Block diagram
Test files for APNG can be found at the following locations:
This page hosts a formal specification of PNG (Portable Network Graphics) file using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
meta:
id: png
title: PNG (Portable Network Graphics) file
file-extension:
- png
- apng
xref:
forensicswiki: portable_network_graphics_(png)
iso: 15948:2004
justsolve:
- PNG
- APNG
loc: fdd000153
mime:
- image/png
- image/apng
- image/vnd.mozilla.apng
pronom:
- fmt/11 # PNG 1.0
- fmt/12 # PNG 1.1
- fmt/13 # PNG 1.2
- fmt/935 # APNG
rfc: 2083
wikidata:
- Q178051 # PNG
- Q433224 # APNG
license: CC0-1.0
ks-version: 0.9
endian: be
doc: |
Test files for APNG can be found at the following locations:
* <https://philip.html5.org/tests/apng/tests.html>
* <http://littlesvr.ca/apng/>
seq:
# https://www.w3.org/TR/png/#5PNG-file-signature
- id: magic
contents: [137, 80, 78, 71, 13, 10, 26, 10]
# https://www.w3.org/TR/png/#11IHDR
# Always appears first, stores values referenced by other chunks
- id: ihdr_len
type: u4
valid: 13
- id: ihdr_type
contents: "IHDR"
- id: ihdr
type: ihdr_chunk
- id: ihdr_crc
size: 4
# The rest of the chunks
- id: chunks
type: chunk
repeat: until
repeat-until: _.type == "IEND" or _io.eof
types:
chunk:
seq:
- id: len
type: u4
- id: type
type: str
size: 4
encoding: UTF-8
- id: body
size: len
type:
switch-on: type
cases:
# Critical chunks
# '"IHDR"': ihdr_chunk
'"PLTE"': plte_chunk
# IDAT = raw
# IEND = empty, thus raw
# Ancillary chunks
'"cHRM"': chrm_chunk
'"gAMA"': gama_chunk
# iCCP
# sBIT
'"sRGB"': srgb_chunk
'"bKGD"': bkgd_chunk
# hIST
# tRNS
'"pHYs"': phys_chunk
# sPLT
'"tIME"': time_chunk
'"iTXt"': international_text_chunk
'"tEXt"': text_chunk
'"zTXt"': compressed_text_chunk
# animated PNG chunks
'"acTL"': animation_control_chunk
'"fcTL"': frame_control_chunk
'"fdAT"': frame_data_chunk
- id: crc
size: 4
ihdr_chunk:
doc-ref: https://www.w3.org/TR/png/#11IHDR
seq:
- id: width
type: u4
- id: height
type: u4
- id: bit_depth
type: u1
- id: color_type
type: u1
enum: color_type
- id: compression_method
type: u1
- id: filter_method
type: u1
- id: interlace_method
type: u1
plte_chunk:
doc-ref: https://www.w3.org/TR/png/#11PLTE
seq:
- id: entries
type: rgb
repeat: eos
rgb:
seq:
- id: r
type: u1
- id: g
type: u1
- id: b
type: u1
chrm_chunk:
doc-ref: https://www.w3.org/TR/png/#11cHRM
seq:
- id: white_point
type: point
- id: red
type: point
- id: green
type: point
- id: blue
type: point
point:
seq:
- id: x_int
type: u4
- id: y_int
type: u4
instances:
x:
value: x_int / 100000.0
y:
value: y_int / 100000.0
gama_chunk:
doc-ref: https://www.w3.org/TR/png/#11gAMA
seq:
- id: gamma_int
type: u4
instances:
gamma_ratio:
value: 100000.0 / gamma_int
srgb_chunk:
doc-ref: https://www.w3.org/TR/png/#11sRGB
seq:
- id: render_intent
type: u1
enum: intent
enums:
intent:
0: perceptual
1: relative_colorimetric
2: saturation
3: absolute_colorimetric
bkgd_chunk:
doc: |
Background chunk stores default background color to display this
image against. Contents depend on `color_type` of the image.
doc-ref: https://www.w3.org/TR/png/#11bKGD
seq:
- id: bkgd
type:
switch-on: _root.ihdr.color_type
cases:
color_type::greyscale: bkgd_greyscale
color_type::greyscale_alpha: bkgd_greyscale
color_type::truecolor: bkgd_truecolor
color_type::truecolor_alpha: bkgd_truecolor
color_type::indexed: bkgd_indexed
bkgd_greyscale:
doc: Background chunk for greyscale images.
seq:
- id: value
type: u2
bkgd_truecolor:
doc: Background chunk for truecolor images.
seq:
- id: red
type: u2
- id: green
type: u2
- id: blue
type: u2
bkgd_indexed:
doc: Background chunk for images with indexed palette.
seq:
- id: palette_index
type: u1
phys_chunk:
doc: |
"Physical size" chunk stores data that allows to translate
logical pixels into physical units (meters, etc) and vice-versa.
doc-ref: https://www.w3.org/TR/png/#11pHYs
seq:
- id: pixels_per_unit_x
type: u4
doc: |
Number of pixels per physical unit (typically, 1 meter) by X
axis.
- id: pixels_per_unit_y
type: u4
doc: |
Number of pixels per physical unit (typically, 1 meter) by Y
axis.
