画像圧縮
画像ツール · 無料オンラインツール
品質スライダーで JPEG/WebP 画像を圧縮。
画像圧縮の使い方
- 1Upload your image file or drag and drop it into the tool.
- 2Adjust the tool-specific settings (quality, size, format, etc.).
- 3Click the process button to apply changes.
- 4Download the processed image.
機能と特徴
- ✓完全無料 — 登録不要、サブスクリプションなし、広告なし
- ✓ブラウザ上で完全に動作 — データは端末から送信されません
- ✓高速・軽量 — 即座に結果を表示
- ✓クロスプラットフォーム — デスクトップ・タブレット・モバイル対応
画像圧縮とは？
Image compression reduces image file size while maintaining acceptable visual quality. Lossy compression (JPEG, WebP) discards data permanently; lossless compression (PNG) preserves all data. Modern web performance requires optimized images — uncompressed images are the #1 cause of slow page loads.
画像圧縮の仕組み
JPEG uses Discrete Cosine Transform (DCT) to discard high-frequency visual data. WebP uses VP8 intra-frame encoding with better compression. The browser's Canvas API decodes the image, re-encodes it at a chosen quality level (0-1), and outputs a compressed Blob. Higher quality = larger file; lower quality = artifacts.
よくある使用例
- ✓Web performance — reduce LCP (Largest Contentful Paint) time
- ✓E-commerce — faster product image loading increases conversions
- ✓Email attachments — fit images within size limits
- ✓Storage savings — reduce cloud storage costs for user uploads
JPEG vs WebP vs PNG
JPEG: lossy, best for photos, no transparency. WebP: lossy or lossless, smaller than JPEG, supports transparency, 97% browser support. PNG: lossless, supports transparency, larger files, best for graphics/screenshots. For web photos use WebP with JPEG fallback; for graphics use PNG or SVG.
セキュリティとプライバシー
This tool processes images entirely in your browser using the Canvas API — your images are NEVER uploaded to any server. This is critical for sensitive images (medical, legal, personal). Server-based compressors see your images; browser-based ones cannot.
技術的な詳細
JPEG: DCT-based, quality 1-100, no alpha channel, baseline/progressive. WebP: VP8-based, 25-35% smaller than JPEG at equivalent quality, supports alpha. PNG: DEFLATE compression, lossless, supports 8/16-bit per channel. Canvas API: toBlob() with quality parameter for JPEG/WebP. EXIF metadata is stripped during re-encoding.
よくある質問
What quality level should I use?
For web photos: 75-85 (good balance). For thumbnails: 60-70. For high-quality displays: 90-95. Below 50 shows visible artifacts. WebP at 80 usually matches JPEG at 90 with smaller size.
Does compression affect image dimensions?
No. Compression reduces file size, not pixel dimensions. To resize, use a separate image resize tool. Compression only changes quality and encoding efficiency.
Is my image uploaded to a server?
No. This tool uses the browser's Canvas API — all processing happens locally. Your images never leave your device. This is safer than server-based compressors.
関連ツール
概念を学ぶ
概念を学ぶ
このツールの背後にあるプログラミングの概念を理解します。