The latest versions of this document, the PNG specification, and related
information can always be found at the PNG archive
sites,
ftp://ftp.simplesystems.org/pub/png/
and
http://libpng.download/
.
The maintainers of the PNG specification can be contacted
by e-mail at
png-mng-misc @ lists.sourceforge.net
.
This document is an extension to the Portable Network Graphics
(PNG) specification, version 1.2 [PNG-1.2]
,
and in
"Portable Network Graphics (PNG) Specification (Second Edition)"
[PNG-ISO]
.
It describes additional public chunk
types and contains additional information for use in PNG images.
This document, together with the PNG specification, contains the
entire list of registered "public" PNG chunks. The additional
registered
chunks appearing in this document are the oFFs
, pCAL
,
sCAL
, gIFg
, gIFs
, sTER
, and fRAc
chunks, plus the deprecated gIFt
chunk.
Additional chunk types may
be proposed for inclusion in this list by contacting the PNG
specification maintainers at
png-mng-misc @ lists.sourceforge.net
.
Chunks described here are expected to be less widely supported
than those defined in the basic specification. However, application
authors are encouraged to use these chunk types whenever
appropriate for their applications.
This document also describes data representations that do not occur in the core PNG format, but are used in one or more special-purpose chunks. New chunks should use these representations whenever applicable, in order to maximize portability and simplify decoders.
If "2^{31}
" looks like
the number "231
"
instead of 2
raised to the power
31
, your viewer is not
recognizing the HTML <SUP> tag that was
introduced in HTML version 3.2; you need to look at
the HTML 2.0, ASCII text, or PostScript version
of this document instead, or use another browser.
The words "must", "required", "should", "recommended", "may", and "optional" in this document are to be interpreted as described in RFC-2119 which is consistent with their plain English meanings. The word "can" carries the same force as "may".
1. Data Representation 1.1. Integer values 1.2. Floating-point values 2. Summary of Special-Purpose Chunks 3. Chunk Descriptions 3.1. oFFs Image offset 3.2. pCAL Calibration of pixel values 3.3. sCAL Physical scale of image subject 3.4. gIFg GIF Graphic Control Extension 3.5. gIFx GIF Application Extension 3.6. sTER Indicator of Stereo Image 3.7. eXIf Exchangeable Image Format (Exif) Profile 4. Chunks Not Described Here 4.1. dSIG Digital Signature 4.2. fRAc Fractal image parameters 5. Text Chunk Keywords 5.1 Additional Registered Keywords 5.1.1. Collection 5.2. Keyword Syntax 6. Deprecated Chunks 6.1. gIFt GIF Plain Text Extension 7. Security Considerations 8. Appendix: Sample code 8.1. pCAL Sample code 8.2. Fixed-point gamma correction 9. Appendix: Rationale 9.1. pCAL Rationale 9.2. eXIf Rationale 10. Appendix: Revision History 11. References 11.1 dSIG References 11.2 eXIf References 12. Credits
Refer to Section 2.1 of the PNG specification for the format and range of integer values.
The core of PNG does not use floating-point numbers anywhere; it uses integers or, where applicable, fixed-point fractional values. However, special-purpose chunks may need to represent values that do not fit comfortably in fixed-point notation. The textual floating-point notation defined here is recommended for use in all such cases. This representation is simple, has no a priori limits on range or precision, and is portable across all machines.
A floating-point value in this notation is represented by an ASCII text string in a standardized decimal floating-point format. The string is variable-length and must be terminated by a null (zero) character unless it is the last item in its chunk. The string consists of an optional sign ("+" or "-"), an integer part, a fraction part beginning with a decimal point ("."), and an exponent part beginning with an "E" or "e" and optional sign. The integer, fraction, and exponent parts each contain one or more digits (ASCII "0" to "9"). Either the integer part or the fraction part, but not both, may be omitted. A decimal point is allowed, but not required, if there is no fraction part. The exponent part may be omitted. No spaces or any other character besides those specified may appear.
Note in particular that C-language "F" and "L" suffixes are not allowed, the string "." is not allowed as a shorthand for 0 as in some other programming languages, and no commas or underscores are allowed. This format ought to be easily readable in all programming environments.
This table summarizes some properties of the chunks described in this document.
Name Multiple Ordering constraints OK? oFFs No Before IDAT pCAL No Before IDAT sCAL No Before IDAT gIFg Yes None gIFt Yes None (this chunk is deprecated) gIFx Yes None sTER No Before IDAT dSIG Yes In pairs, immediately after IHDR and before IEND eXIf No None fRAc Yes None
oFFs
Image offsetThe oFFs
chunk gives the position on a printed page at which
the image should be output when printed alone. It can also be used to
define the image's location with respect to a larger screen or other
application-specific coordinate system.
The oFFs
chunk contains:
X position: 4 bytes (signed integer) Y position: 4 bytes (signed integer) Unit specifier: 1 byte
Both position values are signed. The following values are legal for the unit specifier:
0: unit is the pixel (true dimensions unspecified) 1: unit is the micrometer
Conversion note: one inch is equal to exactly 25400 micrometers. A
micrometer (also called a micron) is 10^{-6}
meter.
