- What does PNG stand for and when was it created?
PNG stands for "Portable Network Graphics." It was initially released on October 1, 1996, as a W3C Recommendation, and later published as RFC 2083 in January 1997. The format was developed by the PNG Development Group led by Thomas Boutell as a patent-free replacement for GIF. - What type of compression does PNG use?
PNG uses lossless data compression. Specifically, it employs:- Deflate compression algorithm (LZ77-based)
- A sliding window of at most 32,768 bytes
- Adaptive filtering with five filter types (None, Sub, Up, Average, Paeth) applied before compression to improve compressibility
- What color types and bit depths does PNG support?
PNG supports five image types with sample depths ranging from 1 to 16 bits:- Greyscale (color type 0)
- Truecolor RGB (color type 2)
- Indexed-color with palette (color type 3)
- Greyscale with alpha channel (color type 4)
- Truecolor RGB with alpha channel (color type 6)
- What is the structure of a PNG file?
A PNG file consists of an 8-byte signature (89 50 4E 47 0D 0A 1A 0A) followed by a series of chunks. Each chunk contains a length field, chunk type (4 bytes), chunk data, and a CRC-32 checksum. Critical chunks include IHDR (image header), PLTE (palette), IDAT (image data), and IEND (image trailer). - What are some key features that distinguish PNG from GIF?
PNG was designed as a superior replacement for GIF and includes several important features not available in GIF:- Truecolor images up to 48 bits per pixel (vs. GIF's 256 colors)
- Full alpha channel for variable transparency (vs. GIF's binary transparency)
- Grayscale images up to 16 bits per pixel
- Gamma correction information for consistent color display across platforms
- Better compression for most image types
- No patent restrictions (GIF used LZW compression which was patented)
