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webp-container-spec: minor wording updates

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Mostly grammatical and addition/subtraction of commas from the AUTH48
portion of the RFC review process.

The serial comma changes are based on the Chicago Manual of Style
(CMOS), 17th edition.

Change-Id: Ic75abf2e53e09c8a849e28e9c40e16c127515287
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James Zern 2023-06-02 19:52:36 -07:00
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doc/webp-container-spec.txt
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@ -21,7 +21,7 @@ Introduction
------------ ------------
WebP is an image format that uses either (i) the VP8 key frame encoding to WebP is an image format that uses either (i) the VP8 key frame encoding to
compress image data in a lossy way, or (ii) the WebP lossless encoding. These compress image data in a lossy way or (ii) the WebP lossless encoding. These
encoding schemes should make it more efficient than older formats such as JPEG, encoding schemes should make it more efficient than older formats such as JPEG,
GIF and PNG. It is optimized for fast image transfer over the network (for GIF and PNG. It is optimized for fast image transfer over the network (for
example, for websites). The WebP format has feature parity (color profile, example, for websites). The WebP format has feature parity (color profile,
@ -31,19 +31,20 @@ the structure of a WebP file.
The WebP container (that is, the RIFF container for WebP) allows feature support The WebP container (that is, the RIFF container for WebP) allows feature support
over and above the basic use case of WebP (that is, a file containing a single over and above the basic use case of WebP (that is, a file containing a single
image encoded as a VP8 key frame). The WebP container provides additional image encoded as a VP8 key frame). The WebP container provides additional
support for: support for the following:
* **Lossless compression.** An image can be losslessly compressed, using the * Lossless Compression: An image can be losslessly compressed, using the
WebP Lossless Format. WebP Lossless Format.
* **Metadata.** An image may have metadata stored in Exif or XMP formats. * Metadata: An image may have metadata stored in Exchangeable Image File
Format (Exif) or Extensible Metadata Platform (XMP) format.
* **Transparency.** An image may have transparency, that is, an alpha channel. * Transparency: An image may have transparency, that is, an alpha channel.
* **Color Profile.** An image may have an embedded ICC profile as described * Color Profile: An image may have an embedded ICC profile as described
by the [International Color Consortium][iccspec]. by the [International Color Consortium][iccspec].
* **Animation.** An image may have multiple frames with pauses between them, * Animation: An image may have multiple frames with pauses between them,
making it an animation. making it an animation.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
@ -52,15 +53,15 @@ document are to be interpreted as described in BCP 14 [RFC 2119][] [RFC 8174][]
when, and only when, they appear in all capitals, as shown here. when, and only when, they appear in all capitals, as shown here.
Bit numbering in chunk diagrams starts at `0` for the most significant bit Bit numbering in chunk diagrams starts at `0` for the most significant bit
('MSB 0') as described in [RFC 1166][]. ('MSB 0'), as described in [RFC 1166][].
Terminology & Basics Terminology & Basics
-------------------- --------------------
A WebP file contains either a still image (that is, an encoded matrix of pixels) A WebP file contains either a still image (that is, an encoded matrix of pixels)
or an [animation](#animation). Optionally, it can also contain transparency or an [animation](#animation). Optionally, it can also contain transparency
information, color profile and metadata. We refer to the matrix of pixels as the information, a color profile and metadata. We refer to the matrix of pixels as
_canvas_ of the image. the _canvas_ of the image.
Below are additional terms used throughout this document: Below are additional terms used throughout this document:
@ -83,7 +84,7 @@ _uint32_
_FourCC_ _FourCC_
: A _FourCC_ (four-character code) is a _uint32_ created by concatenating four : A four-character code (FourCC) is a _uint32_ created by concatenating four
ASCII characters in little-endian order. This means 'aaaa' (0x61616161) and ASCII characters in little-endian order. This means 'aaaa' (0x61616161) and
'AAAA' (0x41414141) are treated as different _FourCCs_. 'AAAA' (0x41414141) are treated as different _FourCCs_.