The X position is measured rightwards from the left edge of the page
to the left edge of the image; the Y position is measured downwards
from the top edge of the page to the top edge of the image. Note that
negative values are permitted, and denote displacement in the opposite
directions. Although oFFs
can specify an image placement that
is partially or wholly outside the page boundaries, the result of such
placement is application-dependent.
If present, this chunk must precede the first IDAT
chunk.
pCAL
Calibration of pixel valuesWhen a PNG file is being used to store physical data other than color
values, such as a two-dimensional temperature field, the pCAL
chunk can be used to record the relationship (mapping) between stored
pixel samples, original samples, and actual physical values. The
pCAL
data might be used to construct a reference color bar
beside the image, or to extract the original physical data values
from the file. It is not expected to affect the way the pixels are
displayed. Another method should be used if the encoder wants the
decoder to modify the sample values for display purposes.
The pCAL
chunk contains:
Calibration name: 1-79 bytes (character string) Null separator: 1 byte Original zero (x0): 4 bytes (signed integer) Original max (x1): 4 bytes (signed integer) Equation type: 1 byte Number of parameters: 1 byte Unit name: 0 or more bytes (character string) Null separator: 1 byte Parameter 0 (p0): 1 or more bytes (ASCII floating-point) Null separator: 1 byte Parameter 1 (p1): 1 or more bytes (ASCII floating-point) ...etc...
There is no null separator after the final parameter (or after the unit name, if there are zero parameters). The number of parameters field must agree with the actual number of parameters present in the chunk, and must be correct for the specified equation type (see below).
The calibration name can be any convenient name for referring
to the mapping, and is subject to the same restrictions as the keyword in a
PNG text
chunk: it must contain only printable Latin-1
[ISO/IEC-8859-1]
characters (33-126 and 161-255) and spaces (32), but no leading,
trailing, or consecutive spaces. The calibration name can permit
applications or people to choose the appropriate pCAL
chunk
when more than one is present (this could occur in a multiple-image
file, but not in a PNG file). For example, a calibration name
of "SI"
or "English" could be used to identify the system of units in the
pCAL
chunk as well as in other chunk types, to permit a decoder
to select an appropriate set of chunks based on their names.
The pCAL
chunk defines two mappings:
0..max
, where max=2^{bitdepth}-1
, to
the original samples,
which are signed integers. The x0
and x1
fields,
together with the bit depth for the image, define this mapping.
x0
, x1
,
the equation type, parameters, and unit name.
The mapping between the stored samples and the original samples is given by the following equations:
original_sample = (stored_sample * (x1-x0) + max/2) / max + x0 stored_sample = ((original_sample - x0) * max + (x1-x0)/2) / (x1-x0) clipped to the range 0..max
In these equations, "/
" means integer division
that rounds toward
negative infinity, so n/d = integer(floor(real(a)/real(b))))
.
Note that
this is the same as the "/
" operator in the
C programming language
when n
and d
are nonnegative, but not necessarily
when n
or d
is negative.
Notice that x0
and x1
are the original samples
that correspond to
the stored samples 0 and max
, respectively. Encoders will usually
set x0=0
and x1=max
to indicate that the stored samples
are equal to the
original samples. Note that x0
is not constrained to be less
than x1
,
and neither is constrained to be positive, but they must be different
from each other.
This mapping is lossless and reversible when abs(x1-x0) <= max
and the original sample is in the range x0..x1
.
If abs(x1-x0) > max
then there can be no lossless reversible mapping, but the functions
provide the best integer approximations to floating-point affine
transformations.
The mapping between the original samples and the physical values is given by one of several equations, depending on the equation type, which may have the following values:
0: Linear mapping 1: Base-e exponential mapping 2: Arbitrary-base exponential mapping 3: Hyperbolic mapping
For equation type 0:
physical_value = p0 + p1 * original_sample / (x1-x0)
For equation type 1:
physical_value = p0 + p1 * exp(p2 * original_sample / (x1-x0))
For equation type 2:
physical_value = p0 + p1 * pow(p2, (original_sample / (x1-x0)))
For equation type 3:
physical_value = p0 + p1 * sinh(p2 * (original_sample - p3) / (x1-x0))
For these physical value equations, "/
" means
floating-point division.
The function exp(x)
is e
raised to the power
of x
, where e
is
the base of the natural logarithms, approximately 2.71828182846. The
exponential function exp()
is the inverse the natural logarithm
function ln()
.
The function pow(x,y)
is x
raised to the
power of y
.
pow(x,y) = exp(y * ln(x))
The function sinh(x)
is the hyperbolic sine of x
.
sinh(x) = 0.5 * (exp(x) - exp(-x))
The units for the physical values are given by the unit name, which may contain any number of printable Latin-1 characters, with no limitation on the number and position of blanks. For example, "K", "population density", "MPa". A zero-length string can be used for dimensionless data.
