Pinning animated characters to a background's ground line in a responsive web game

VpsTrial057 — Sun, 08 Nov 2026 15:09:28 +0000

I build a small browser idle game, OreSpark, where miners walk around a painted mine scene and keep digging while you are away. The scene is a single illustration, and the characters are separate animated sprites on top of it.

The first version used flexbox to line the miners up in a row. It looked fine at one width and broke everywhere else: miners floated above the floor, stood on the wall, or sank into a cart. The problem is simple once you see it. A flex row knows nothing about the picture behind it. The floor in the illustration is not a straight line at a fixed pixel height, it is a shape painted into the image.

Here is the approach that fixed it.

1. Measure the ground on the image, not on the screen

Open the background at its native size and note, for each spot where a character should stand, the point under its feet. Store it as a fraction of the image, not in pixels:

// x, y = foot point as a fraction of the background image (0..1)
// s = scale, smaller for characters further back
const SPOTS = [
  { x: 0.18, y: 0.86, s: 1.00 }, // front left, near the cart
  { x: 0.47, y: 0.79, s: 0.85 }, // middle, on the rail level
  { x: 0.71, y: 0.73, s: 0.70 }, // back, near the tunnel
];

Because these are fractions of the image, they stay correct at any size, as long as the image itself keeps its proportions.

2. Keep the scene at the image's aspect ratio

The scene box must have exactly the same ratio as the illustration, otherwise the fractions point at the wrong place:

.scene {
  position: relative;
  width: 100%;
  aspect-ratio: 1600 / 900; /* the background's native size */
  background: url(mine.webp) center / 100% 100% no-repeat;
}

No cover, no cropping. If you crop the image on small screens, the ground moves and every spot is wrong.

3. Anchor each sprite by its feet

Position the sprite with left / top in percent, then shift it so the bottom centre of the sprite sits on the spot:

.miner {
  position: absolute;
  transform: translate(-50%, -100%) scale(var(--s));
  transform-origin: 50% 100%;
}

SPOTS.forEach((p, i) => {
  const el = scene.children[i];
  el.style.left = p.x * 100 + '%';
  el.style.top  = p.y * 100 + '%';
  el.style.setProperty('--s', p.s);
  el.style.zIndex = Math.round(p.y * 100); // lower on screen = closer = on top
});

Two details matter here:

transform-origin: 50% 100% makes the scale grow up from the feet, so a smaller character in the back still touches the floor.
zIndex from y gives you depth for free: characters lower on the screen are closer to the viewer and are drawn on top.

4. Size sprites relative to the scene

If the sprite has a fixed pixel width, it looks tiny on a wide screen and huge on a phone. Give it a width in percent of the scene instead, so the whole picture scales as one:

.miner { width: 9%; }
.miner img { width: 100%; height: auto; display: block; }

5. Animate inside, place outside

Walking, swinging a pickaxe or bobbing should be done on an inner element. The outer element only holds the position. That way an animation can never push the character off its ground point:

<div class="miner" style="left:18%;top:86%">
  <div class="miner__body"><img src="miner.webp" alt=""></div>
</div>

Result

The same scene now works from a 360px phone to a wide desktop, and adding a character is just one more {x, y, s} line measured on the image. The rule that came out of it: anything that stands on a picture should be positioned in the picture's coordinates.

If you want to see the scene running, it is the home screen of OreSpark, a free play-to-earn idle mining game.