Tutorials · Physics and Mechanisms · Chapter 8

Chains and welds

The clockworks, finished

1. Select knight and set his Position to 488, 1120: the clockworks’ east floor, where chapter 6’s climb leaves him. Press F5 to play from there, and F5 again to stop.
2. Afterwards press Ctrl+Z until his Position reads 48, 1792 again, or type 48, 1792 back, so the tower still starts at its foot.
3. In the strip under the Viewport, open the Gizmos menu: Collision, Bodies, Triggers and Forces stay ticked, as chapter 7 left them. Forces draws every joint in orange; Bodies draws each body’s box in rose, with a rose arrow for its own velocity and an orange one for its Drift. The strips on this page are drawn with those two alone, so on your screen there is more than in them: the Colliders’ green outlines, and the yellow box of the shutter’s Area, a little wider than the shutter. (The crate’s Area is the same box as its body, under the rose.)

A chain bridge

1. Select clockworks, click + Add node and choose Sprite. Rename it link_1 and set its Position to 104, 980.
2. In its Sprite, drag art/chain-link.png from Assets onto Texture, and set Size to 12, 6 and Anchor to 0.5, 1: hung from its feet.
3. Click Add component and choose CharacterBodyComponent. Set its Size to 12, 6, Driven By to Script and Gravity to 400.
4. Click Add component and choose ColliderComponent. Set its Size to 12, 6 and Offset to 0, -3. Solid stays ticked.
5. Click Add component and choose JointComponent. Set Kind to Distance, type gallery_west into Target, and set Anchor to -6, 0 and Target Anchor to 40, -8.
6. In the Joint’s Limits fold, tick Use Limits. Min and Max appear under it: leave Min at 0 and set Max to 6.

1. In the Hierarchy, right-click link_1 and choose Duplicate. Rename the copy link_2, set its Position to 120, 980, type link_1 into its Joint’s Target, and set its Joint’s Target Anchor to 6, 0: the east foot of the link before.
2. Duplicate link_2 six times in the same way, each copy’s Joint Target the link before it: link_3 at 136, 980, link_4 at 152, 980, link_5 at 168, 980, link_6 at 184, 980, link_7 at 200, 980 and link_8 at 216, 980. Their Target Anchor stays 6, 0.
3. Select gallery_east, the east ledge. Click Add component and choose JointComponent: Kind Distance, Target link_8, Anchor -40, -8, Target Anchor 6, 0; tick Use Limits and set Max to 6. Press Ctrl+S.

Kind › Distance, with limits

The Joint’s Kind, as chapter 7’s rod has it. Without limits a Distance joint holds its two ends exactly Length apart, a rod. With Use Limits ticked it holds them anywhere from Min to Max apart, and pulls only when they are further than Max: a slack tie, a link of chain.

Target

The Joint’s Target: the node at its other end, typed by name. A node with no Character Body, like the ledges, is an anchor the joint never moves. The east ledge holds a Joint of its own whose Target is link_8: a ledge has no body, so that joint holds the link and never moves the ledge.

Anchor and Target Anchor

The Joint’s Anchor is its end on this node, and Target Anchor its end on the Target, each in that node’s own units from its position. Chapter 7’s rod used the pivot’s own point, 0, 0. Here both are off the node: -6, 0 is a link’s west foot, 6, 0 the east foot of the link before, and 40, -8 on gallery_west is its east end, 4 above its top.

Use Limits

The Joint’s Use Limits, in its Limits fold: ticked, the joint keeps its measure between Min and Max, and shows those two rows. For a Distance joint the measure is how far apart its ends are. A Distance, a Slider and a Target read it; on other kinds it is dim. It comes unticked.

Min

The Joint’s Min: the near end of its range, shown only with Use Limits ticked. For a Distance joint, the least its ends may be apart, in world units. The ties leave it at 0, as it comes: a link may close right up to the one before.

Max

The Joint’s Max: the far end of its range, in world units. For a slack tie, the slack: each tie lets its ends be up to 6 apart, and pulls them back past that. A new Joint’s is 64.

