PNG Specification and File Structure

PNG Specification

• PNG stands for Portable Network Graphics, a file format for storing raster images losslessly and compressively.
• It was designed as a replacement for GIF and can replace many uses of TIFF.
• The PNG specification is on a standards track under the purview of ISO/IEC JTC 1 SC 24 and is expected to be released eventually as ISO/IEC International Standard 15948.
• It is the intent of the standards bodies to maintain backward compatibility with this specification.
• PNG supports indexed-color, grayscale, and truecolor images, plus an optional alpha channel.
• Sample depths range from 1 to 16 bits.

PNG File Structure and Chunks

• A PNG file is composed of a series of chunks.
• Chunks are categorized as critical or ancillary.
• Critical chunks are required for proper image decoding.
• Ancillary chunks provide additional information about the image.
• Every chunk begins with a 4-byte length field, followed by a 4-byte chunk type field.
• The chunk type is a 4-letter identifier (e.g., “IHDR”).
• Critical chunks follow the naming convention of all uppercase letters.
• The chunk type is followed by the actual chunk data, and then a 4-byte CRC (Cyclic Redundancy Check) calculated over the chunk type and data.
• The safe/unsafe mechanism is intended for use with ancillary chunks.
• The safe-to-copy bit will always be 0 for critical chunks.
• The maximum value at a given sample depth is 2^sampledepth-1, not 2^sampledepth.

PNG Image Layout

• A PNG image is a rectangular pixel array.
• Pixels appear left-to-right within each scanline, and scanlines appear top-to-bottom.
• The size of each pixel is determined by the bit depth.
• The image bit depth determines the maximum number of palette entries, but not the color precision within the palette.
• Truecolor pixels are represented by three samples: red, green, and blue.
• The bit depth specifies the size of each sample, not the total pixel size.
• Pixels smaller than a byte never cross byte boundaries; they are packed into bytes with the leftmost pixel in the high-order bits of a byte, the rightmost in the low-order bits.

PNG Color Spaces

• PNG supports different color spaces such as sRGB.
• sRGB is a standard RGB color space.
• iCCP embeds ICC profiles, which are used to define color spaces.

PNG Compression and Filtering

• PNG uses the Deflate/Inflate compression algorithm.
• The image data is filtered using various algorithms before compression.
• Filter type 0: None
• Filter type 1: Sub
• Filter type 2: Up
• Filter type 3: Average
• Filter type 4: Paeth
• The PNG specification also defines rules for chunk ordering, with specific behaviors expected from PNG editors.

Encoding and Decoding

• PNG encoders are responsible for choosing the sample depth, filtering, and compression methods.
• Decoders are expected to handle rescaling of sample depths, if necessary.

Conclusion

• PNG is a widely used file format for storing and transmitting images.
• It offers a number of advantages, including lossless compression, support for various color spaces, and a robust file structure.
• PNG is also extensible, with the ability to include additional chunk types as needed.
• Online Resources are available for PNG software developers.

This quiz explores the Portable Network Graphics (PNG) format, its specifications, and file structure. Learn about its role as a replacement for GIF, its support for various color types, and the organization of chunks within a PNG file. Test your knowledge on the critical and ancillary chunks essential for image decoding.