@ -94,9 +95,8 @@ _1-based_
_ChunkHeader('ABCD')_ _ChunkHeader('ABCD')_
: This is used to describe the _FourCC_ and _Chunk Size_ header of individual : Used to describe the _FourCC_ and _Chunk Size_ header of individual chunks,
chunks, where 'ABCD' is the FourCC for the chunk. This element's size is 8 where 'ABCD' is the FourCC for the chunk. This element's size is 8 bytes.
bytes.
RIFF File Format RIFF File Format
@ -124,11 +124,11 @@ Chunk FourCC: 32 bits
Chunk Size: 32 bits (_uint32_) Chunk Size: 32 bits (_uint32_)
: The size of the chunk in bytes, not including this field, the chunk : The size of the chunk in bytes, not including this field, the chunk
identifier or padding. identifier, or padding.
Chunk Payload: _Chunk Size_ bytes Chunk Payload: _Chunk Size_ bytes
: The data payload. If _Chunk Size_ is odd, a single padding byte -- that MUST : The data payload. If _Chunk Size_ is odd, a single padding byte -- which MUST
be `0` to conform with RIFF -- is added. be `0` to conform with RIFF -- is added.
**Note:** RIFF has a convention that all-uppercase chunk FourCCs are standard **Note:** RIFF has a convention that all-uppercase chunk FourCCs are standard
@ -151,17 +151,17 @@ WebP File Header
'RIFF': 32 bits 'RIFF': 32 bits
: The ASCII characters 'R' 'I' 'F' 'F'. : The ASCII characters 'R', 'I', 'F', 'F'.
File Size: 32 bits (_uint32_) File Size: 32 bits (_uint32_)
: The size of the file in bytes starting at offset 8. The maximum value of : The size of the file in bytes, starting at offset 8. The maximum value of
this field is 2^32 minus 10 bytes and thus the size of the whole file is at this field is 2^32 minus 10 bytes and thus the size of the whole file is at
most 4GiB minus 2 bytes. most 4 GiB minus 2 bytes.
'WEBP': 32 bits 'WEBP': 32 bits
: The ASCII characters 'W' 'E' 'B' 'P'. : The ASCII characters 'W', 'E', 'B', 'P'.
A WebP file MUST begin with a RIFF header with the FourCC 'WEBP'. The file size A WebP file MUST begin with a RIFF header with the FourCC 'WEBP'. The file size
in the header is the total size of the chunks that follow plus `4` bytes for in the header is the total size of the chunks that follow plus `4` bytes for
@ -206,15 +206,15 @@ VP8 data: _Chunk Size_ bytes
: VP8 bitstream data. : VP8 bitstream data.
Note the fourth character in the 'VP8 ' FourCC is an ASCII space (0x20). Note that the fourth character in the 'VP8 ' FourCC is an ASCII space (0x20).
The VP8 bitstream format specification can be found at [VP8 Data Format and The VP8 bitstream format specification is described in [VP8 Data Format and
Decoding Guide][vp8spec]. Note that the VP8 frame header contains the VP8 frame Decoding Guide][rfc 6386]. Note that the VP8 frame header contains the VP8 frame
width and height. That is assumed to be the width and height of the canvas. width and height. That is assumed to be the width and height of the canvas.
The VP8 specification describes how to decode the image into Y'CbCr format. To The VP8 specification describes how to decode the image into Y'CbCr format. To
convert to RGB, Rec. 601 SHOULD be used. Applications MAY use another convert to RGB, [Recommendation BT.601][rec601] SHOULD be used. Applications MAY
conversion method, but visual results may differ among decoders. use another conversion method, but visual results may differ among decoders.
