アスペクト比計算機 - 画像・動画サイズ計算 オンライン

// Aspect Ratio Calculator in JavaScript

// Calculate GCD using Euclidean algorithm
function gcd(a, b) {
  a = Math.abs(Math.round(a));
  b = Math.abs(Math.round(b));
  while (b) {
    [a, b] = [b, a % b];
  }
  return a;
}

// Calculate aspect ratio from width and height
function calculateAspectRatio(width, height) {
  if (width <= 0 || height <= 0) {
    throw new Error('Width and height must be positive numbers');
  }

  const divisor = gcd(width, height);
  return {
    ratioWidth: width / divisor,
    ratioHeight: height / divisor,
    decimal: +(width / height).toFixed(4),
    label: `${width / divisor}:${height / divisor}`,
  };
}

// Calculate missing dimension
function calculateMissingDimension(ratioW, ratioH, knownDimension, type) {
  if (type === 'width') {
    return Math.round((knownDimension * ratioW) / ratioH);
  } else {
    return Math.round((knownDimension * ratioH) / ratioW);
  }
}

// Scale dimensions proportionally
function scaleDimensions(originalW, originalH, targetW, targetH) {
  if (targetW) {
    const factor = targetW / originalW;
    return { width: targetW, height: Math.round(originalH * factor), factor };
  }
  if (targetH) {
    const factor = targetH / originalH;
    return { width: Math.round(originalW * factor), height: targetH, factor };
  }
}

// Example usage
const ratio = calculateAspectRatio(1920, 1080);
console.log(ratio);
// { ratioWidth: 16, ratioHeight: 9, decimal: 1.7778, label: "16:9" }

const height = calculateMissingDimension(16, 9, 2560, 'height');
console.log(`2560 x ${height}`);
// "2560 x 1440"

const scaled = scaleDimensions(1920, 1080, 1280, null);
console.log(`${scaled.width} x ${scaled.height} (${(scaled.factor * 100).toFixed(1)}%)`);
// "1280 x 720 (66.7%)"