14, -30, where on the post it is nailed, and Weld Angle to -17.19. Press Ctrl+S.

Kind › Fixed

The Joint’s Kind for a weld, as chapter 8’s lantern has it: it holds its Anchor on its Target Anchor and its node turned at the target’s turn plus Weld Angle, set true again each step. The post never turns, so the board is held at the Weld Angle itself.

Weld Angle

The Joint’s Weld Angle, which only a Fixed joint reads: how far the weld holds its node turned from its Target, in degrees in the Inspector. 0, as it comes and as the lantern keeps it, is turned as the target is. The board’s -17.19 (-0.3 of a radian, as the scene file keeps it) tips its west end down. A few degrees reads as a crooked nail.

Target Anchor

The Joint’s Target Anchor: its end on the post, in the post’s own units from its node, the post’s foot. 14, -30 is 30 up the post and 14 east of it, where the board’s middle is laid.

1. Select crooked_sign and set its Joint’s Weld Angle to 0.
2. Press F5: the board hangs square on its post. Press F5 to stop.
3. Set Weld Angle back to -17.19.

A tavern sign and its shadow

1. Select gallery, click + Add node and choose Node. Rename it sign_hook and set its Position to 432, 96: the foot of its pillar.
2. Select gallery and add a Sprite, tavern_sign. Drag art/swing-sign.png onto Texture, and set Size to 32, 28 and Sort Layer to 1. Set its Scale to 1.5, 1.5, its Rotation to 20 and its Position to 425.8436, 112.9145: its ring on the hook, the sign let go 20° out.
3. Click Add component and choose CharacterBodyComponent: Size 8, 8, Driven By Script, Gravity 300.
4. Click Add component and choose JointComponent. Leave Kind at Pin. Type sign_hook into Target, set Anchor to 0, -18, and tick Turns Body.
5. Click Add component and choose RemoteTransformComponent. Type sign_shadow into Target. Position stays ticked; tick Rotation and Scale. Set Offset to -10, 8, and untick Enabled.
6. Select gallery and add a Sprite, sign_shadow, at Position 415.8436, 120.9145. Drag art/swing-sign-shadow.png onto Texture, and set Size to 32, 28 and Sort Layer to -2, behind the sign. Leave it unturned at its own size, and untick its Visible.
7. In Assets, click Create, choose New script, type shade and press Enter. Replace everything in shade.rgs with the script below, and press Ctrl+S.
8. Select tavern_sign, click Add component and choose Shade. Click the picker beside Shadow and click sign_shadow in the Hierarchy. Press Ctrl+S.

/// The tavern sign's shadow. The lamp's `lit` switches on the Remote
/// Transform on this node, which puts the shadow where the sign is, by its
/// Offset, turned and sized as the sign is, every step; and it shows the
/// shadow. The lamp's `dark` switches both off. A step that hears both ends
/// lit.
component Shade {
    /// The shadow: the node the Remote Transform copies onto.
    #[node_path]
    shadow: i32 = 0,

    fn on_signal() {
        if heard("dark") {
            set_flag("RemoteTransformComponent", "enabled", false);
            set_visible(node(shadow), false);
        }
        if heard("lit") {
            set_flag("RemoteTransformComponent", "enabled", true);
            set_visible(node(shadow), true);
        }
    }
}

Anchor and Turns Body

The Joint’s Anchor, on the sign: 0, -18 is its ring, 12 over its middle times its Scale of 1.5. With Turns Body ticked, a Pin is a hinge: it holds the ring on the hook and the sign’s middle the Anchor’s length from it, wherever the swing has it, and turns the sign to hang from its ring, as chapter 7’s bob hangs from its pivot.

Gravity

The Character Body’s Gravity, on the sign: 300, light, so a swing there and back takes about 1.5 s, as a pendulum 18 long takes under that gravity, 2π√(18 ÷ 300) = 1.54 s: the strip below has it back on step 92, 1.53 s.

The Remote Transform’s Target

The node the sign’s Remote Transform copies the sign onto: sign_shadow, typed by name.

The Remote Transform’s Position

Whether it copies the sign’s place onto the shadow. It comes ticked, and the sign keeps it so: the shadow goes where the sign is, by the Offset.