- id: unit
type: u1
enum: phys_unit
time_chunk:
doc: |
Time chunk stores time stamp of last modification of this image,
up to 1 second precision in UTC timezone.
doc-ref: https://www.w3.org/TR/png/#11tIME
seq:
- id: year
type: u2
- id: month
type: u1
- id: day
type: u1
- id: hour
type: u1
- id: minute
type: u1
- id: second
type: u1
international_text_chunk:
doc: |
International text chunk effectively allows to store key-value string pairs in
PNG container. Both "key" (keyword) and "value" (text) parts are
given in pre-defined subset of iso8859-1 without control
characters.
doc-ref: https://www.w3.org/TR/png/#11iTXt
seq:
- id: keyword
type: strz
encoding: UTF-8
doc: Indicates purpose of the following text data.
- id: compression_flag
type: u1
doc: |
0 = text is uncompressed, 1 = text is compressed with a
method specified in `compression_method`.
- id: compression_method
type: u1
enum: compression_methods
- id: language_tag
type: strz
encoding: ASCII
doc: |
Human language used in `translated_keyword` and `text`
attributes - should be a language code conforming to ISO
646.IRV:1991.
- id: translated_keyword
type: strz
encoding: UTF-8
doc: |
Keyword translated into language specified in
`language_tag`. Line breaks are not allowed.
- id: text
type: str
encoding: UTF-8
size-eos: true
doc: |
Text contents ("value" of this key-value pair), written in
language specified in `language_tag`. Line breaks are
allowed.
text_chunk:
doc: |
Text chunk effectively allows to store key-value string pairs in
PNG container. Both "key" (keyword) and "value" (text) parts are
given in pre-defined subset of iso8859-1 without control
characters.
doc-ref: https://www.w3.org/TR/png/#11tEXt
seq:
- id: keyword
type: strz
encoding: iso8859-1
doc: Indicates purpose of the following text data.
- id: text
type: str
size-eos: true
encoding: iso8859-1
compressed_text_chunk:
doc: |
Compressed text chunk effectively allows to store key-value
string pairs in PNG container, compressing "value" part (which
can be quite lengthy) with zlib compression.
doc-ref: https://www.w3.org/TR/png/#11zTXt
seq:
- id: keyword
type: strz
encoding: UTF-8
doc: Indicates purpose of the following text data.
- id: compression_method
type: u1
enum: compression_methods
- id: text_datastream
process: zlib
size-eos: true
animation_control_chunk:
doc-ref: https://wiki.mozilla.org/APNG_Specification#.60acTL.60:_The_Animation_Control_Chunk
seq:
- id: num_frames
type: u4
doc: Number of frames, must be equal to the number of `frame_control_chunk`s
- id: num_plays
type: u4
doc: Number of times to loop, 0 indicates infinite looping.
frame_control_chunk:
doc-ref: https://wiki.mozilla.org/APNG_Specification#.60fcTL.60:_The_Frame_Control_Chunk
seq:
- id: sequence_number
type: u4
doc: Sequence number of the animation chunk
- id: width
type: u4
valid:
min: 1
max: _root.ihdr.width
doc: Width of the following frame
- id: height
type: u4
valid:
min: 1
max: _root.ihdr.height
doc: Height of the following frame
- id: x_offset
type: u4
valid:
max: _root.ihdr.width - width
doc: X position at which to render the following frame
- id: y_offset
type: u4
valid:
max: _root.ihdr.height - height
doc: Y position at which to render the following frame
- id: delay_num
type: u2
doc: Frame delay fraction numerator
- id: delay_den
type: u2
doc: Frame delay fraction denominator
- id: dispose_op
type: u1
enum: dispose_op_values
doc: Type of frame area disposal to be done after rendering this frame
- id: blend_op
type: u1
enum: blend_op_values
doc: Type of frame area rendering for this frame
instances:
delay:
value: 'delay_num / (delay_den == 0 ? 100.0 : delay_den)'
doc: Time to display this frame, in seconds
frame_data_chunk:
doc-ref: https://wiki.mozilla.org/APNG_Specification#.60fdAT.60:_The_Frame_Data_Chunk
seq:
- id: sequence_number
type: u4
doc: |
Sequence number of the animation chunk. The fcTL and fdAT chunks
have a 4 byte sequence number. Both chunk types share the sequence.
The first fcTL chunk must contain sequence number 0, and the sequence
numbers in the remaining fcTL and fdAT chunks must be in order, with
no gaps or duplicates.
- id: frame_data
size-eos: true
doc: |
Frame data for the frame. At least one fdAT chunk is required for
each frame. The compressed datastream is the concatenation of the
contents of the data fields of all the fdAT chunks within a frame.
enums:
color_type:
0: greyscale
2: truecolor
3: indexed
4: greyscale_alpha
6: truecolor_alpha
phys_unit:
0: unknown
1: meter
compression_methods:
0: zlib
dispose_op_values:
0:
id: none
doc: |
No disposal is done on this frame before rendering the next;
the contents of the output buffer are left as is.
doc-ref: https://wiki.mozilla.org/APNG_Specification#.60fcTL.60:_The_Frame_Control_Chunk
1:
id: background
doc: |
The frame's region of the output buffer is to be cleared to
fully transparent black before rendering the next frame.
doc-ref: https://wiki.mozilla.org/APNG_Specification#.60fcTL.60:_The_Frame_Control_Chunk
2:
id: previous
doc: |
The frame's region of the output buffer is to be reverted
to the previous contents before rendering the next frame.
doc-ref: https://wiki.mozilla.org/APNG_Specification#.60fcTL.60:_The_Frame_Control_Chunk
blend_op_values:
0:
id: source
doc: |
All color components of the frame, including alpha,
overwrite the current contents of the frame's output buffer region.
1:
id: over
doc: |
The frame is composited onto the output buffer based on its alpha