Simple File Format (Lossless) Simple File Format (Lossless)
@ -326,7 +326,7 @@ Reserved (Rsv): 2 bits
ICC profile (I): 1 bit ICC profile (I): 1 bit
: Set if the file contains an ICC profile. : Set if the file contains an 'ICCP' Chunk.
Alpha (L): 1 bit Alpha (L): 1 bit
@ -396,14 +396,14 @@ Background Color: 32 bits (_uint32_)
: The default background color of the canvas in \[Blue, Green, Red, Alpha\] : The default background color of the canvas in \[Blue, Green, Red, Alpha\]
byte order. This color MAY be used to fill the unused space on the canvas byte order. This color MAY be used to fill the unused space on the canvas
around the frames, as well as the transparent pixels of the first frame. around the frames, as well as the transparent pixels of the first frame.
Background color is also used when disposal method is `1`. The background color is also used when the Disposal method is `1`.
**Note**: **Note**:
* Background color MAY contain a non-opaque alpha value, even if the _Alpha_ * The background color MAY contain a non-opaque alpha value, even if the
flag in [VP8X chunk](#extended_header) is unset. _Alpha_ flag in the ['VP8X' Chunk](#extended_header) is unset.
* Viewer applications SHOULD treat the background color value as a hint, and * Viewer applications SHOULD treat the background color value as a hint and
are not required to use it. are not required to use it.
* The canvas is cleared at the start of each loop. The background color MAY be * The canvas is cleared at the start of each loop. The background color MAY be
@ -411,7 +411,8 @@ Background Color: 32 bits (_uint32_)
Loop Count: 16 bits (_uint16_) Loop Count: 16 bits (_uint16_)
: The number of times to loop the animation. `0` means infinitely. : The number of times to loop the animation. If it is `0`, this means
infinitely.
This chunk MUST appear if the _Animation_ flag in the VP8X chunk is set. This chunk MUST appear if the _Animation_ flag in the VP8X chunk is set.
If the _Animation_ flag is not set and this chunk is present, it MUST be If the _Animation_ flag is not set and this chunk is present, it MUST be
@ -459,10 +460,10 @@ Frame Height Minus One: 24 bits (_uint24_)
Frame Duration: 24 bits (_uint24_) Frame Duration: 24 bits (_uint24_)
: The time to wait before displaying the next frame, in 1 millisecond units. : The time to wait before displaying the next frame, in 1-millisecond units.
Note the interpretation of frame duration of 0 (and often <= 10) is Note that the interpretation of the Frame Duration of 0 (and often <= 10) is
implementation defined. Many tools and browsers assign a minimum duration defined by the implementation. Many tools and browsers assign a minimum
similar to GIF. duration similar to GIF.
Reserved: 6 bits Reserved: 6 bits
@ -473,10 +474,10 @@ Blending method (B): 1 bit
: Indicates how transparent pixels of _the current frame_ are to be blended : Indicates how transparent pixels of _the current frame_ are to be blended
with corresponding pixels of the previous canvas: with corresponding pixels of the previous canvas:
* `0`: Use alpha blending. After disposing of the previous frame, render the * `0`: Use alpha-blending. After disposing of the previous frame, render the
current frame on the canvas using [alpha-blending](#alpha-blending). If current frame on the canvas using [alpha-blending](#alpha-blending). If
the current frame does not have an alpha channel, assume alpha value of the current frame does not have an alpha channel, assume the alpha value
255, effectively replacing the rectangle. is 255, effectively replacing the rectangle.
* `1`: Do not blend. After disposing of the previous frame, render the * `1`: Do not blend. After disposing of the previous frame, render the
current frame on the canvas by overwriting the rectangle covered by the current frame on the canvas by overwriting the rectangle covered by the
@ -489,20 +490,20 @@ Disposal method (D): 1 bit
* `0`: Do not dispose. Leave the canvas as is. * `0`: Do not dispose. Leave the canvas as is.
