PNG Proposed Chunks, draft 0.960710



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This document is an informal draft of the PNG development group.

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Copyright © 1996 Thomas Boutell

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This document describes some special-purpose chunk types that have been proposed for use in various PNG (Portable Network Graphics) applications. They have not yet been approved for registration by the PNG developers, and therefore are experimental and their format is subject to change. The proposed chunks are alIG, fiNG, and spLT.

Table of Contents

1. Introduction

Chunks described here have been proposed to the PNG development group. Their definitions are subject to change until such time as the group formally registers them. No decoder is required or expected to implement these chunks except for experimental or evaluation purposes. Comments on these proposals, and new proposals for additional chunk types, should be sent to the PNG specification maintainers at The basic PNG specification is available from the W3C archive at

2. Using Proposed PNG Chunks

No chunks described in this document have yet been registered by the PNG maintainers. Therefore they have a lower-case letter in the second position of the chunkname. They are experimental chunks and the format is subject to change. If and when any become registered, the second letter of the chunk name will become uppercase, the chunk description will be moved either to a PNG extensions document or to the core PNG specification, and there will be no further change to the format.

Whenever you use any of these unregistered chunks you should also include a tEXt chunk describing it, for example:

This file contains a spLT chunk written according to the
format given in Version 0.960406 of the PNG Proposed Chunks document.

3. Summary of Proposed Chunks

This table summarizes some properties of the proposed chunks described in this document.
   Chunk name      Multiple   Ordering
                      OK?     constraints
   alIG               No      Before IDAT
   fiNG               No      None
   spLT               Yes     Before IDAT

4. Proposed Chunk Specifications

This section provides the detailed specifications for the proposed chunks.

4.1. alIG Alignment

This chunk may be used to provide alignment information to applications. It is especially useful for transparent images in which the edges of the interesting part of the picture do not correspond exactly to the edges of the image data. It can be helpful in aligning the picture with adjacent text, if there is a specific location in the picture that should be aligned with the text baseline. (We use "picture" to mean the thing depicted, as opposed to "image" which is the entire width x height rectangle described by the PNG stream.)

The format of this chunk has not changed since version 0.960406 of this document.

This chunk's contents are

left      4 bytes  left side of picture
center    4 bytes  center of picture (horizontal)
right     4 bytes  right side of picture
top       4 bytes  top of picture
middle    4 bytes  middle of picture (vertical)
baseline  4 bytes  typographic baseline of picture (vertical)
bottom    4 bytes  bottom of picture
All values are signed 32-bit integer values, measured in pixels downward from the top of the image or rightward from the left edge of the image.

Applications will not normally try to use all of the alignment values at once. A browser might use the "center" value when it wishes to center the image left-to-right on a page. A typographic system might use the "top" value to line up an image that contains a fancy capital letter with the top of the adjacent text, or the "baseline" value to line up images containing mathematical equations. Typographic systems could also determine the typographic "height", "depth", "width", and reference point values:

font_height = baseline - top
font_width  = right - left
font_depth  = bottom - baseline
font_ref_x  = left
font_ref_y  = baseline
If the alIG chunk is not present, applications can assume the following values:
left     = 0
center   = width/2
right    = width
top      = 0
middle   = height/2
baseline = 3/4 * height
bottom   = height
If an encoder writes the alIG chunk, it must supply all of the fields, and should use these values as defaults. Only one alIG chunk is permitted in a PNG datastream. If present, it must appear prior to the first IDAT chunk.

4.2. fiNG Fingerprint

This chunk contains an MD5 fingerprint of the uncompressed, unfiltered, uninterlaced image data, that can be used for rapidly determining whether two files contain the same image, without having to actually decode the IDAT chunks. The MD5 fingerprint is calculated according to the RSA Data Security, Inc. MD5 Message-Digest Algorithm, copyright by RDS Data Security, Inc., 1991-1992. A reference implementation of the algorithm and the license for its use are given in [MD5].

The format of this chunk has not changed since version 0.960710 of this document.

The fiNG chunk's contents are

fingerprint 16 bytes
The fingerprint is the MD5 digest value computed on the image data, promoted by left-bit-replication (see bit_depth scaling in PNG spec) to 16-bit RGBA (colortype 6). Scaling is done without regard to the sBIT chunk, if present. It is calculated without regard to any ancillary chunks. The value is computed on the data in the same order as the bytes would appear in a color type 6 datastream, except that no filter bytes are included in the calculation. The MD5 fingerprint value is computed as described in RFC 1321 [RFC1321].

