WD-png-960502

PNG (Portable Network Graphics) Specification

Version 0.97

W3C Working Draft 2-May-1996

This version:

http://www.w3.org/pub/WWW/TR/WD-png-960502

Latest version:

http://www.w3.org/pub/WWW/TR/WD-png

Status of this document

This is a W3C Working Draft for review by W3C members and other interested parties. It is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of current W3C working drafts can be found at: http://www.w3.org/pub/WWW/TR

Note: Since working drafts are subject to frequent change, you are advised to reference the above URL, rather than the URLs for working drafts themselves.

Abstract

PNG is designed to work well in online viewing applications, such as the World Wide Web, and 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 a proposed 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 Image Histogram
    • 4.2.5. pHYs Physical Pixel Dimensions
    • 4.2.6. sBIT Significant Bits
    • 4.2.7. tEXt Textual Data
    • 4.2.8. tIME Image Last-Modification Time
    • 4.2.9. tRNS Transparency
    • 4.2.10. 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 type 0: None
  • 6.2. Filter type 1: Sub
  • 6.3. Filter type 2: Up
  • 6.4. Filter type 3: Average
  • 6.5. 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. Bit 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. Bit 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 XYZ?
  • 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
• 14. Appendix: Color Tutorial
• 15. Appendix: Sample CRC Code
• 16. Appendix: Online Resources
• 17. Appendix: Revision History
• 18. References
• 19. Credits