Length

The Joint’s Length, dim on every link: a Distance joint with Use Limits ticked keeps its range and never asks it. It stays 64, as it comes.

Gravity

The Character Body’s Gravity, as in chapter 2: how fast it gathers speed falling, in units a second each second. The links’ 400 is light against the knight’s 2000, so his weight shapes the chain more than its own does. A new body’s is 1800.

1. Set the knight’s Position to 488, 1120 and press F5. Ride the pendulum to the east ledge as in chapter 7, and wait there a moment: the bridge has hung into its sag.
2. Hold A. He walks out onto the bridge, and the chain dips under him as he goes, the link he stands on lowest. Let go of A over the middle: he stands there, the bridge dipped under him a little.
3. Press Space from there: he jumps off it until his head meets the ceiling, and lands on it again. Hold A: he walks on to the west ledge. Press F5 to stop, and Ctrl+Z back to 48, 1792.

A rotten plank

1. Select clockworks, click + Add node and choose Node. Rename it nail and set its Position to 224, 1124: the middle floor’s west edge, 4 under its top.
2. Select clockworks and add a Sprite, plank, at Position 200, 1128. Drag art/rotten-plank.png onto Texture, and set Size to 48, 8 and Anchor to 0.5, 1. Its top lies flush with the floor’s.
3. Click Add component and choose CharacterBodyComponent: Size 48, 8, Driven By Script. Then a ColliderComponent: Size 48, 8, Offset 0, -4.
4. Click Add component and choose JointComponent. Leave Kind at Pin, as it comes. Type nail into Target, set Anchor to 24, -4, its east end, and in the Breaking fold set Break Stretch to 1.3.

Kind › Pin

The Joint’s Kind as a new Joint comes: a Pin holds its Anchor on its Target Anchor, a nail or a hinge. The plank, pinned by its east end, hangs from that end like a shelf from its bracket.

Anchor

The Joint’s Anchor, on the plank: 24, -4 from its node, which is the middle of its bottom edge, is the middle of its east end, where the nail is. Left at 0, 0, the plank would hang from its middle, half of it over the floor.

Turns Body

The Joint’s Turns Body, which chapter 7 ticked for the bob: unticked here, as it comes, so the plank never turns about its nail. It stays level while the nail holds, and falls level when it gives.

Break Stretch

The Joint’s Break Stretch, in its Breaking fold, in world units: how far out of true the joint may be pulled in one step before it breaks. When it breaks, its Enabled is unticked for you and the node hears joint:broke. 0, as a new Joint comes, never breaks. Every kind reads it.

joint:broke

The signal a node hears on the step its own Joint breaks, carrying the measure that broke it. Broken by its Break Stretch, that is how far out of true it was, in world units, as the plank’s is. A weld broken by its Break Twist alone carries how far it was wrenched off its angle instead, in radians, not the Inspector’s degrees, and signed: less than 0 for a wrench one way, more for the other. It needs no Signal component: the joint says it to its own node. plank.rgs, below, listens for it.

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

/// The rotten plank. The Pin on this node is its nail, and the Pin's Break
/// Stretch is how far out of true the nail can be pulled before it gives.
/// When it gives, the joint says `joint:broke` to this node with how far out
/// of true it was: the plank keeps that, and turns its picture over as it
/// falls.
component Plank {
    /// Whether the nail has given way.
    #[readonly]
    snapped: bool = false,
    /// How far out of true the nail was when it gave, in world units.
    #[readonly]
    stretch: f32 = 0.0,

    fn on_signal() {
        if heard("joint:broke") {
            snapped = true;
            stretch = heard_value("joint:broke");
            set_flag("SpriteComponent", "flip.y", true);
        }
    }
}

component Plank

A script with no base, like chapter 4’s door: it goes on any node, and hears signals. Its name, Plank, is what Add component lists.

