JPEG 압축기

What the quality setting actually changes

A JPEG encoder splits the image into 8×8 pixel blocks and describes each block as a sum of frequency patterns. High-frequency detail — fine texture, sharp edges, noise — is stored with the least precision, because that is where human vision is least sensitive. The quality setting controls how aggressively that precision is dropped.

• Quality 80–90 is usually indistinguishable from the original at normal viewing size, at roughly a quarter of the file size.
• Below about quality 60, flat areas start to show 8×8 blocking and sharp edges pick up a halo of "mosquito noise".
• Photographs with lots of texture compress well; screenshots of text compress badly, because text is nothing but sharp edges.

Every re-save costs you something

Compression artifacts are baked into the pixels, so opening a compressed JPEG and saving it again compresses artifacts that are already there. Do it repeatedly and the image degrades visibly — the generational loss that makes heavily reshared images look muddy. Always compress from the highest-quality original you have rather than from a copy that has already been through the process.

When JPEG is the wrong format

JPEG has no transparency and handles flat colour and hard edges poorly. If your image is a logo, a screenshot, a chart, or anything with a transparent background, a compressed PNG will usually be both smaller and sharper. JPEG earns its place on photographs and photographic textures, where its assumptions about human vision hold.

• Photographs, product shots, hero images → JPEG.
• Logos, icons, UI screenshots, flat illustrations → PNG.
• Anything that needs a transparent background → PNG, never JPEG.

How small does it actually need to be?

Pick a target from the constraint you are actually under, not from a round number. A photo emailed as an attachment only needs to clear the mail provider's limit. A hero image on a landing page is competing for Largest Contentful Paint and wants to be as small as it can be while still looking sharp on a high-density display. A form that rejects uploads over a fixed size gives you the target directly.