* `1`: Dispose to background color. Fill the _rectangle_ on the canvas * `1`: Dispose to the background color. Fill the _rectangle_ on the canvas
covered by the _current frame_ with background color specified in the covered by the _current frame_ with the background color specified in the
[ANIM chunk](#anim_chunk). ['ANIM' Chunk](#anim_chunk).
**Notes**: **Notes**:
* The frame disposal only applies to the _frame rectangle_, that is, the * The frame disposal only applies to the _frame rectangle_, that is, the
rectangle defined by _Frame X_, _Frame Y_, _frame width_ and _frame height_. rectangle defined by _Frame X_, _Frame Y_, _frame width_, and _frame
It may or may not cover the whole canvas. height_. It may or may not cover the whole canvas.
{:#alpha-blending} {:#alpha-blending}
* **Alpha-blending**: * Alpha-blending:
Given that each of the R, G, B and A channels is 8-bit, and the RGB Given that each of the R, G, B, and A channels is 8 bits, and the RGB
channels are _not premultiplied_ by alpha, the formula for blending channels are _not premultiplied_ by alpha, the formula for blending
'dst' onto 'src' is: 'dst' onto 'src' is:
@ -518,8 +519,8 @@ Disposal method (D): 1 bit
* Alpha-blending SHOULD be done in linear color space, by taking into account * Alpha-blending SHOULD be done in linear color space, by taking into account
the [color profile](#color-profile) of the image. If the color profile is the [color profile](#color-profile) of the image. If the color profile is
not present, sRGB is to be assumed. (Note that sRGB also needs to be not present, standard RGB (sRGB) is to be assumed. (Note that sRGB also
linearized due to a gamma of ~2.2). needs to be linearized due to a gamma of ~2.2.)
Frame Data: _Chunk Size_ - `16` bytes Frame Data: _Chunk Size_ - `16` bytes
@ -532,7 +533,7 @@ Frame Data: _Chunk Size_ - `16` bytes
* An optional list of [unknown chunks](#unknown-chunks). * An optional list of [unknown chunks](#unknown-chunks).
**Note**: The 'ANMF' payload, _Frame Data_ above, consists of individual **Note**: The 'ANMF' payload, _Frame Data_ above, consists of individual
_padded_ chunks as described by the [RIFF file format](#riff-file-format). _padded_ chunks, as described by the [RIFF file format](#riff-file-format).
#### Alpha #### Alpha
@ -549,18 +550,18 @@ Reserved (Rsv): 2 bits
: MUST be `0`. Readers MUST ignore this field. : MUST be `0`. Readers MUST ignore this field.
Pre-processing (P): 2 bits Preprocessing (P): 2 bits
: These _informative_ bits are used to signal the pre-processing that has : These _informative_ bits are used to signal the preprocessing that has
been performed during compression. The decoder can use this information to been performed during compression. The decoder can use this information to
for example, dither the values or smooth the gradients prior to display. for example, dither the values or smooth the gradients prior to display.
* `0`: No pre-processing. * `0`: No preprocessing.
* `1`: Level reduction. * `1`: Level reduction.
Filtering method (F): 2 bits Filtering method (F): 2 bits
: The filtering method used: : The filtering methods used are described as follows:
* `0`: None. * `0`: None.
* `1`: Horizontal filter. * `1`: Horizontal filter.
@ -584,8 +585,8 @@ made depending on the filtering method:
where `clip(v)` is equal to: where `clip(v)` is equal to:
* 0 if v < 0 * 0 if v < 0,
* 255 if v > 255 * 255 if v > 255, or
* v otherwise * v otherwise
The final value is derived by adding the decompressed value `X` to the The final value is derived by adding the decompressed value `X` to the
@ -594,9 +595,10 @@ into the \[0..255\] one:
`alpha = (predictor + X) % 256` `alpha = (predictor + X) % 256`
There are special cases for the left-most and top-most pixel positions: There are special cases for the left-most and top-most pixel positions. For
example, the top-left value at location (0, 0) uses 0 as the predictor value.