Snapped and Stretch

The Plank’s two fields, drawn in the Inspector as Snapped and Stretch. Both are #[readonly] (attributes): drawn, and not for typing into, so in Play you can watch them change. Snapped starts unticked and Stretch at 0.

heard and heard_value

heard("joint:broke") is true on the step the signal reaches this node; heard_value("joint:broke") is the number it carried, here how far out of true the nail was, or 0 if it carried none.

set_flag

Writes true or false onto another component on this node. "flip.y" reaches one part of the Sprite’s Flip: its Y, which draws the picture upside down. The plank is a body and never turns, so its picture is turned over instead.

1. In the Hierarchy, right-click crate_1, chapter 5’s first crate, and choose Duplicate.
2. Drag the copy onto clockworks, rename it crate_5, and set its Position to 264, 1120: on the middle floor, east of the plank. Press Ctrl+S.

1. Select knight, set his Position to 240, 1120, west of the crate, and press F5.
2. Hold A: he walks off the floor onto the plank. Let go once he is wholly on it. It holds him, and stays where it is.
3. Press F5 to stop, and Ctrl+Z back to 48, 1792.

1. Select knight and set his Position to 300, 1120, east of the crate. Then select plank, so the Inspector shows its Snapped and Stretch while the game runs, and press F5.
2. Hold A: he shoves the crate west onto the plank, and it holds the crate. Keep holding: the moment he steps onto it after the crate, the nail gives, and the plank falls with the crate on it, upside down. Let go of A at once, or he follows it down. In the Inspector, Snapped is ticked and Stretch holds how far out of true the nail was.
3. To catch the step, press Pause or F6 as the crate goes onto the plank, then Step or F7 one step at a time, and watch the step counter on the Play bar. Press F5 to stop, and Ctrl+Z back to 48, 1792.

A lantern on a weld

1. Select clockworks and add a Sprite, lantern, at Position 430, 1068, and set its Rotation to -53.13, the bob’s. Drag art/lantern.png onto Texture, and set Size to 14, 22, Anchor to 0, 0.5, its outer edge, and Sort Layer to 3.
2. Click Add component and choose CharacterBodyComponent: Size 4, 4, Driven By Script.
3. Click Add component and choose JointComponent. Set Kind to Fixed, type bob into Target, and set Target Anchor to -30, 0, the bob’s west side. Anchor stays 0, 0 and Weld Angle 0. In the Breaking fold, set Break Twist to 28.65.

Kind › Fixed

The Joint’s Kind for a weld: it holds its Anchor on the Target Anchor, as a Pin does, and its turn too, at the target’s turn plus Weld Angle. Welded onto a swinging body, it moves only itself and is set true again each step, so the bob keeps its swing with the lantern on it.

Weld Angle

The Joint’s Weld Angle, which only a Fixed joint reads: how far this node is turned from its Target, in degrees in the Inspector. The lantern’s is 0: it is turned as the bob is, both at -53.13, so in the Inspector you turn the lantern itself to match. The gallery’s signpost, in chapter 14, has one that is not 0.

Break Twist

The Joint’s Break Twist, in its Breaking fold, in degrees: how far a weld may be wrenched off its angle in one step before it breaks. Only a Fixed joint reads it. The lantern’s 28.65 is half a radian. The swing wrenches it about 4° a step empty and under 6° with him riding the bob, so it is set and never reached. 0 never breaks.

Size

The Character Body’s Size, on the lantern: 4 by 4, far smaller than its picture. It only rides its weld, and a small box stays clear of the bob’s round Collider as the two turn.

1. Select lantern and set its Rotation to 0.
2. Press F5, and at once F6 to pause. The game camera is on the knight, far below: move the pointer over the Viewport and press F to frame the lantern. On the first step the weld is wrenched past its Break Twist and breaks: the lantern drops straight down through gap A while the bob swings on.
3. Press F7 a step at a time, or F6 to run on, and watch the Play bar’s step counter. Press F5 to stop.
4. Set the lantern’s Rotation back to -53.13.

