ナレッジハブ・技術ガイド
画像圧縮：画質を維持したWeb最適化テクニック
画像圧縮：画質を維持したWeb最適化テクニック
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コアアーキテクチャ & 計算式
Compression Ratio = (Original Bytes — Compressed Bytes) / Original Bytes * 100
Setting a JPEG or WebP compression quality between 75% and 80% usually delivers optimal byte reduction (often 60% smaller) without any perceptible visual degradation to the human eye.
ベストプラクティスとガイドライン
- Use Modern Formats (WebP): Whenever possible, compress your images as WebP rather than JPEG. WebP consistently achieves 25% to 35% superior compression density compared to legacy formats.
- Strip Hidden EXIF Metadata: Smartphones embed GPS coordinates, camera models, and heavy thumbnail previews inside photo files. Compressing the image natively strips this EXIF metadata, protecting your privacy and saving bytes.
- Don't Recompress Lossy Files: If you take a highly compressed JPEG and compress it again, you introduce compounding compression artifacts (ugly blocky halos around text and edges). Keep your original masters lossless.
よくある質問 (FAQ)
What is the difference between Lossy and Lossless?
Lossless compression (like PNG) perfectly reconstructs pixel data with zero quality change. Lossy compression (like JPEG) permanently throws away subtle chromatic details for vastly higher byte savings.
Why did my PNG file get bigger?
If you try to compress a complex photograph as a PNG, it will be massive. PNG is designed for flat vector graphics, logos, and screenshots. Always use JPEG or WebP for real world photography.
Is WebP supported everywhere?
Yes. WebP is fully supported by all modern versions of Chrome, Safari, Firefox, and Edge. The legacy format barrier has been completely eliminated.