For color types 0 (gray) and 4 (gray-alpha), the mappings apply to the gray sample values (but not to the alpha sample). For color types 2 (RGB), 3 (indexed RGB), and 6 (RGBA), the mappings apply independently to each of the red, green, and blue sample values (but not the alpha sample). In the case of color type 3 (indexed RGB), the mapping refers to the RGB samples and not to the index values.
Linear data can be expressed with equation type 0.
Pure logarithmic data can be expressed with either equation type 1 or 2:
Equation type 1 Equation type 2 x0 = 0 x0 = 0 x1 = max x1 = max p0 = 0 p0 = 0 p1 = bottom p1 = bottom p2 = ln(top/bottom) p2 = top/bottom
Equation types 1 and 2 are functionally equivalent; both are defined because authors may find one or the other more convenient.
Using equation type 3, floating-point data can be reduced (with
loss) to a set of integer samples such that the resolution of the
stored data is roughly proportional to its magnitude. For example,
floating-point data ranging from -10^{31}
to 10^{31}
(the usual range of
32-bit floating-point numbers) can be represented with:
Equation type 3 x0 = 0 x1 = 65535 p0 = 0.0 p1 = 1.0e-30 p2 = 280.0 p3 = 32767.0
The resolution near zero is
about 10^{-33}
, while the resolution near
10^{31}
or -10^{31}
is about 10^{28}
.
Everywhere the resolution is about 0.4 percent of the magnitude.
Note that those floating-point parameters could be stored in the chunk more compactly as follows:
p0 = 0 p1 = 1e-30 p2 = 280 p3 = 32767
Applications should use double precision arithmetic (or take other
precautions) while performing the mappings for equation types 1, 2, and
3, to prevent overflow of intermediate results when p1 is small and the
exp()
, pow()
, or sinh()
function is large.
If present, the pCAL
chunk must appear before the first
IDAT
chunk. Only one instance of the pCAL
chunk is
permitted in a PNG datastream.
sCAL
Physical scale of image subjectWhile the pHYs
chunk is used to record the physical size
of the image itself as it was scanned or as it should be printed,
certain images (such as maps, photomicrographs, astronomical surveys,
floor plans, and others) may benefit from knowing the actual physical
dimensions of the image's subject for remote measurement and other
purposes. The sCAL
chunk serves this need. It contains:
Unit specifier: 1 byte Pixel width: 1 or more bytes (ASCII floating-point) Null separator: 1 byte Pixel height: 1 or more bytes (ASCII floating-point)
The following values are legal for the unit specifier:
1: unit is the meter 2: unit is the radian
Following the unit specifier are two ASCII strings. The first
string defines the physical width represented by one image pixel;
the second string defines the physical height represented by
one pixel. The two strings are separated by a zero byte (null
character). As in the text
chunks, there is no trailing
zero byte for the final string. Each of these strings contains
a floating-point constant in the format specified above
(Floating-point values, Section 1.2).
Both values are required to be greater than zero.
If present, this chunk must precede the first IDAT
chunk.
gIFg
GIF Graphic Control ExtensionThe gIFg
chunk is provided for backward compatibility with
the GIF89a Graphic Control Extension. It contains:
Disposal Method: 1 byte User Input Flag: 1 byte Delay Time: 2 bytes (byte order converted from GIF)
The Disposal Method indicates the way in which the graphic is to be treated after being displayed. The User Input Flag indicates whether user input is required before continuing. The Delay Time specifies the number of hundredths (1/100) of a second to delay before continuing with the processing of the datastream. Note that this field is to be byte-order-converted.
The "Transparent Color Flag" and "Transparent Color
Index" fields
found in the GIF89a Graphic Control Extension are omitted from
gIFg
. These fields should be converted using the transparency
features of basic PNG.
The GIF specification allows at most one Graphic Control Extension to
preceed each graphic rendering block. Because each PNG file holds only
one image, it is expected that gIFg
will appear at most once,
before IDAT
, but there is no strict requirement.
gIFx
GIF Application ExtensionThe gIFx
chunk is provided for backward compatibility with
the GIF89a Application Extension. The Application Extension contains
application-specific information. This chunk contains:
Application Identifier: 8 bytes Authentication Code: 3 bytes Application Data: n bytes
The Application Identifier is a sequence of eight printable ASCII characters used to identify the application creating the Application Extension. The Authentication Code is three additional bytes that the application may use to further validate the Application Extension. The remainder of the chunk is application-specific data whose content is not defined by the GIF specification.
Note that GIF-to-PNG converters should not attempt to perform byte reordering on the contents of the Application Extension. The data is simply transcribed without any processing except for de-blocking GIF sub-blocks.
Applications that formerly used GIF Application Extensions may define
special-purpose PNG chunks to replace their application extensions.
