PNG images
PNG stores losslessly coded raster samples with optional transparency, colour information, metadata and, in the current specification, frame-based animation.
A transparent logo or a diagram with sharp edges often benefits from preserving its prepared samples exactly. PNG supplies lossless image coding rather than the usual JPEG photo quantization.
Prediction can be reversible
For an invented eight-bit grayscale row containing 10, 13, 15, 18, store the first byte and then differences from the previous byte:
Original: 10 13 15 18
Differences: 10 3 2 3
Recovered: 10 13 15 18
Adding each difference back to the already recovered byte reconstructs the row. A real PNG filter operates on bytes with specified arithmetic and predictors; compression then codes that reversible representation.
This explains why prediction is different from rounding several values to one level. The example intentionally uses a one-byte-per-pixel row; it is not a complete PNG file.
Chunks describe different parts of the resource
| Information | Example chunk role |
|---|---|
| Grid and sample representation | IHDR |
| Coded image data | IDAT |
| Palette, where applicable | PLTE |
| Additional transparency, where applicable | tRNS |
| ICC colour profile | iCCP |
| Exif record | eXIf |
| Animation control and frames | acTL, fcTL, fdAT |
Some transparency is carried directly in alpha-bearing samples rather than tRNS. Choosing an indexed representation can also require palette preparation before encoding.
The current Third Edition W3C Recommendation, dated 24 June 2025, includes animated PNG and HDR colour signaling. An old summary that treats PNG as universally static and SDR misses those defined uses.
A lossless derivative can still differ from its master
If a source is cropped, resized, palette-reduced or converted to another colour space, PNG can retain the resulting samples exactly. It cannot reverse those earlier operations.
For an Emby artwork comparison, examine the original and the delivered derivative. A correctly decoded logo can have lost alpha during a conversion, and a high-precision master can have been reduced before the final lossless save.