◑ Photo editing

Gaussian blur

가우시안 블러

Replaces every pixel with a weighted average of its neighbors — the most fundamental blur algorithm there is.

Also known as: Gaussian smoothing가우시안 스무딩

···

html

<div class="wrap">
  <div class="stage"><canvas id="c" width="300" height="180"></canvas></div>
  <div class="hud"><span>왼쪽 원본 / 오른쪽 블러</span><b id="r">0px</b></div>
</div>

css

.wrap{position:absolute;inset:0;padding:clamp(8px,3vmin,20px);display:flex;flex-direction:column;gap:6px}
.stage{position:relative;flex:1;min-height:0}
canvas{position:absolute;inset:0;width:100%;height:100%;object-fit:contain;border-radius:10px;border:1px solid var(--line);background:#0a0a0a}
.hud{display:flex;justify-content:space-between;gap:8px;font:700 clamp(9px,2.2vmin,11px)/1 ui-monospace,monospace;color:var(--muted)}
.hud b{color:var(--fg)}

js

var W = 300, H = 180;
var c = document.getElementById('c'), ctx = c.getContext('2d');

function paintPhoto(cx, w, h) {
  var g = cx.createLinearGradient(0, 0, 0, h * 0.62);
  g.addColorStop(0, '#5b86c9'); g.addColorStop(0.55, '#bcd6ea'); g.addColorStop(1, '#f5e0ad');
  cx.fillStyle = g; cx.fillRect(0, 0, w, h);
  var sx = w * 0.74, sy = h * 0.22, sr = h * 0.11;
  var sg = cx.createRadialGradient(sx, sy, 0, sx, sy, sr * 3.2);
  sg.addColorStop(0, 'rgba(255,247,214,0.9)'); sg.addColorStop(1, 'rgba(255,247,214,0)');
  cx.fillStyle = sg; cx.fillRect(0, 0, w, h * 0.62);
  cx.fillStyle = '#fff8df'; cx.beginPath(); cx.arc(sx, sy, sr, 0, 7); cx.fill();
  cx.fillStyle = '#6f9152';
  cx.beginPath(); cx.moveTo(0, h * 0.64);
  for (var x = 0; x <= w; x += 14) cx.lineTo(x, h * 0.64 - Math.sin(x * 0.014) * h * 0.05 - Math.sin(x * 0.032 + 1.2) * h * 0.02);
  cx.lineTo(w, h); cx.lineTo(0, h); cx.closePath(); cx.fill();
  cx.fillStyle = '#405f38';
  cx.beginPath(); cx.moveTo(0, h * 0.79);
  for (var x2 = 0; x2 <= w; x2 += 14) cx.lineTo(x2, h * 0.79 - Math.sin(x2 * 0.022 + 0.6) * h * 0.035);
  cx.lineTo(w, h); cx.lineTo(0, h); cx.closePath(); cx.fill();
  var tx = w * 0.2, ty = h * 0.8;
  cx.strokeStyle = '#3a2a1c'; cx.lineWidth = Math.max(1.5, w * 0.008);
  cx.beginPath(); cx.moveTo(tx, ty); cx.lineTo(tx - w * 0.01, ty - h * 0.13); cx.stroke();
  cx.fillStyle = '#2e4a28';
  for (var i = 0; i < 5; i++) { cx.beginPath(); cx.arc(tx + (i - 2) * w * 0.013, ty - h * 0.15 - (i % 2) * h * 0.018, w * 0.028, 0, 7); cx.fill(); }
  cx.fillStyle = '#1e1e1c';
  cx.beginPath(); cx.arc(w * 0.56, h * 0.745, h * 0.018, 0, 7); cx.fill();
  cx.fillRect(w * 0.555, h * 0.76, w * 0.01, h * 0.06);
  var img = cx.getImageData(0, 0, w, h), d = img.data;
  for (var p = 0; p < d.length; p += 4) { var n = (Math.random() - 0.5) * 10; d[p] += n; d[p + 1] += n; d[p + 2] += n; }
  cx.putImageData(img, 0, 0);
}
paintPhoto(ctx, W, H);
var orig = ctx.getImageData(0, 0, W, H);
function boxBlur(src, w, h, r) {
  var tmp = new Float32Array(src.length), out = new Float32Array(src.length);
  for (var y = 0; y < h; y++) {
    for (var x = 0; x < w; x++) {
      var sr = 0, sg = 0, sb = 0, n = 0;
      for (var k = -r; k <= r; k++) { var xx = x + k; if (xx < 0 || xx >= w) continue; var idx = (y * w + xx) * 4; sr += src[idx]; sg += src[idx + 1]; sb += src[idx + 2]; n++; }
      var o = (y * w + x) * 4; tmp[o] = sr / n; tmp[o + 1] = sg / n; tmp[o + 2] = sb / n;
    }
  }
  for (var x2 = 0; x2 < w; x2++) {
    for (var y2 = 0; y2 < h; y2++) {
      var sr2 = 0, sg2 = 0, sb2 = 0, n2 = 0;
      for (var k2 = -r; k2 <= r; k2++) { var yy = y2 + k2; if (yy < 0 || yy >= h) continue; var idx2 = (yy * w + x2) * 4; sr2 += tmp[idx2]; sg2 += tmp[idx2 + 1]; sb2 += tmp[idx2 + 2]; n2++; }
      var o2 = (y2 * w + x2) * 4; out[o2] = sr2 / n2; out[o2 + 1] = sg2 / n2; out[o2 + 2] = sb2 / n2; out[o2 + 3] = 255;
    }
  }
  return out;
}
var t = 0;
function frame() {
  t += 0.02;
  var radius = Math.round((Math.sin(t) + 1) / 2 * 6);
  var blurred = boxBlur(orig.data, W, H, radius);
  var out = ctx.createImageData(W, H);
  for (var i = 0; i < blurred.length; i++) out.data[i] = blurred[i];
  ctx.putImageData(out, 0, 0);
  var frac = (Math.sin(t * 0.55) + 1) / 2 * 0.7 + 0.15;
  var sw = Math.round(W * frac);
  if (sw > 0) ctx.putImageData(orig, 0, 0, 0, 0, sw, H);
  ctx.strokeStyle = '#fff'; ctx.lineWidth = 2; ctx.beginPath(); ctx.moveTo(sw, 0); ctx.lineTo(sw, H); ctx.stroke();
  document.getElementById('r').textContent = radius + 'px';
  requestAnimationFrame(frame);
}
frame();

A true Gaussian blur averages neighboring pixels with bell-curve weights — nearby pixels count a lot, distant ones barely at all. This demo uses a box blur (a plain average) passed horizontally and vertically several times instead, since repeated box blurs converge toward a Gaussian result (a consequence of the central limit theorem) at a fraction of the cost. A larger radius averages over a wider neighborhood and blurs more.

Use it to soften a background into a fake shallow-depth-of-field look, to build the low-frequency layer in skin retouching, or to suppress noise (noise is itself high-frequency, so blur buries it). It's also the simplest blur filter there is — one radius value in pixels, nothing else to tune.

Push the radius too far and edges that should stay crisp — an eye, a single strand of hair — dissolve into mush, and the photo stops looking like a photo. Applying it uniformly across the whole frame rarely looks like real out-of-focus blur either, because actual lens blur varies with distance — which is why a mask or a dedicated lens-blur filter that varies the radius by position looks more convincing.

When to use

Use it to soften a background or bury noise. Applying it uniformly rarely reads as real lens blur — mask it so the strength varies by position.