Otherwise:
* The top-left value at location (0, 0) uses 0 as predictor value. Otherwise,
* For horizontal or gradient filtering methods, the left-most pixels at * For horizontal or gradient filtering methods, the left-most pixels at
location (0, y) are predicted using the location (0, y-1) just above. location (0, y) are predicted using the location (0, y-1) just above.
* For vertical or gradient filtering methods, the top-most pixels at * For vertical or gradient filtering methods, the top-most pixels at
@ -622,27 +624,27 @@ containing a 'VP8L' chunk SHOULD NOT contain this chunk.
**Rationale**: The transparency information is already part of the 'VP8L' **Rationale**: The transparency information is already part of the 'VP8L'
chunk. chunk.
The alpha channel data is stored as uncompressed raw data (when The alpha channel data is stored as uncompressed raw data (when the
compression method is '0') or compressed using the lossless format compression method is '0') or compressed using the lossless format
(when the compression method is '1'). (when the compression method is '1').
* Raw data: consists of a byte sequence of length width * height, * Raw data: This consists of a byte sequence of length = width * height,
containing all the 8-bit transparency values in scan order. containing all the 8-bit transparency values in scan order.
* Lossless format compression: the byte sequence is a compressed * Lossless format compression: The byte sequence is a compressed
image-stream (as described in the [WebP Lossless Bitstream Format] image-stream (as described in ["WebP Lossless Bitstream Format"]
[webpllspec]) of implicit dimension width x height. That is, this [webpllspec]) of implicit dimensions width x height. That is, this
image-stream does NOT contain any headers describing the image dimension. image-stream does NOT contain any headers describing the image dimensions.
**Rationale**: the dimension is already known from other sources, **Rationale**: The dimensions are already known from other sources,
so storing it again would be redundant and error-prone. so storing them again would be redundant and prone to error.
Once the image-stream is decoded into ARGB color values, following Once the image-stream is decoded into Alpha, Red, Green, Blue (ARGB) color
the process described in the lossless format specification, the values, following the process described in the lossless format
transparency information must be extracted from the *green* channel specification, the transparency information must be extracted from the
of the ARGB quadruplet. *green* channel of the ARGB quadruplet.
**Rationale**: the green channel is allowed extra transformation **Rationale**: The green channel is allowed extra transformation
steps in the specification -- unlike the other channels -- that can steps in the specification -- unlike the other channels -- that can
improve compression. improve compression.
@ -650,13 +652,13 @@ compression method is '0') or compressed using the lossless format
This chunk contains compressed bitstream data for a single frame. This chunk contains compressed bitstream data for a single frame.
A bitstream chunk may be either (i) a VP8 chunk, using "VP8 " (note the A bitstream chunk may be either (i) a 'VP8 ' Chunk, using 'VP8 ' (note the
significant fourth-character space) as its tag _or_ (ii) a VP8L chunk, using significant fourth-character space) as its FourCC, _or_ (ii) a 'VP8L' Chunk,
"VP8L" as its tag. using 'VP8L' as its FourCC.
The formats of VP8 and VP8L chunks are as described in sections The formats of 'VP8 ' and 'VP8L' Chunks are as described in sections
[Simple File Format (Lossy)](#simple-file-format-lossy) [Simple File Format (Lossy)](#simple-file-format-lossy)
and [Simple File Format (Lossless)](#simple-file-format-lossless) respectively. and [Simple File Format (Lossless)](#simple-file-format-lossless), respectively.
#### Color Profile #### Color Profile
@ -720,59 +722,59 @@ XMP Metadata: _Chunk Size_ bytes
: Image metadata in XMP format. : Image metadata in XMP format.
Note the fourth character in the 'XMP ' FourCC is an ASCII space (0x20). Note that the fourth character in the 'XMP ' FourCC is an ASCII space (0x20).
