What is a PNG file?

PNG does not throw information away. Before compressing, the encoder runs a filter over each row of pixels. The five filter types in the specification (None, Sub, Up, Average and Paeth) each predict a byte from its neighbours to the left and above, and store only the difference from that prediction. On an image with large areas of one colour those differences are mostly zero, and the deflate step that follows packs long runs of zeros into very few bytes.

The same mechanism explains why PNG struggles with photographs. Sensor noise and fine texture make every pixel slightly different from its neighbours, so the predictions miss, the differences are not zero, and deflate has little repetition to exploit. A PNG of a photo is often many times the size of a JPEG or WebP of the same picture, and the measurement on this page shows by how much for one sample.

Because the filters are reversible and deflate is lossless, decoding returns the exact samples that were stored. Saving a PNG again does not degrade it, which is why PNG is a common working format for images that will be edited more than once.

Drag the file onto any open browser tab. Every current major browser displays PNG, including its transparency and, for APNG, its animation, so a browser is the quickest viewer on a computer that has no image editor installed.

If a file ending in .png refuses to open anywhere, look at its first eight bytes in a hex viewer. A real PNG always starts with 89 50 4E 47 0D 0A 1A 0A; the second to fourth bytes spell "PNG" in ASCII. If the bytes say something else, the file is another format with the wrong extension, or it is damaged. The specification notes that the signature also allows early detection of some transmission errors, so a damaged first few bytes are caught before any pixel is read.

To send a PNG to someone whose software only takes JPEG, convert it in the Image Converter. JPEG has no alpha channel, so transparent areas are filled with a background colour (white by default) during the conversion.

The Third Edition brought three things that had lived outside the core specification into it. Animated PNG, which browsers had supported for years through unofficial chunks, is now defined by the acTL, fcTL and fdAT chunks in the main text. The eXIf chunk, which carries camera metadata, moved in from the PNG extensions document. And the new cICP, mDCV and cLLI chunks let a PNG describe High Dynamic Range and wide colour gamut content and the display it was mastered on.

None of this changes how an ordinary PNG is decoded. An older decoder ignores ancillary chunks it does not understand, so it still shows an APNG's static image and an HDR PNG's pixel data, just without the animation or the extra colour information.