What the number means
JPEG quality is not a percentage of anything. It is the knob an encoder turns to scale how coarsely it rounds each 8×8 block's frequencies: at 90 the rounding is fine and invisible, at 60 smooth areas start to show squares, by 40 they are obvious. The estimate printed here is the quality that would have produced the tables in your file, which is why it can be read without guessing at the content.
Deblocking, step by step
| Input | Value | Used for |
|---|---|---|
| Estimated quality | q≈ from the file | the quantiser step |
| Quantiser step | from the IJG scaling | how large a boundary step can be compression |
| Grid offset | 0–7 px across and down | where the boundaries are |
| Strength | 0–100, default 60 | how far each boundary is smoothed |
The filter is the same shape as the in-loop deblocking filters video codecs use: small, symmetric corrections across each boundary, clipped so no pixel moves more than a fraction of a quantiser step.
Check it at 1:1
Blocks are a pixel-scale defect, so they are hidden by the downsampled view at Fit. The loupe opens on the busiest patch and shows the full-resolution result beside the original; flat areas — sky, walls, skin — are where to look.
Limits
- Compression loss is permanent: deblocking hides the grid, it does not recover the detail rounded away.
- Heavily compressed colour (4:2:0 at low quality) can leave colour smears that deblocking on brightness steps does not remove.
Questions people ask
- How can you tell my JPEG's quality?
- Every JPEG stores the quantisation tables its encoder used. Most encoders scale one standard table by the quality setting, so the table in your file can be compared with the standard one at every quality from 1 to 100 and the closest match read off. Files from encoders that use their own tables are reported as approximate.
- Will re-saving it make it worse?
- Re-saving any JPEG at a lower quality than it already has adds more loss. The Export panel defaults to quality 92; if your file reads q≈60, saving at 92 will not make it worse, but it will not undo the original compression either. Saving as PNG avoids new loss at the cost of a much larger file.
- What does deblocking actually change?
- Only pixels on either side of an 8×8 boundary, and only where the step across that boundary is smaller than the compression could have produced. A real edge that happens to land on a boundary is left alone. With de-ringing on, pixels inside blocks that hold a strong edge are also smoothed toward neighbours within one quantiser step.
- Does it change the size of my picture?
- No. This route is same-size by design; the HD and fix-pixelated routes are the ones that enlarge.
Related
- Fix pixelated imageBlocks cleaned, light and colour corrected, then 2× — each step listed and switchable.
- Improve image qualityA quality report first: grain, JPEG quality, blocking, edge energy, clipping and resolution, each with its fix.
- Denoise photoGrain measured first, colour noise removed, luminance grain reduced to taste, fine detail kept.
- JPEG quality and the 8×8 blocksWhat the quality number in a JPEG means, how it can be read from the file, and what deblocking can undo.
