Melt guide

Compress an image on Mac without visible quality loss

There's a sweet spot where image compression saves enormous space but stays visually identical. Here's where it is on macOS.

4 min read

You’d love a smaller file. You’d hate a fuzzier one. The good news is the curve between “no compression” and “obviously degraded” has a wide flat section in the middle — and most images live there at quality 80.

Why this happens

JPEG and PNG-with-quantization both exploit gaps between what the encoder discards and what your eye can detect. At quality 100, every micro-detail is preserved, including detail your monitor can’t even render. Drop to quality 90 and you discard mostly invisible information. Drop to 80 and you discard slightly more invisible information. Below about 70 you start removing details that humans can occasionally spot.

So “visually lossless” isn’t a marketing trick — it’s a real region of the quality curve. The trick is knowing where it ends.

The native way

For JPEG. Preview’s Export window lets you preview the file size live. Set quality to 85 for the safest “visually identical” tier, 80 for the standard sweet spot:

sips -s format jpeg -s formatOptions 85 input.jpg —out output.jpg

For PNG. macOS’s native sips won’t help here — it only does zlib re-compression, which gets you 5-15%. You need quantization to get real savings without visible loss. Install pngquant:

brew install pngquant && pngquant —quality=85-95 —strip input.png

The 85-95 range means: quantize aggressively, but if the result would drop below quality 85, skip and keep the original.

The faster way

Download Melt does the same thing in a drag — and applies it across formats consistently.

  1. Download Melt and open it.
  2. Drag images in.
  3. For "no visible difference," pick quality 90.
  4. For "you'd need to pixel-peep to find it," pick 80.
  5. Click Compress.

The “never makes a file bigger” check means already-optimised images aren’t re-saved — useful when you mix new and old files in a batch.

How much smaller will it really get?

The numbers depend on the source, but for typical photographs and screenshots:

Honestly — the difference between quality 100 and quality 85 on a typical photo is something you can verify only by toggling between them in a viewer and looking for the JPEG mosquito artefacting near sharp edges. On a phone, on a busy slide, in an email — invisible.

When to break the rule

If the asset is going to be printed at high DPI, or it’s a source file that’ll be re-edited downstream (each re-encode adds artefacting), keep the quality high or stay genuinely lossless. For anything displayed on a screen exactly once, quality 80 is the answer.

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