Additional guidance about handling metadata can be found in the Additional guidance about handling metadata can be found in the
Metadata Working Group's [Guidelines for Handling Metadata][metadata]. Metadata Working Group's ["Guidelines for Handling Metadata"][metadata].
#### Unknown Chunks #### Unknown Chunks
A RIFF chunk (described in [this](#terminology-amp-basics) section) whose _chunk A RIFF chunk (described in the [RIFF File Format](#riff-file-format) section)
tag_ is different from any of the chunks described in this document, is whose FourCC is different from any of the chunks described in this document, is
considered an _unknown chunk_. considered an _unknown chunk_.
**Rationale**: Allowing unknown chunks gives a provision for future extension **Rationale**: Allowing unknown chunks gives a provision for future extension
of the format, and also allows storage of any application-specific data. of the format and also allows storage of any application-specific data.
A file MAY contain unknown chunks: A file MAY contain unknown chunks:
* At the end of the file as described in [Extended WebP file * at the end of the file, as described in [Extended WebP file
header](#extended_header) section. header](#extended_header) section, or
* At the end of ANMF chunks as described in the * at the end of 'ANMF' Chunks, as described in the
[Animation](#animation) section. [Animation](#animation) section.
Readers SHOULD ignore these chunks. Writers SHOULD preserve them in their Readers SHOULD ignore these chunks. Writers SHOULD preserve them in their
original order (unless they specifically intend to modify these chunks). original order (unless they specifically intend to modify these chunks).
### Assembling the Canvas From Frames ### Canvas Assembly from Frames
Here we provide an overview of how a reader MUST assemble a canvas in the case Here we provide an overview of how a reader MUST assemble a canvas in the case
of an animated image. of an animated image.
The process begins with creating a canvas using the dimensions given in the The process begins with creating a canvas using the dimensions given in the
'VP8X' chunk, `Canvas Width Minus One + 1` pixels wide by `Canvas Height Minus 'VP8X' Chunk, `Canvas Width Minus One + 1` pixels wide by `Canvas Height Minus
One + 1` pixels high. The `Loop Count` field from the 'ANIM' chunk controls how One + 1` pixels high. The `Loop Count` field from the 'ANIM' Chunk controls how
many times the animation process is repeated. This is `Loop Count - 1` for many times the animation process is repeated. This is `Loop Count - 1` for
non-zero `Loop Count` values or infinitely if `Loop Count` is zero. nonzero `Loop Count` values or infinite if the `Loop Count` is zero.
At the beginning of each loop iteration the canvas is filled using the At the beginning of each loop iteration, the canvas is filled using the
background color from the 'ANIM' chunk or an application defined color. background color from the 'ANIM' Chunk or an application-defined color.
'ANMF' chunks contain individual frames given in display order. Before rendering 'ANMF' Chunks contain individual frames given in display order. Before rendering
each frame, the previous frame's `Disposal method` is applied. each frame, the previous frame's `Disposal method` is applied.
The rendering of the decoded frame begins at the Cartesian coordinates (`2 * The rendering of the decoded frame begins at the Cartesian coordinates (`2 *
Frame X`, `2 * Frame Y`) using the top-left corner of the canvas as the origin. Frame X`, `2 * Frame Y`), using the top-left corner of the canvas as the origin.
`Frame Width Minus One + 1` pixels wide by `Frame Height Minus One + 1` pixels `Frame Width Minus One + 1` pixels wide by `Frame Height Minus One + 1` pixels
high are rendered onto the canvas using the `Blending method`. high are rendered onto the canvas using the `Blending method`.
The canvas is displayed for `Frame Duration` milliseconds. This continues until The canvas is displayed for `Frame Duration` milliseconds. This continues until
all frames given by 'ANMF' chunks have been displayed. A new loop iteration is all frames given by 'ANMF' Chunks have been displayed. A new loop iteration is
then begun or the canvas is left in its final state if all iterations have been then begun, or the canvas is left in its final state if all iterations have been
completed. completed.
