ガイド
JPG→WebPとPNG→WebPの違い
JPG変換とPNG変換でWebPの画質・サイズ結果が変わる理由を整理します。
JPGとPNGは元データの性質が異なるため、WebP変換後の結果も同じにはなりません。
JPG and PNG start from different data
JPG is already lossy, so converting it to WebP usually gives incremental savings rather than dramatic quality improvements.
PNG is often lossless or transparency-heavy, so converting to WebP can reduce size a lot, but settings must be chosen carefully.
What to expect from JPG to WebP
For photo content, WebP often cuts size further while keeping acceptable quality.
If the original JPG is already heavily compressed, pushing quality too low can quickly look worse.
What to expect from PNG to WebP
UI graphics and transparent assets can shrink significantly with WebP.
But text edges and logos need closer review, especially with lossy settings.
- Keep transparency checks in your QA step.
- Use higher quality for text-heavy graphics.
- Use lossless mode for critical brand assets.
Build two simple conversion rules
Use one default for photos (usually lossy) and another for graphics/transparency (often lossless or higher quality).
This keeps automation simple and avoids quality surprises.
Useful when
- Planning bulk conversion jobs.
- Comparing before/after image weight.
- Choosing export rules by asset type.
Start from the source format
If you separate JPG and PNG workflows, your WebP results become more predictable and easier to control.