JPEG images
Common JPEG photo files use block-transform lossy coding, while the wider JPEG 1 standard includes additional coding modes and file-interchange conventions.
JPEG is commonly used for photographic still images, including library artwork. The usual JPEG photo route uses lossy transform coding. JPEG 1's broader standard also defines other modes, so “every JPEG is inherently lossy” is too broad.
A photo's gradual tones often tolerate some reduced precision. Thin lettering and sharp graphic edges can make the same coding changes much easier to see.
Blocks become frequency coefficients
The common DCT-based route divides component samples into 8 × 8 blocks and expresses each block through coefficients. Those coefficients describe a constant contribution and variations at different spatial frequencies.
Quantization reduces their precision before further coding. Coefficients that become zero can be represented efficiently; their discarded detail cannot be reconstructed from the encoded result.
For a small invented coefficient example:
| Original coefficient | Quantization step | Quantized integer | Reconstructed coefficient |
|---|---|---|---|
| 112 | 8 | 14 | 112 |
| −17 | 10 | −2 | −20 |
| 3 | 5 | 1 | 5 |
| 1 | 5 | 0 | 0 |
The table illustrates round/divide/multiply arithmetic rather than a full JPEG block. Reconstruction retains some values exactly and changes others.
Strong reduction can make neighboring blocks disagree visibly or create ringing around edges. These effects are different from blur already present in the photographed scene.
Chroma and alpha are separate choices
A common photographic representation reduces chroma sampling relative to luma. Chroma subsampling explains that representation and why fine coloured text can be sensitive to it.
The ordinary JPEG photo path does not supply a PNG-style alpha channel. A transparent logo converted to that path must use some background or another presentation arrangement. For a rectangular poster this can be appropriate; for a floating logo it changes the design.
Exif and an ICC profile can accompany JPEG picture data. They describe capture/orientation and colour interpretation respectively. Removing those records can change downstream presentation without changing the compressed image coefficients.
Sequential and progressive describe scan organization
A sequential JPEG supplies its coded components in the relevant sequential scan arrangement. A progressive JPEG supplies coefficient information in stages, allowing an increasingly complete reconstruction.
Progressive organization does not inherently change the final dimensions or make the image lossless. Its browsing benefit depends on a consumer presenting intermediate scans and on how data reaches that consumer.
An Emby route may decode and generate a new artwork image before the app receives it. In that case, the original source's scan arrangement and metadata are distinct from the delivered derivative's properties.
A high quality setting is not a reversible-edit guarantee
Encoder quality labels influence choices such as quantization, and sometimes subsampling through additional options. Their interpretation belongs to the encoder. Even a high setting does not establish exact recovery of the original editing buffer.
After another resize or edit, decoding and re-encoding commonly quantizes the new result again. Keeping the master separates archive quality from the particular small JPEG useful for a browsing tile.