If a GIF-to-PNG converter recognizes the Application Identifier and
is aware of a corresponding PNG chunk, it may choose to convert the
Application Extension into that PNG chunk type rather than using
gIFx
.
sTER
Indicator of Stereo Image
When present, the sTER
chunk indicates that the datastream
contains a stereo pair of subimages within a single PNG image.
The sTER
chunk contains:
Mode: 1 byte 0: cross-fuse layout 1: diverging-fuse layout
The sTER
chunk with mode==0
or mode==1
indicates that the datastream
contains two subimages, encoded within a single PNG image.
They are arranged side-by-side, with one subimage intended
for presentation to the right eye and the other subimage
intended for presentation to the left eye. The left edge of
the right subimage must be on a column that is evenly divisible
by eight, so that if interlacing is employed the two images
will have coordinated interlacing. Padding columns between
the two subimages must be introduced by the encoder if
necessary.
The sTER
chunk imposes no requirements on the contents of the
padding pixels. For compatibility with software not supporting
sTER
, it does not exempt the padding pixels from existing
requirements; for example, in palette images, the padding pixels
must be valid palette indices.
The two subimages must have the same dimensions
after removal of any padding.
When mode==0
, the right-eye image appears at the left and
the left-eye image appears at the right, suitable for
cross-eyed free viewing. When mode==1
, the left-eye image
appears at the left and the right-eye image appears at the
right, suitable for divergent (wall-eyed) free viewing.
Decoders that are aware of the sTER
chunk may display the two images
in any suitable manner, with or without the padding.
Decoders that are not
aware of the sTER
chunk, and those that recognize the chunk
but choose not to treat stereo pairs differently from regular
PNG images, will naturally display them side-by-side in a
manner suitable for free viewing.
If present, the sTER
chunk must appear before the first
IDAT
chunk.
Given two subimages with width subimage_width, encoders can calculate the inter-subimage padding and total width W using the following pseudocode:
padding := 7 - ((subimage_width - 1) mod 8) W := 2 * subimage_width + padding
Given an image with width W, decoders can calculate the subimage width and inter-subimage padding using the following pseudocode:
padding := 15 - ((W - 1) mod 16) if (padding > 7) then error subimage_width := (W - padding) / 2
Decoders can assume that the samples in the left and right subimages are cosited, such that the subimages and their centers are coincident at the projection plane. Decoders can also assume that the left and right subimages are intended to be presented directly to the right and left eyes of the user/viewer without independent scaling, rotation or displacement. I.e., the subimages will be presented at the same size in the same relative position and orientation to each eye of the viewer.
Encoders should use the pHYs
chunk to indicate the pixel's
size ratio when it is not 1:1.
It is recommended that encoders use the cross-fusing layout
(mode==0
),
especially when the image centers are separated by more than 65 millimeters
when displayed on a typical monitor.
eXIf
Exchangeable Image File (Exif) Profile 101 88 73 102 (ASCII "eXIf")
The data segment of the eXIf chunk contains an Exif profile in the format specified in "4.7.2 Interoperability Structure of APP1 in Compressed Data" of [CIPA DC-008-2016] except that the JPEG APP1 marker, length, and the "Exif ID code" described in 4.7.2(C), i.e., "Exif", NULL, and padding byte, are not included.
The eXIf chunk may appear anywhere between the IHDR and IEND chunks except between IDAT chunks. The eXIf chunk size is constrained only by the maximum of 2^31-1 bytes imposed by the PNG specification. Only one eXIf chunk is allowed in a PNG datastream.
The eXIf chunk contains metadata concerning the original image data. If the image has been edited subsequent to creation of the Exif profile, this data might no longer apply to the PNG image data. It is recommended that unless a decoder has independent knowledge of the validity of the Exif data, the data should be considered to be of historical value only. It is beyond the scope of this specification to resolve potential conflicts between data in the eXIf chunk and in other PNG chunks.
73 73 42 0 (ASCII "II", 16-bit little-endian integer 42)or
77 77 0 42 (ASCII "MM", 16-bit big-endian integer 42)All other values are reserved for possible future definition.
The definitions of some public chunks are being maintained by groups other than the core PNG group. In general, these are chunks that are useful to more than one application (and thus are not private chunks), but are considered too specialized to list in the core PNG documentation.
dSIG
Digital signatureThe dSIG
chunk provides a digital signature
that guarantees that the contents of the prtion of the entire datastream
enclosed in a pair of such chunks has not changed since the digital signature
was added. This chunk is described in detail in a separate document,
[dSIG-spec]
,
which is accompanied by an example provided in
[dSIG-example]
.
fRAc
Fractal image parametersThe fRAc
chunk will describe the parameters used to
generate a fractal image. The specification for the contents of
the fRAc
chunk is being developed by Tim Wegner,
twegner @ phoenix.net
.
In the future, chunks will be fully specified before they are registered.
All registered textual keywords in text
chunks and all other chunk types are limited to the ASCII
characters A-Z, a-z, 0-9, space,
and the following 20 symbols:
! " % & ' ( ) * + , - . / : ; < = > ? _
but not the remaining 12 symbols:
# $ @ [ \ ] ^ ` { | } ~
This restricted set is the ISO-646 "invariant" character set
[ISO-646]
.