The Remote Transform’s Rotation

Whether it copies the sign’s turn. Ticked, the shadow swings as the sign does.

The Remote Transform’s Scale

Whether it copies the sign’s size. Ticked, the shadow is drawn half as big again as its own picture, because the sign is: its Scale is 1.5. It comes unticked.

The Remote Transform’s Offset

Added to the copied place, in world units: -10, 8 sets the shadow 10 west of the sign and 8 lower, on the wall behind, as a lamp up and east of it would throw it. It comes 0, 0.

The Remote Transform’s Enabled

Whether it copies right now. The sign’s is saved unticked, and shade.rgs ticks it when the lamp says lit and unticks it at dark. Unticked, the shadow stays wherever it was last left.

set_visible

Shows or hides a node: set_visible(node(shadow), true) shows the shadow picked in Shadow. A Remote Transform copies place, turn and size, never whether a node is shown, so the script does both.

set_flag

Ticks or unticks a box on another component of this node: set_flag("RemoteTransformComponent", "enabled", true) ticks the Remote Transform’s Enabled. The Remote Transform is on the sign, so the sign’s script switches it.

A lamp that snaps between hooks

1. Select gallery and add a Node, lamp_rail, at Position 528, 100.
2. Click Add component and choose LineComponent. Under Points, press + twice and set the two to -80, 0 and 56, 0: from the sign’s pillar east. Set Width to 2 and Color to 0.08, 0.09, 0.1, alpha 1. Leave Joint at Flat, and set Sort Layer to 1.
3. Click knight in the Hierarchy and press Ctrl+Down until he is just under lamp_rail: nine times, each one slot down among the gallery’s nodes. The lamp and the scale are built after him, as the picture at the top of the page lists them.
4. Select gallery and add a Sprite, lamp, at Position 488, 100. Drag chapter 8’s art/lantern.png onto Texture, and set Size to 14, 22, Anchor to 0.5, 0, hung by its ring, and Sort Layer to 2.
5. Click Add component and choose CharacterBodyComponent: Size 4, 4, Driven By Script, Gravity 0.
6. Click Add component and choose JointComponent. Set Kind to Target, Target Point to 488, 100, Stiffness to 0.05 and Damping to 0.03.
7. In Assets, click Create, choose New script, type lamp and press Enter. Replace everything in lamp.rgs with the script below, and press Ctrl+S. Select lamp, click Add component and choose Lamp; its Near, Far and Stay stay as the script gives them.
8. Click Add component and choose SignalComponent. Add two connections, as chapter 4’s plate has them: Signal lit, To tavern_sign, and Signal dark, To tavern_sign. Press Ctrl+S.

/// The lamp. The Target on this node pulls it toward its Target Point, a hook
/// on its rail: this snaps that point from one hook to the other and back,
/// and the lamp swings across after it. It says `lit` as it goes to the hook
/// by the tavern sign, and `dark` as it leaves it; its Signal wires both to
/// the sign.
component Lamp {
    /// The hook by the sign, where its light casts the sign's shadow.
    near: Vec2 = vec2(488.0, 100.0),
    /// The hook away from it.
    far: Vec2 = vec2(568.0, 100.0),
    /// Seconds it stays on each hook.
    #[suffix("s")]
    stay: f32 = 2.0,

    fn loop() {
        set_point("JointComponent", "target_point", near);
        emit("lit");
        await wait(stay);
        set_point("JointComponent", "target_point", far);
        emit("dark");
        await wait(stay);
    }
}

Target Point

The Joint’s Target Point, which only a Target reads: where it pulls its body, in world units. Saved on the near hook, 488, 100; lamp.rgs sets it to the far hook, 568, 100, and back.

Stiffness

The Joint’s Stiffness: the share of the way to its point a Target pulls each pass, from 0 to 1. The lamp’s 0.05 is soft: after its point jumps, the lamp swings across to it over a few dozen steps, as the strip below shows.

Damping

The Joint’s Damping: the share of its body’s speed each pass takes off, from 0 to 1. Chapter 9’s claw walked its point slowly and its Damping hardly showed. Here the point jumps: at 0.03 the lamp swings 27 past the far hook and is settled on it within 29 steps. A new Joint’s is 0.1.