A shutter in a groove

1. Select clockworks and add a Node, shutter_rail, at Position 88, 938: where the groove starts.
2. Select clockworks and add a Sprite, shutter, at Position 88, 970. Drag art/shutter.png onto Texture, and set Size to 16, 32, Anchor to 0.5, 1 and Sort Layer to 1.
3. Click Add component and choose CharacterBodyComponent: Size 16, 32, Driven By Script; in its Pushing fold tick Pushable and set Push Speed to 0.75. Then a ColliderComponent: Size 16, 32, Offset 0, -16.
4. Click Add component and choose JointComponent. Set Kind to Groove, type shutter_rail into Target, and set Anchor to 0, -32, the middle of its top, Axis to -1, 0 and Length to 48.
5. Click Add component and choose AreaComponent. Open its Shape and set Size to 24, 32 and Offset to 0, -16. Click the button beside Mask, untick World and tick Knight.

Kind › Groove

The Joint’s Kind for a groove: it holds its Anchor on a line that starts at the Target’s point and runs along Axis for Length, and lets it slide anywhere on it. The shutter’s top slides from the rail up to 48 west, and no further either way. Godot’s GrooveJoint2D is the same idea.

Axis

The Joint’s Axis: the way a Groove or a Slider runs, a direction in the Target’s own units. -1, 0 is west. Its length does not matter, only its way. A new Joint’s is 0, 1, down. Only a Groove and a Slider read it.

Length

The Joint’s Length, on a Groove: how long the groove is, in world units, from the Target along Axis. The shutter’s 48 stops it with its top at x 40 and him behind it at x 54, under H4, where his jump up clears the opening.

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

/// The shutter's ring pull. The shutter hangs in its groove by the Joint on
/// this node. With the knight against the shutter, the `action` key (E) pulls
/// the ring: the Joint is switched off, and the shutter drops.
component Lever : Area {
    /// How many things its Area notices against the shutter.
    #[readonly]
    near: i32 = 0,
    /// Whether the ring has been pulled.
    #[readonly]
    pulled: bool = false,

    fn on_enter() {
        near += 1;
    }

    fn on_exit() {
        near -= 1;
    }

    fn loop() {
        if near > 0 && !pulled && pressed("action") {
            pulled = true;
            set_flag("JointComponent", "enabled", false);
        }
    }
}

component Lever : Area

A script with an Area base, like chapter 4’s plate: it goes on a node with an Area, and hears what comes in and goes out. Near counts the Areas against the shutter; Pulled is ticked once the ring is pulled, so it is pulled only once.

loop

Runs once every step the game runs. Here it asks, each step, whether he is near and the ring not yet pulled, and whether action went down.

pressed

pressed("action") is true on the one step an action goes down, not while it is held. action is one of the project’s actions in input.ron, on E and Enter.

Enabled

The Joint’s Enabled: whether it is held right now. Unticked, the joint does nothing, and the shutter is a plain body again, which gravity drops. lever.rgs unticks it with set_flag("JointComponent", "enabled", false); a break unticks it too, as the plank’s did. It comes ticked.

1. Set the knight’s Position to 488, 1120, then select shutter, so the Inspector shows its Joint’s Enabled while the game runs. Press F5, ride the pendulum to the east ledge and hold A across the bridge.
2. He walks into the shutter and shoves it west along its groove, slower than he walks, until it stops at the groove’s end with him behind it. Let go of A.
3. Press E. Enabled is unticked, the groove is drawn dashed, and the shutter drops onto the ledge. Press F5 to stop, and Ctrl+Z back to 48, 1792.

The way up

1. Set the knight’s Position to 488, 1120 and press F5. Ride the pendulum to the east ledge, as in chapter 7.
2. Hold A across the bridge into the shutter, and on until it stops at the groove’s end. Let go: he stands under H4.
3. Hold D and Space together: he jumps up through H4 and lands on the lift shaft’s floor, east of the opening, where chapter 9 starts. Press F5 to stop, and Ctrl+Z back to 48, 1792.

The bridge, the plank, the lantern and the shutter

Check it works