These characters have the same numeric codes in all ISO character sets,
including all national variants of ASCII.
The chunks listed in this section are registered, but deprecated. Encoders are discouraged from using them, and decoders are not encouraged to support them.
gIFt
GIF Plain Text ExtensionThe gIFt
chunk was originally provided for backward
compatibility with the GIF89a Plain Text Extension, but gIFt
is
now deprecated because it suffers from some fundamental design flaws.
IDAT
and gIFt
simultaneously. Since IDAT
is
required, gIFt
must be discouraged.
The gIFt
chunk contains:
Text Grid Left Position: 4 bytes (signed integer, byte order and size converted) Text Grid Top Position: 4 bytes (signed integer, byte order and size converted) Text Grid Width: 4 bytes (unsigned integer, byte order and size converted) Text Grid Height: 4 bytes (unsigned integer, byte order and size converted) Character Cell Width: 1 byte Character Cell Height: 1 byte Text Foreground Color: 3 bytes (R,G,B samples) Text Background Color: 3 bytes (R,G,B samples) Plain Text Data: n bytes
Text Grid Left Position, Top Position, Width, and Height specify the
text area position and size in pixels. The converter must reformat
these fields from 2-byte LSB-first unsigned integers to 4-byte
MSB-first signed or unsigned integers. Note that GIF defines the
position to be relative to the upper left corner of the logical screen.
If an oFFs
chunk is also present, a decoder should assume
that the oFFs
chunk defines the offset of the image relative
to the GIF logical screen; hence subtracting the oFFs
values
(converted from micrometers to pixels if necessary) from the Text Grid
Left and Top Positions gives the text area position relative to the main
PNG image.
Character Cell Width and Height give the dimensions of each character in pixels.
Text Foreground and Background Color give the colors to be used to render text foreground and background. Note that the GIF-to-PNG converter must replace the palette index values found in the GIF Plain Text Extension block with the corresponding palette entry.
The remainder of the chunk is the text to be displayed. Note that this data is not in GIF sub-block format, but is a continuous datastream.
The normal precautions (see the Security considerations section of
the PNG specification) should be taken when displaying text contained
in the sCAL
calibration name, pCAL
unit name, or
any ASCII floating-point fields.
Applications must take care to avoid underflow and overflow of
intermediate results when converting data from one form to another
according to the pCAL
mappings.
The Exif specification does not contain a requirement that tag "value offset" pointers actually point to a valid address within the file (see Paragraph 4.6.2). Offsets are measured in bytes from the beginning of the TIFF header (the "II" or "MM"). Although this seems to be an implicit requirement, decoders should be prepared to encounter invalid pointers and deal with them appropriately.
This appendix provides some sample code that can be used in encoding and decoding PNG chunks. It does not form a part of the specification. In the event of a discrepancy between the sample code in this appendix and the chunk definition, the chunk definition prevails.
pCAL
Sample codeThe latest version of this code, including test
routines not shown here, is available at
ftp://ftp.simplesystems.org/pub/png/src/pcal.c
.
#if 0 pcal.c 0.2.2 (Sat 19 Dec 1998) Adam M. Costello <amc @ cs.berkeley.edu> This is public domain example code for computing the mappings defined for the PNG pCAL chunk. #endif #if __STDC__ != 1 #error This code relies on ANSI C conformance. #endif #include <limits.h> #include <math.h> #include <stdio.h> #include <stdlib.h> /* In this program a type named uintN denotes an unsigned */ /* type that handles at least all values 0 through (2^N)-1. */ /* A type named intN denotes a signed type that handles at */ /* least all values 1-2^(N-1) through 2^(N-1)-1. It is not */ /* necessarily the smallest such type; we are more concerned */ /* with speed. */ typedef unsigned int uint16; #if UINT_MAX >= 0xffffffff typedef unsigned int uint32; #else typedef unsigned long uint32; #endif #if INT_MAX >= 0x7fffffff && INT_MIN + 0x7fffffff <= 0 typedef int int32; #else typedef long int32; #endif /* Testing for 48-bit integers is tricky because we cannot */ /* safely use constants greater than 0xffffffff. Also, */ /* shifting by the entire width of a type is undefined, so */ /* for unsigned int, which might be only 16 bits wide, we */ /* must shift in two steps. */ #if (UINT_MAX - 0xffff) >> 8 >> 8 >= 0xffffffff typedef unsigned int uint48; #define HAVE_UINT48 1 #elif (ULONG_MAX - 0xffff) >> 16 >= 0xffffffff typedef unsigned long uint48; #define HAVE_UINT48 1 #elif defined(ULLONG_MAX) #if (ULLONG_MAX - 0xffff) >> 16 >= 0xffffffff typedef unsigned long long uint48; #define HAVE_UINT48 1 #endif #else #define HAVE_UINT48 0 #endif /*******************/ /* Program failure */ void fail(const char *msg) { fputs(msg,stderr); fputc('\n', stderr); exit(EXIT_FAILURE); } /*************************/ /* Check max, x0, and x1 */ int samp_params_ok(uint16 max, int32 x0, int32 x1) /* Returns 1 if max, x0, and x1 have */ /* allowed values, 0 otherwise. */ { const int32 xlimit = 0x7fffffff; return max > 0 && max <= 0xffff && x0 <= xlimit && x0 >= -xlimit && x1 <= xlimit && x1 >= -xlimit && x0 != x1; } /***********************************************/ /* Map from stored samples to original samples */ int32 stored_to_orig(uint16 stored, uint16 max, int32 x0, int32 x1) #if 0 Returns the original sample corresponding to the given stored sample, which must be <= max. The parameters max, x0, and x1 must have been approved by samp_params_ok(). The pCAL spec says: orig = (stored * (x1-x0) + max/2) / max + x0 [1] Equivalently: orig = (stored * (x1-x0) + max/2) / max + (x0-x1) - (x0-x1) + x0 orig = (stored * (x1-x0) + max * (x0-x1) + max/2) / max - (x0-x1) + x0 orig = ((max - stored) * (x0-x1) + max/2) / max + x1 So we can check whether x0 < x1 and coerce the formula so that the numerators and denominators are always nonnegative: orig = (offset * xspan + max/2) / max + xbottom [2] This will come in handy later. But the multiplication and the subtraction can overflow, so we have to be trickier. For the subtraction, we can convert to unsigned integers. For the multiplication, we can use 48-bit integers if we have them, otherwise observe that: b = (b/c)*c + b%c a*b = a*(b/c)*c + a*(b%c) ; let d = a*(b%c) (a*b)/c = a*(b/c) + d/c remainder d%c [3] These are true no matter which way the division rounds. If (a*b)/c is in-range, a*(b/c) is guaranteed to be in-range if b/c rounds toward zero. Here is another observation: sum{x_i} / c = sum{x_i / c} + sum{x_i % c} / c [4] This one also avoids overflow if the division rounds toward zero. The pCAL spec requires rounding toward -infinity. ANSI C leaves the rounding direction implementation-defined except when both the numerator and denominator are nonnegative, in which case it rounds downward. So if we arrange for all numerators and denominators to be nonnegative, everything works. Starting with equation 2 and applying identity 4, then 3, we obtain the final formula: d = offset * (xspan % max) xoffset = offset * (xspan / max) + d/max + (d%max + max/2) / max orig = xoffset + xbottom #endif { uint16 offset; uint32 xspan, q, r, d, xoffset; int32 xbottom; if (stored > max) fail("stored_to_orig: stored > max"); if (x1 >= x0) { xbottom = x0; xspan = (uint32)x1 - (uint32)x0; offset = stored; } else { xbottom = x1; xspan = (uint32)x0 - (uint32)x1; offset = max - stored; } /* We knew xspan would fit in a uint32, but we needed to */ /* cast x0 and x1 before subtracting because otherwise the */ /* subtraction could overflow, and ANSI doesn't say what */ /* the result will be in that case. */ /* Let's optimize two common simple cases */ /* before handling the general case: */ if (xspan == max) { xoffset = offset; } else if (xspan <= 0xffff) { /* Equation 2 won't overflow and does only one division. */ xoffset = (offset * xspan + (max>>1)) / max; } else { #if HAVE_UINT48 /* We can use equation 2 and do one uint48 */ /* division instead of three uint32 divisions. */ xoffset = (offset * (uint48)xspan + (max>>1)) / max; #else q = xspan / max; r = xspan % max; /* Hopefully those were compiled into one instruction. */ d = offset * r; xoffset = offset * q + d/max + (d%max + (max>>1)) / max; #endif } /* xoffset might not fit in an int32, but we know the sum */ /* xbottom + xoffset will, so we can do the addition on */ /* unsigned integers and then cast. */ return (int32)((uint32)xbottom + xoffset); } /***********************************************/ /* Map from original samples to stored samples */ uint16 orig_to_stored(int32 orig, uint16 max, int32 x0, int32 x1) #if 0 Returns the stored sample corresponding to the given original sample. The parameters max, x0, and x1 must have been approved by samp_params_ok(). The pCAL spec says: stored = ((orig - x0) * max + (x1-x0)/2) / (x1-x0) clipped to the range 0..max Notice that all three terms are nonnegative, or else all are nonpositive. Just as in stored_to_orig(), we can avoid overflow and rounding problems by transforming the equation to use unsigned quantities: stored = (xoffset * max + xspan/2) / xspan #endif { uint32 xoffset, xspan; if (x0 < x1) { if (orig < x0) return 0; if (orig > x1) return max; xspan = (uint32)x1 - (uint32)x0; xoffset = (uint32)orig - (uint32)x0; } else { if (orig < x1) return 0; if (orig > x0) return max; xspan = (uint32)x0 - (uint32)x1; xoffset = (uint32)x0 - (uint32)orig; } /* For 16-bit xspan the calculation is straightforward: */ if (xspan <= 0xffff) return (xoffset * max + (xspan>>1)) / xspan; /* Otherwise, the numerator is more than 32 bits and the */ /* denominator is more than 16 bits. The tricks we played */ /* in stored_to_orig() depended on the denominator being */ /* 16-bit, so they won't help us here. */ #if HAVE_UINT48 return ((uint48)xoffset * max + (xspan>>1)) / xspan; #else /* Doing the exact integer calculation with 32-bit */ /* arithmetic would be very difficult. But xspan > 0xffff */ /* implies xspan > max, in which case the pCAL spec says */ /* "there can be no lossless reversible mapping, but the */ /* functions provide the best integer approximations to */ /* floating-point affine transformations." So why insist */ /* on using the integer calculation? Let's just use */ /* floating-point. */ return ((double)xoffset * max + (xspan>>1)) / xspan; #endif } /*********************************************/ /* Check x0, x1, eqtype, n, and p[0]..p[n-1] */ int phys_params_ok(int32 x0, int32 x1, int eqtype, int n, double *p) /* Returns 1 if x0, x1, eqtype, n, and p[0]..p[n-1] */ /* have allowed values, 0 otherwise. */ { if (!samp_params_ok(1,x0,x1)) return 0; switch (eqtype) { case 0: return n == 2; case 1: return n == 3; case 2: break; case 3: return n == 4; } /* eqtype is 2, check for pow() domain error: */ if (p[2] > 0) return 1; if (p[2] < 0) return 0; return (x0 <= x1) ? (x0 > 0 && x1 > 0) : (x0 < 0 && x1 < 0); } /************************************************/ /* Map from original samples to physical values */ double orig_to_phys(int32 orig, int32 x0, int32 x1, int eqtype, double *p) /* Returns the physical value corresponding to the given */ /* original sample. The parameters x0, x1, eqtype, and p[] */ /* must have been approved by phys_params_ok(). The array */ /* p[] must hold enough parameters for the equation type. */ { double xdiff, f; xdiff = (double)x1 - x0; switch (eqtype) { case 0: f = orig / xdiff; break; case 1: f = exp(p[2] * orig / xdiff); break; case 2: f = pow(p[2], orig / xdiff); break; case 3: f = sinh(p[2] * (orig - p[3]) / xdiff); break; default: fail("orig_to_phys: unknown equation type"); } return p[0] + p[1] * f; }
The latest version of this code, including test
routines not shown here, is available at
ftp://ftp.simplesystems.org/pub/png/src/gamma-lookup.c
.
#if 0 gamma-lookup.c 0.1.4 (Sat 19 Dec 1998) by Adam M. Costello <amc @ cs.berkeley.edu> This is public domain example code for computing gamma correction lookup tables using integer arithmetic. #endif #if __STDC__ != 1 #error This code relies on ANSI C conformance. #endif #include <limits.h> #include <math.h> /* In this program a type named uintN denotes the */ /* smallest unsigned type we can find that handles */ /* at least all values 0 through (2^N)-1. */ typedef unsigned char uint8; #if UCHAR_MAX >= 0xffff typedef unsigned char uint16; #else typedef unsigned short uint16; #endif #if UCHAR_MAX >= 0xffffffff typedef unsigned char uint32; #elif USHRT_MAX >= 0xffffffff typedef unsigned short uint32; #elif UINT_MAX >= 0xffffffff typedef unsigned int uint32; #else typedef unsigned long uint32; #endif /*********************/ /* 16-bit arithmetic */ void precompute16(uint16 L[511]) /* Precomputes the log table (this requires floating point). */ { int j; double f; /* L[j] will hold an integer representation of */ /* -log(j / 510.0). Knowing that L[1] (the largest) is */ /* 0xfe00 will help avoid overflow later, so we set the */ /* scale factor accordingly. */ f = 0xfe00 / log(1 / 510.0); for (j = 1; j <= 510; ++j) L[j] = log(j / 510.0) * f + 0.5; } void gamma16(uint16 L[511], uint8 G[256], uint16 g) /* Makes a 256-entry gamma correction lookup table G[] with */ /* exponent g/pow(2,14), where g must not exceed 0xffff. */ { int i, j; uint16 x, y, xhi, ghi, xlo, glo; j = 1; G[0] = 0; for (i = 1; i <= 255; ++i) { x = L[i << 1]; xhi = x >> 8; ghi = g >> 8; y = xhi * ghi; if (y > 0x3f80) { /* We could have overflowed later. */ /* But now we know y << 2 > L[1]. */ G[i] = 0; continue; } xlo = x & 0xff; glo = g & 0xff; y = (y << 2) + ((xhi * glo) >> 6) + ((xlo * ghi) >> 6); while (L[j] > y) ++j; G[i] = j >> 1; } } /*********************/ /* 32-bit arithmetic */ void precompute32(uint32 L[511]) /* Precomputes the log table (this requires floating point). */ { int j; double f; /* L[j] will hold an integer representation of */ /* -log(j / 510.0). Knowing that L[1] (the largest) */ /* is 0x3ffffff will help avoid overflow later, so we */ /* set the scale factor accordingly. */ f = 0x3fffffff / log(1 / 510.0); for (j = 1; j <= 510; ++j) L[j] = log(j / 510.0) * f + 0.5; } void gamma32(uint32 L[511], uint8 G[256], uint16 g) /* Makes a 256-entry gamma correction lookup table G[] with */ /* exponent g/pow(2,14), where g must not exceed 0xffff. */ { int i, j; uint32 x, y; j = 1; G[0] = 0; for (i = 1; i <= 255; ++i) { x = L[i << 1]; y = (x >> 14) * g; while (L[j] > y) ++j; G[i] = j >> 1; } } /**********************************************/ /* Floating-point arithmetic (for comparison) */ void gamma_fp(uint8 G[256], double g) /* Makes a 256-entry gamma correction */ /* lookup table G[i] with exponent g. */ { int i; G[0] = 0; for (i = 1; i <= 255; ++i) G[i] = pow(i/255.0, g) * 255 + 0.5; }
This appendix gives the reasoning behind some of the design decisions in the PNG extension chunks. It does not form a part of the specification.