Gravity

The Character Body’s Gravity, on the lamp: 0. Nothing but its joint moves it.

Points, Width and Color

The Line’s, as chapter 13’s rope has them: the rail’s two points, 136 apart along the line from the pillar east, in the node’s own units; a stroke 2 thick; and a dark iron colour.

Joint › Flat

The Line’s Joint as it comes: its ends square, and nothing laid on its corners. The rail is open, two ends and no corner, so it is one bar with square ends.

Sort Layer

The Line’s Sort Layer: the rail’s 1 draws it over the wall and the stone, and under the lamp on 2.

set_point

Writes a pair of numbers onto another component of this node at once: set_point("JointComponent", "target_point", far) moves the joint’s Target Point to the far hook in one step. The script moves the point, never the lamp.

emit and wait

emit("lit") says lit, which the lamp’s Signal carries to tavern_sign, where shade.rgs hears it. await wait(stay) holds the loop on that line for Stay seconds, 2, before it carries on.

1. Select lamp and press F5. Move the pointer over the Viewport and press F to frame the lamp, with the tavern sign beside it.
2. Watch the lamp: two seconds on the near hook, then its point jumps to the far hook and the lamp swings across after it, past the hook, back, and settles. Two seconds later it swings back.
3. Watch the sign: it swings on its hook, 20° each way, for good. While the lamp is on the near hook, its shadow swings with it on the wall, faint on the dark stone; while the lamp is away, the shadow is gone. Press F5 to stop.

1. Select lamp and set its Joint’s Damping to 0. Press F5 and press F over the Viewport: the lamp swings further past the far hook, and is longer settling. Stop.
2. Set Damping to 0.3. Press F5: the lamp creeps onto each hook and never passes it. Stop, and set Damping back to 0.03.

1. Select tavern_sign and untick its Joint’s Turns Body. Press F5: the pin holds the sign’s middle straight under the hook, the board still turned as it was saved, and it does not swing. Stop, and tick Turns Body again.
2. Untick its Remote Transform’s Scale. Press F5: the shadow is its picture’s own size, smaller than the sign. Stop, and tick Scale again.

A scale that opens a door

1. Double-click scenes/tower.ron, select chapter 5’s crate_1 and press Ctrl+C. Double-click scenes/gallery.ron, right-click gallery and choose Paste as child. The copy keeps its name, crate_1: set its Position to 724, 224.
2. Right-click gallery and choose Paste as child again. This copy is named crate_1 too: rename it crate_2 and set its Position to 640, 224: on the floor, west of the pit.
3. Select gallery and add a Sprite, pan, at Position 712, 232. Drag art/scaffold-plank.png onto Texture, and set Size to 48, 8 and Anchor to 0.5, 1: its top at 224, flush with the floor.
4. Click Add component and choose CharacterBodyComponent: Size 46, 8, Driven By Script. Then a ColliderComponent: Size 48, 8, Offset 0, -4. Then an AreaComponent: its Shape Size 48, 8, Offset 0, -4, and its Mask set to nothing: the contact below looks for the pan, not the other way.
5. Click Add component and choose JointComponent. Set Kind to Spring, type scale_base into Target, and set Length to 84 and Stiffness to 0.003. Damping stays 0.1.
6. Select gallery and add a Node, scale_base, at Position 712, 256, the pit’s foot. It comes after pan in the Hierarchy.
7. Click Add component and choose JointComponent. Set Kind to Slider and type pan into Target. Axis stays 0, 1. Tick Use Limits, and set Min to 16 and Max to 24.
8. Select gallery and add a Sprite, scale_coil, at Position 712, 256. Drag chapter 9’s art/spring-coil.png onto Texture, and set Size to 16, 20, Anchor to 0.5, 1 and Sort Layer to 1: in the pit, under the pan.
9. Select gallery and add a Sprite, scale_door, at Position 752, 176, in the doorway: chapter 4’s portcullis again. Drag art/portcullis.png onto Texture, set Draw Mode to Tiled, Size to 32, 48, Tile Size to 16, 48, Anchor to 0.5, 0 and Sort Layer to -1. Add a ColliderComponent, Size 32, 48, Offset 0, 24, and chapter 4’s Door script.
10. Select gallery and add a Node, scale_contact, at Position 712, 240, where the pan’s foot is at its bottom stop. Add an AreaComponent with its Shape Size 40, 3; its Mask stays World, the pan’s Area’s layer. Add chapter 4’s Plate script, and a SignalComponent: Signal pressed, To scale_door, and Signal released, To scale_door. Press Ctrl+S.

