REC-png.html

PNG (Portable Network Graphics) Specification

Version 1.1

W3C Recommendation draft-02-Nov-1998

For list of authors, see Credits.

Status of this document

DRAFT, November 2, 1998, not for publication! Please do not post this document to web sites. This incorporates Adam's "PNG-1.1 draft 1.2.8" proposal and his "incorporate sPLT" proposal. Ignore the W3C boilerplate in the remainder of this section.

This document has been reviewed by W3C members and other interested parties and has been endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited as a normative reference from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.

The Consortium staff have encouraged the development of PNG, as have Compuserve, Inc. Most of the work has been done by the PNG Development Group, png-group@w3.org. The Consortium does not currently have plans to work on any future versions of PNG, though were the necessity to arise, and were an activity in that area to receive the support of Members, the Consortium could in principle support some future activity.

Abstract

This document describes PNG (Portable Network Graphics), an extensible file format for the lossless, portable, well-compressed storage of raster images. PNG provides a patent-free replacement for GIF and can also replace many common uses of TIFF. Indexed-color, grayscale, and truecolor images are supported, plus an optional alpha channel. Sample depths range from 1 to 16 bits.

PNG is designed to work well in online viewing applications, such as the World Wide Web, so it is fully streamable with a progressive display option. PNG is robust, providing both full file integrity checking and simple detection of common transmission errors. Also, PNG can store gamma and chromaticity data for improved color matching on heterogeneous platforms.

This specification defines the Internet Media Type "image/png".

Table of Contents

• 1. Introduction
• 2. Data Representation
  • 2.1. Integers and byte order
  • 2.2. Color values
  • 2.3. Image layout
  • 2.4. Alpha channel
  • 2.5. Filtering
  • 2.6. Interlaced data order
  • 2.7. Gamma correction
  • 2.8. Text strings
• 3. File Structure
  • 3.1. PNG file signature
  • 3.2. Chunk layout
  • 3.3. Chunk naming conventions
  • 3.4. CRC algorithm
• 4. Chunk Specifications
  • 4.1. Critical chunks
    • 4.1.1. IHDR Image header
    • 4.1.2. PLTE Palette
    • 4.1.3. IDAT Image data
    • 4.1.4. IEND Image trailer
  • 4.2. Ancillary chunks
    • 4.2.1. bKGD Background color
    • 4.2.2. cHRM Primary chromaticities and white point
    • 4.2.3. gAMA Image gamma
    • 4.2.4. hIST Palette histogram
    • 4.2.5. iCCP Embedded ICC profile
    • 4.2.6. pHYs Physical pixel dimensions
    • 4.2.7. sBIT Significant bits
    • 4.2.8. sPLT Suggested palette
    • 4.2.9. sRGB Standard RGB color space
    • 4.2.10. tEXt Textual data
    • 4.2.11. tIME Image last-modification time
    • 4.2.12. tRNS Transparency
    • 4.2.13. zTXt Compressed textual data
  • 4.3. Summary of standard chunks
  • 4.4. Additional chunk types
• 5. Deflate/Inflate Compression
• 6. Filter Algorithms
  • 6.1. Filter types
  • 6.2. Filter type 0: None
  • 6.3. Filter type 1: Sub
  • 6.4. Filter type 2: Up
  • 6.5. Filter type 3: Average
  • 6.6. Filter type 4: Paeth
• 7. Chunk Ordering Rules
  • 7.1. Behavior of PNG editors
  • 7.2. Ordering of ancillary chunks
  • 7.3. Ordering of critical chunks
• 8. Miscellaneous Topics
  • 8.1. File name extension
  • 8.2. Internet media type
  • 8.3. Macintosh file layout
  • 8.4. Multiple-image extension
  • 8.5. Security considerations
• 9. Recommendations for Encoders
  • 9.1. Sample depth scaling
  • 9.2. Encoder gamma handling
  • 9.3. Encoder color handling
  • 9.4. Alpha channel creation
  • 9.5. Suggested palettes
  • 9.6. Filter selection
  • 9.7. Text chunk processing
  • 9.8. Use of private chunks
  • 9.9. Private type and method codes
• 10. Recommendations for Decoders
  • 10.1. Error checking
  • 10.2. Pixel dimensions
  • 10.3. Truecolor image handling
  • 10.4. Sample depth rescaling
  • 10.5. Decoder gamma handling
  • 10.6. Decoder color handling
  • 10.7. Background color
  • 10.8. Alpha channel processing
  • 10.9. Progressive display
  • 10.10. Suggested-palette and histogram usage
  • 10.11. Text chunk processing
• 11. Glossary
• 12. Appendix: Rationale
  • 12.1. Why a new file format?
  • 12.2. Why these features?
  • 12.3. Why not these features?
  • 12.4. Why not use format X?
  • 12.5. Byte order
  • 12.6. Interlacing
  • 12.7. Why gamma?
  • 12.8. Non-premultiplied alpha
  • 12.9. Filtering
  • 12.10. Text strings
  • 12.11. PNG file signature
  • 12.12. Chunk layout
  • 12.13. Chunk naming conventions
  • 12.14. Palette histograms
• 13. Appendix: Gamma Tutorial
  • 13.1. Nonlinear transfer functions
  • 13.2. Combining exponents
  • 13.3. End-to-end exponent
  • 13.4. CRT exponent
  • 13.5. Gamma correction
  • 13.6. Benefits of gamma encoding
  • 13.7. General gamma handling
  • 13.8. Some specific examples
  • 13.9. Video camera transfer functions
  • 13.10. Further reading
• 14. Appendix: Color Tutorial
  • 14.1. About chromaticity
  • 14.2. The problem of color
  • 14.3. Device-dependent color
  • 14.4. Device-independent color
  • 14.5. Calibrated device-dependent color
  • 14.6. Chromaticity and luminance
  • 14.7. Characterizing computer monitors
  • 14.8. Uses for XYZ
  • 14.9. Converting between RGB and XYZ
  • 14.10. Device gamut
  • 14.11. Further reading
• 15. Appendix: Sample CRC Code
• 16. Appendix: Online Resources
• 17. Appendix: Revision History
• 18. References
• 19. Credits