Fundamentally different compression strategies
JPEG and PNG don't just differ in file size — they use entirely different compression approaches, which is the actual root of every other difference between them.
JPEG uses a lossy, DCT-based approach: the image is divided into small blocks, transformed into frequency-domain data (via the Discrete Cosine Transform), and high-frequency detail — information the human eye is less sensitive to — is quantized more aggressively, discarding it in exchange for smaller file size. This is why JPEG compression is tunable (a "quality" slider) and why over-compressing produces visible blocky artifacts concentrated at block boundaries.
PNG uses lossless deflate compression (the same underlying algorithm family as ZIP) — it finds and eliminates redundancy in the pixel data without discarding any information at all. Decompressing a PNG always reproduces the exact original pixels, which is why PNG has no "quality" setting — only a compression effort level that trades encoding time for how thoroughly it searches for redundancy, with no effect on the final decoded image.
Why this determines color depth and transparency support
JPEG's DCT-based approach was designed around continuous-tone photography and doesn't include an alpha (transparency) channel at all — transparency requires a separate mechanism, which JPEG's format doesn't define. PNG's lossless, pixel-exact approach naturally extends to a full 8-bit (or 16-bit) alpha channel per pixel, since there's no quantization step working against precise value preservation.
| JPEG | PNG | |
|---|---|---|
| Compression type | Lossy (DCT + quantization) | Lossless (deflate) |
| Transparency | Not supported | Full alpha channel |
| Best for | Photographs, continuous tone | Screenshots, flat-color graphics, anything needing transparency |
| Repeated save cycles | Degrades further each time | No degradation (lossless) |
Why JPEG looks worse on flat-color graphics specifically
JPEG's block-based DCT compression is tuned for the kind of smooth, gradual color variation typical of photographs — it performs noticeably worse on sharp edges and flat color regions (text, logos, UI screenshots), producing visible ringing artifacts around hard edges precisely because the frequency-domain transform struggles to represent abrupt transitions efficiently. This is the underlying reason "never save a screenshot as JPEG" is good practical advice, not just convention — PNG's lossless approach has no such weakness against sharp edges.
Common mistakes
- Saving screenshots or logos as JPEG "to save space." The DCT-based compression actively fights against sharp edges and flat color, often producing a larger, visibly worse result than PNG for this specific content type.
- Re-saving a JPEG multiple times during editing. Each save re-applies lossy quantization on top of the already-lossy previous version — generation loss compounds with every round trip.
- Expecting PNG's file size to shrink with a "quality" setting. PNG has no lossy quality parameter — size reduction comes from choosing a better encoder/effort level or reducing color depth, not sacrificing pixel accuracy.
FAQ
Why does JPEG have a quality slider but PNG doesn't?
JPEG's compression is lossy and quantization-based, so quality is a genuine, adjustable tradeoff against file size; PNG is lossless — its compression effort setting only affects encoding time and search thoroughness, not the final pixel-accurate output.
Can PNG represent transparency the way JPEG can't?
Yes — PNG supports a full alpha channel per pixel; JPEG's format has no native transparency mechanism at all.
Why do JPEG screenshots often look bad around text and sharp edges?
JPEG's DCT-based compression is tuned for smooth photographic gradients, not abrupt transitions — sharp edges (like text) produce visible ringing artifacts that PNG's lossless approach doesn't have.
Convert between JPEG and PNG (and preview the quality tradeoff) with the Image Converter — entirely in your browser.