Kind › Spring and Slider

The Joint’s Kind, as chapter 9’s board and lift have them: a Spring pushes and pulls its ends toward Length apart, by Stiffness; a Slider holds its Target on a line along Axis and, with Use Limits, between Min and Max along it. A Spring does not read the limits; a Slider does. So the pan has both, on two nodes.

Use Limits

The Joint’s Use Limits: ticked on the base’s Slider, it keeps the pan’s height over the base, along Axis, between Min and Max.

Min

The Joint’s Min: the near end of a Slider’s range, in world units, shown only with Use Limits ticked. The base’s 16 is the pan’s bottom stop, 8 under the floor.

Max

The Joint’s Max: the far end of the range. The base’s 24 is the pan’s top stop, its top flush with the floor.

Length

The Joint’s Length, on the pan’s Spring: how far apart it pushes its ends, in world units. 84 is over three times the 24 it is laid at, so the spring is squashed: it presses the pan up against its top stop, and what stands on the pan must outweigh that before the pan moves.

Stiffness

The Joint’s Stiffness, on the Spring: how hard it pushes toward its Length, from 0 to 1. 0.003 is very soft; squashed by 60, it is still enough to hold one crate up.

Axis

The Joint’s Axis, on the Slider: the way its line runs from the base, 0, 1, down, as it comes. Along it, the pan’s height over the base is its measure.

1. Select pan and press F5. Hold D: the knight walks east to crate_2 and shoves it east onto the pan, beside crate_1. Let go of D when the two crates meet.
2. Hold A a moment to walk him back off the pan. The two crates take it down to its bottom stop, the contact under it says pressed, and the door goes up. Press F5 to stop.

1. Select scale_base and set its Joint’s Min to 18. Press F5 and shove the second crate on as before: the pan stops 2 higher, the contact never feels it, and the door stays shut. Stop, and set Min back to 16.
2. Select pan and set its Joint’s Length to 64. Press F5: the saved crate alone sinks the pan, and the door opens at once. Stop, and set Length back to 84.

The crate puzzle

1. Choose File › New scene. Add a Node, rename it crate_puzzle, and choose File › Save as…: scenes/crate_puzzle.ron.
2. Select crate_puzzle and add a Camera Confiner, confiner, at Position 192, 144, with Bounds 384, 288. Until the camera arrives with the knight, confiner carries a warning, and Problems reads “This Confiner works on the active camera and this scene has no active Camera”: leave it.
3. Select crate_puzzle and add a Tilemap, stone. In the Tilemap tab, drag the castle’s tileset onto New tilemap…: this time the file is art/old-dark-castle-interior-tileset-3.tilemap, since the gallery’s tilemap kept the id the -2 name gave it, so that name is not free. Rename it crate-puzzle.tilemap in Assets.
4. Set the Tileset Margin y to 8. In the Layers fold, double-click ground, press Ctrl+A, type stone, and press Enter; then click stone, so you paint on it.
5. Click Rect, which turns on Paint mode, and paint the wall round the room as the table below sets out, as you painted the gallery’s: each tile by its number in the picker, and a run of two by turns with Pick and Stamp. Press the tab’s Save, and Ctrl+S.

1. In Assets, click Create, choose New script, type mark and press Enter. Replace everything in mark.rgs with the script below, and press Ctrl+S.
2. Select crate_puzzle and add a Sprite, mark_1, at Position 296, 30: straight up from where the gate will be, its top edge on the north wall’s face. Drag art/floor-mark.png onto Texture, set Size to 28, 28 and Sort Layer to 1, a decal on the floor, and its Color’s alpha to 0.3, dim.
3. Add an AreaComponent: Shape Size 20, 20, Mask Crate. Add the Mark script; its Fill stays 1.
4. Duplicate mark_1, rename the copy mark_2, and set its Position to 236, 52: against the pen’s west side. Press Ctrl+S.