pCAL
RationaleThis section gives the reasoning behind some of the design
decisions in the pCAL
chunk. It does not form a part of the
specification.
Equation types 1 and 2 seem to be equivalent. Why have both?
ln()
and exp()
, since pow()
may provide
better accuracy in some
floating-point math libraries. We also don't want to force people using
base-10 logs to store a sufficiently accurate value of ln(10)
in the
pCAL
chunk.
e
, we don't want to force people to encode a
sufficiently accurate value of e
in the pCAL
chunk, or
to use pow()
when exp()
is sufficient.
x0
and x1
provide a way to recover the
original data,
losslessly, when the original range is not a power of two. Sometimes
the digitized values do not have a range that fills the full depth of a
PNG. For example, if the original samples range from 0 (corresponding
to black) to 800 (corresponding to white), PNG requires that these
samples be scaled to the range 0 to 65535. By recording x0=0
and x1=800
we can recover the original samples, and we indicate the precision of
the data.
x0=46000
and
x1.4.200
,
we can recover the original data samples that fell between 46000 and
47000.
Why define integer divison to round toward negative infinity? This is different from many C implementations and from all Fortran implementations, which round toward zero.
We cannot leave the choice unspecified. If we were to specify
rounding toward zero, we'd have to account for a discontinuity at
zero. A division by positive d
would map the 2d-1
values from -(d-1)
through d-1
to zero, but would map only d values to any other value;
for example, 3d
through 4d-1
would be mapped
to 3
. Achieving lossless
mappings in spite of this anomaly would be difficult.
eXIf
RationaleeXIf
chunk
dSIG
chunk.
sTER
chunk.
iTXt
chunk, which has been moved to the core spec.
iTXt
chunk
pCAL
chunk and related sample code
gIFT
chunk
htp://libpng.download/documents/signatures/
.
http://libpng.download/documents/signatures/
.
http://libpng.download/documents/iso_8859-1.*
http://libpng.download/documents/
.
http://png-mng.sourceforge.net/pub/png/spec/iso/
glennrp @ users.sourceforge.net
tgl @ sss.pgh.pa.us
(edited the first release
of this document)
Names of contributors not already listed in the PNG specification are presented in alphabetical order:
tfrench @ sandia.gov
alaric @ alaric-snell.com
cosmin @ cs.toronto.edu
GIF is a service mark of CompuServe Incorporated. PostScript is a trademark of Adobe Systems.
Copyright © 1998, 1999, 2006, 2008, 2017 by: Glenn Randers-Pehrson
This specification is being provided by the copyright holder under the following license. By obtaining, using and/or copying this specification, you agree that you have read, understood, and will comply with the following terms and conditions:
Permission to use, copy, and distribute this specification for any purpose and without fee or royalty is hereby granted, provided that the full text of this NOTICE appears on ALL copies of the specification or portions thereof, including modifications, that you make.
THIS SPECIFICATION IS PROVIDED "AS IS," AND COPYRIGHT HOLDER MAKES NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED. BY WAY OF EXAMPLE, BUT NOT LIMITATION, COPYRIGHT HOLDERS MAKE NO REPRESENTATIONS OR WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE OR THAT THE USE OF THE SPECIFICATION WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER RIGHTS. COPYRIGHT HOLDER WILL BEAR NO LIABILITY FOR ANY USE OF THIS SPECIFICATION.
The name and trademarks of copyright holder may NOT be used in advertising or publicity pertaining to the specification without specific, written prior permission. Title to copyright in this specification and any associated documentation will at all times remain with copyright holder.
The "Appendix: Sample Code" has been placed in the public domain, and the conditions described above do not apply to that appendix.