smooth-the-motion

name	smooth-the-motion
description	Make movement look smooth instead of stepped by drawing between updates with BT.renderAlpha. Use when motion looks jerky, jittery, choppy, stuttering, or 'not smooth', when the game feels rough on a fast monitor, or when the user asks how to get smooth scrolling or smooth movement.

Smooth the motion

Your game thinks at a fixed speed (60 times a second) but the screen may draw much faster than that – 120 or 144 times a second on a modern monitor. So some drawn frames land in between two thinking steps, and the engine has to guess where things are. BT.renderAlpha is that guess, and using it is what turns stepped, jerky motion into smooth motion.

When to use

Use when movement looks jerky, choppy, or stuttering, when the game feels rough on a fast monitor, or when the user asks for smooth scrolling or smooth movement.

The idea

Think of a flip-book. update() draws the pages, always the same distance apart. render() is your thumb, and it can stop halfway between two pages. BT.renderAlpha tells you how far it stopped: 0 means "exactly on the old page", 1 means "exactly on the new page", 0.5 means "halfway between the two".

To use it you need to remember where a thing was, not just where it is. So you keep two positions.

How to do it

Three steps. Almost everyone forgets the first one.

import { BT, Rect2i, Vector2i } from 'blit386';

async init() {
    this.pos = new Vector2i(50, 100);
    this.prevPos = this.pos.clone(); // where it was last step
    this.vel = new Vector2i(2, 0);
    return true;
}

update() {
    // 1. Remember where it WAS, before you move it. This must be the first line.
    this.prevPos = this.pos.clone();

    // 2. Move it as usual.
    this.pos = this.pos.add(this.vel);
}

render() {
    // 3. Draw somewhere between the two, using renderAlpha as the "how far" number.
    const drawPos = Vector2i.lerp(this.prevPos, this.pos, BT.renderAlpha);
    BT.drawRectFill(new Rect2i(drawPos.x, drawPos.y, 8, 8), COLOR_PLAYER);
}

Vector2i.lerp(a, b, t) means "give me the point t of the way from a to b". Feeding it BT.renderAlpha gives you exactly the in-between position the screen wants.

A smooth camera

A camera scrolls the same way – remember where it was, then draw from the in-between position:

update() {
    this.prevCamera = this.camera.clone();
    this.camera = BT.cameraClamp(this.player.pos, this.worldSize);
}

render() {
    BT.cameraSet(Vector2i.lerp(this.prevCamera, this.camera, BT.renderAlpha));
    this.drawWorld();

    BT.cameraReset(); // back to screen space - the HUD must NOT be smoothed
    BT.systemPrint(new Vector2i(6, 6), COLOR_TEXT, `Score ${this.score}`);
}

Key calls

• BT.renderAlpha (getter) – how far this drawn frame sits between the last two updates, from 0 to 1.
• Vector2i.lerp(a, b, t) (static) – the point t of the way from a to b.
• Vector2i.lerpTo(a, b, t, out) (static) – the same thing, but writes into a vector you already made instead of making a new one. Use it if you are smoothing hundreds of things; see the keep-it-fast skill.
• Vector2i.clone() (method) – a copy you can keep while the original changes.

Notes

• Only smooth things that move in update(). Never smooth the HUD, the score, or anything you draw after BT.cameraReset() – those should sit still.
• Snapshot before you move, not after. If it still looks stepped, that line is in the wrong place: this.prevPos = this.pos.clone() must be the first line of update(), not the last.
• Vector2i always holds whole numbers and rounds down for you, so you do not need Math.floor after a lerp.
• When something teleports (a respawn, a wrap-around at the screen edge), set prevPos to the new position too. Otherwise the engine smoothly slides it across the whole screen instead of jumping.
• This only changes what you see. The game still thinks at a fixed speed, so collisions and scores are unaffected.

See docs/basics.md.