/// A mark where a crate belongs. Its Area looks for crates. While a crate
/// stays on it the mark fills, a step's share of its Fill seconds every step,
/// and its picture brightens from dim to whole as it does. A crate shoved off
/// leaves it as full as it got.
component Mark : Area {
    /// Seconds a crate must stay on it to fill it.
    #[suffix("s")]
    fill: f32 = 1.0,
    /// How full it is, from 0 to 1.
    #[readonly]
    filled: f32 = 0.0,

    fn while_in() {
        filled = min(filled + dt / fill, 1.0);
        set_colour("SpriteComponent", "color", vec4(1.0, 1.0, 1.0, 0.3 + 0.7 * filled));
    }
}

while_in

The third hook of a script with an Area base, beside on_enter and on_exit: it runs every step another Area is still on this one, with other and the step’s dt. The step a crate comes on is an on_enter’s step, and Mark has none, so nothing happens yet; from the next step, every step the crate stays, while_in fills the mark by the step’s length ÷ Fill: with Fill 1 s, a sixtieth of the way each step.

dt

The step’s length in seconds, 1 ÷ 60 at the project’s sixty steps a second. while_in and loop are given it; on_enter is not.

set_colour, vec4 and min

set_colour writes a colour onto another component of this node, here the Sprite’s Color, red, green, blue and alpha, vec4(…). Its alpha goes from the 0.3 it was saved with to whole as Filled goes to 1; min(…, 1.0) stops Filled at 1.

1. Select crate_puzzle and add a Node, fence. Its Position stays 0, 0, so every place under it is the world’s.
2. Click Add component and choose LineComponent. Under Points, press + four times and set them to 224, 18; 366, 18; 366, 112; and 224, 112. Set Width to 3, Color to 0.22, 0.12, 0.05, alpha 1, Joint to Round, tick Closed, and set Sort Layer to 2.
3. Select fence and add four Nodes, each with a ColliderComponent: fence_west at Position 224, 65, its Collider Size 4, 98; fence_south_1 at 248, 112, Size 48, 4; fence_south_2 at 344, 112, Size 48, 4; and gate at 296, 112, Size 48, 4. In the gate’s Collider, untick Solid.
4. Select fence and add five Sprites, the posts: drag art/fence-post.png onto each one’s Texture, and set Size to 8, 24, Anchor to 0.5, 1 and Sort Layer to 3. post_1 at 224, 18 and post_2 at 224, 112, the west side’s ends; post_3 at 272, 112 and post_4 at 320, 112, the gate’s sides; post_5 at 366, 112, the pen’s south-east corner.
5. In Assets, click Create, choose New script, type bell and press Enter. Replace everything in bell.rgs with the script below, and press Ctrl+S.
6. Select fence and add a Sprite, gate_bell, at Position 344, 114: hung under the fence’s south side, east of the gateway. Drag chapter 13’s art/bell.png onto Texture, and set Size to 32, 34, Anchor to 0.5, 0 and Sort Layer to 4.
7. Add an AreaComponent. Open its Shape: Kind Circle, Size 48, 48, Offset -48, -2, round the gateway and centred in it. Set its Mask to Knight and Crate. Add the Bell script. Press Ctrl+S.

/// The bell by the pen's gate. Its Area reaches across the gateway: each
/// thing that comes into it, the knight or a crate, rings it — it swings
/// over — and is counted.
component Bell : Area {
    /// How many times it has rung.
    #[readonly]
    rung: i32 = 0,

    fn on_enter() {
        rung += 1;
        set_flag("SpriteComponent", "flip.x", rung % 2 == 1);
    }
}

Closed

The Line’s Closed, ticked on the fence: its last point joins back to its first, so four points draw all four sides of the pen. Unticked, as it comes and as the rail and chapter 13’s rope keep it, the line has two ends.

Joint › Round

The Line’s Joint set to Round: a disc on every corner, the joining one too, so the corners show no gap at a slant. Flat, as the rail has it, puts nothing there; RoundCaps, the rope’s, rounds its two ends as well.