An image's fingerprint will not change if the image is changed from interlaced to noninterlaced, or compressed with a different method, or filtered differently, or if any ancillary chunks are added, modified, or removed. It will change if there is any lossy conversion of the pixel data or if the PLTE data is changed in an indexed-color image.

While the fiNG chunk does not have any ordering requirements, you should put it early in the file for quick access.

4.3. spLT Suggested Palette

This chunk may be used to suggest a reduced palette to be used when the display device is not capable of displaying the full range of colors present in the image. If present, it provides a recommended set of colors, with alpha and frequency information, that may be used to construct a reduced palette to which the truecolor image may be quantized.

The format of this chunk has not changed since version 0.960406 of this document.

This chunk's contents are a zero-byte-terminated text string that names the palette, followed by a series of palette entries, each a ten-byte series, containing five unsigned integers:

    name:      n bytes (ASCII text)
    null byte  1 byte  (terminator)

    red:       2 bytes (0 = black, 65535 = red)
    green:     2 bytes (0 = black, 65535 = green)
    blue:      2 bytes (0 = black, 65535 = blue)
    alpha:     2 bytes (0 = fully transparent, 65535 = fully opaque)
    frequency: 2 bytes (relative frequency of occurrence)

There can be any number of entries; a decoder determines the number of entries from the remaining chunk length after the null-terminated "name" string. This length not divisible by ten is an error. Entries must appear in decreasing order of "frequency".

The "name" (e.g. "rgba512 8-8-4-2 color cube", "rgb256 winter scenery", "rgb242 6-6-6 color cube plus 26 gray levels", "Windows white background", "50-color rgb palette for use with early versions of Mosaic") identifies the palette, which may permit applications or people to choose the appropriate one when more than one suggested palette appears in a PNG file. The "name" string must consist only of printable ASCII characters and may not have leading or trailing blanks, but may have single embedded blanks. There must be at least one and no more than 79 characters in the name. Names are case-sensitive.

The red, green, and blue values are not premultiplied by alpha, nor are they precomposited against any background. A decoder can build a palette by compositing those palette entries against any background color or set of background colors that it chooses. See [link to bKGD]

Each frequency entry is proportional to the fraction of pixels in the image that are closest to that palette entry, without regard to any compositing against a background palette. The exact scale factor is chosen by the encoder, but should be chosen so that the range of individual values reasonably fills the range 0 to 65535. It is acceptable to artificially inflate the "frequency" values for "important" colors such as those in a company logo or in the facial features of a portrait. Zero is a valid value for frequency.

Note that the palette uses 16 bits (2 bytes) per value regardless of the image bit depth specification. Decoders wishing to construct 8-bit palettes can accomplish this by scaling down the RGB entries to 8 bits. [link to bit_depth rescaling].

Note: Earlier versions of the PNG specification recommended that the PLTE [link to PLTE] and hIST chunks be used for this purpose. While this is still allowed, to maintain backward compatibility, the spLT chunk is preferable, particularly when transparent pixels are present. When both the PLTE and spLT chunks are present, the PLTE data should only be used for decoding the indexed-color (color type 3) pixels, and the spLT data should be used for constructing the display palette. If the hIST chunk is also present, decoders that process the spLT chunk should ignore it.

This chunk may appear for any color type. If this chunk does appear, it must precede the first IDAT chunk. There may be multiple spLT chunks, with different names.

5. Chunks not described here

Additional chunks have been proposed for scientific visualization purposes. Since they are not expected to be implemented by general-purpose decoders, the are described separately in the PNG Scientific Visualization Chunks document. The proposed chunks described there are drNG/DrNG, faLS, loGE/LoGE, xySC, and zsCL.

6. Security considerations

Security considerations are addressed in the basic PNG specification,

The same precautions taken when displaying tEXt data should be taken when displaying the text contained in the "name" strings of the spLT chunk. Viewers should not display these strings directly without first checking for the presence of nonprintable characters, and for the <ESC> character in particular.

No known additional security hazards are posed by the chunks described here.

7. References

[1] RFC 1321, The MD5 Message-Digest Algorithm.

8. Credits



Contributors' names are presented in alphabetical order:


End of PNG Proposed Chunks Listing