Sort Layer

The Line’s Sort Layer on the fence: 2, over the marks on 1, so where mark_2 runs under the fence’s west side the fence is on top, and under the posts and the crates on 3 and the knight. Set to 0, it goes under them: a try-it below.

Points, Width and Color

The Line’s, on the fence: its four corners in the world’s units, since fence sits at the origin; a stroke 3 thick; a dark wood.

Solid

The Collider’s Solid, unticked on the gate: a Collider with Solid off stops nothing and notices nothing. It is not a trigger, as it would be in Unity or Godot, and no script hears it. The gate is a swing gate saved open, the fence’s Line drawn across it shut; what notices what comes through is the bell’s Area beside it. The fence’s sides keep Solid ticked, as it comes.

Kind › Circle

The Area’s Shape › Kind set to Circle: round, its radius the smaller half of its Size, 24, centred where the box would be. What is inside the box round the circle but outside the circle, near its corners, is not noticed.

Offset

The Area’s Shape › Offset: where its shape sits from its node, in world units. The bell hangs east of the gate, so -48, -2 moves its reach west, into the gateway.

Mask

The Area’s Mask on the bell: Knight and Crate, so it rings for him and for each crate.

component Bell : Area

Rings for each thing that comes into its Area: on_enter counts it in Rung and turns its picture over with set_flag("SpriteComponent", "flip.x", …), so the bell swings from one side to the other.

1. Double-click scenes/gallery.ron in Assets, select crate_1 and press Ctrl+C. Double-click scenes/crate_puzzle.ron.
2. Right-click crate_puzzle in the Hierarchy and choose Paste as child. The copy keeps its name, crate_1: set its Position to 296, 176, under the gate.
3. Right-click crate_puzzle and choose Paste as child again. This copy is named crate_1 too: rename it crate_2 and set its Position to 296, 232, under the first.
4. In each crate, set its Sprite’s Sort Layer to 3 and its Character Body’s Motion Mode to Floating. Press Ctrl+S.
5. Double-click scenes/gallery.ron, select knight and press Ctrl+C. Double-click scenes/crate_puzzle.ron, right-click crate_puzzle and choose Paste as child. Set his Position to 296, 268, and in his Character Body set Motion Mode to Floating. Select his camera and set its Position to 0, -15, his middle. Press Ctrl+S.

Motion Mode › Floating

The Character Body’s Motion Mode set to Floating: it has no down. Its Gravity is not applied, it never stands, and every surface is a wall to it. A Keys body walks all four ways at its Speed, W north and S south as A and D west and east, and stays where it is when let go of. A Floating crate is shoved both ways too, and stays where the shove leaves it. Grounded, as it comes, is the tower’s.

1. Press F5. Hold W: the knight shoves the column of crates north through the gate, until the first is against the north wall, on mark_1. Let go.
2. Hold D until he is clear of the second crate’s east side, then W to line him up beside it: his head north of the second crate’s south side, and not north of its north side, 1 to 8 steps of W from where the row left him. S brings him back if he goes too far.
3. Hold A: he shoves the second crate west until the fence’s side stops it, on mark_2. Press F5 to stop.

1. Select gate and tick its Collider’s Solid. Press F5 and hold W: the gate is latched, and the crates stop against it. Stop, and untick Solid.
2. Select fence and untick its Line’s Closed. Press F5: the fence’s west side is gone from the picture, though its Collider still stands. Stop, and tick Closed. Set its Sort Layer to 0 and look at mark_2: it now covers the fence’s west side. Set Sort Layer back to 2.
3. Select gate_bell and set its Shape › Kind to Rectangle. Press F5, hold D until he stands under the gate’s east post, outside the pen, and walk him north a step at a time with Pause and Step and W held: it rings four steps sooner than the circle did, while he is still beside the post, out of the gateway. Stop, and set Kind back to Circle.
4. In mark.rgs, write on_enter for while_in, and (1.0 / 60.0) for dt, which only while_in is given, and press Ctrl+S. Press F5 and shove the first crate onto its mark: it fills a sixtieth and stops. Stop, put the script back as it was, and press Ctrl+S.

Who moves what

The gallery and the puzzle, node by node

Check it works