The following pseudocode illustrates the rendering process. The notation The following pseudocode illustrates the rendering process. The notation
_VP8X.field_ means the field in the 'VP8X' chunk with the same description. _VP8X.field_ means the field in the 'VP8X' Chunk with the same description.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
assert VP8X.flags.hasAnimation assert VP8X.flags.hasAnimation
@ -785,7 +787,7 @@ if loop_count == 0:
frame_params ← nil frame_params ← nil
assert next chunk in image_data is ANMF assert next chunk in image_data is ANMF
for loop = 0..loop_count - 1 for loop = 0..loop_count - 1
clear canvas to ANIM.background_color or application defined color clear canvas to ANIM.background_color or application-defined color
until eof or non-ANMF chunk until eof or non-ANMF chunk
frame_params.frameX = Frame X frame_params.frameX = Frame X
frame_params.frameY = Frame Y frame_params.frameY = Frame Y
@ -805,11 +807,11 @@ for loop = 0..loop_count - 1
frame_params.bitstream = bitstream_data frame_params.bitstream = bitstream_data
render frame with frame_params.alpha and frame_params.bitstream render frame with frame_params.alpha and frame_params.bitstream
on canvas with top-left corner at (frame_params.frameX, on canvas with top-left corner at (frame_params.frameX,
frame_params.frameY), using blending method frame_params.frameY), using Blending method
frame_params.blendingMethod. frame_params.blendingMethod.
canvas contains the decoded image. canvas contains the decoded image.
Show the contents of the canvas for Show the contents of the canvas for
frame_params.frameDuration * 1ms. frame_params.frameDuration * 1 ms.
dispose_method = frame_params.disposeMethod dispose_method = frame_params.disposeMethod
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -817,7 +819,7 @@ for loop = 0..loop_count - 1
Example File Layouts Example File Layouts
-------------------- --------------------
A lossy encoded image with alpha may look as follows: A lossy-encoded image with alpha may look as follows:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RIFF/WEBP RIFF/WEBP
@ -826,7 +828,7 @@ RIFF/WEBP
+- VP8 (bitstream) +- VP8 (bitstream)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A losslessly encoded image may look as follows: A lossless-encoded image may look as follows:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RIFF/WEBP RIFF/WEBP
@ -835,7 +837,7 @@ RIFF/WEBP
+- VP8L (lossless bitstream) +- VP8L (lossless bitstream)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A lossless image with ICC profile and XMP metadata may A lossless image with an ICC profile and XMP metadata may
look as follows: look as follows:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -859,10 +861,11 @@ RIFF/WEBP
+- EXIF (metadata) +- EXIF (metadata)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[vp8spec]: https://datatracker.ietf.org/doc/html/rfc6386
[webpllspec]: https://chromium.googlesource.com/webm/libwebp/+/HEAD/doc/webp-lossless-bitstream-spec.txt [webpllspec]: https://chromium.googlesource.com/webm/libwebp/+/HEAD/doc/webp-lossless-bitstream-spec.txt
[iccspec]: https://www.color.org/icc_specs2.xalter [iccspec]: https://www.color.org/icc_specs2.xalter
[metadata]: https://web.archive.org/web/20180919181934/http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf [metadata]: https://web.archive.org/web/20180919181934/http://www.metadataworkinggroup.org/pdf/mwg_guidance.pdf
[rec601]: https://www.itu.int/rec/R-REC-BT.601
[rfc 1166]: https://datatracker.ietf.org/doc/html/rfc1166 [rfc 1166]: https://datatracker.ietf.org/doc/html/rfc1166
[rfc 2119]: https://datatracker.ietf.org/doc/html/rfc2119 [rfc 2119]: https://datatracker.ietf.org/doc/html/rfc2119
[rfc 6386]: https://datatracker.ietf.org/doc/html/rfc6386
[rfc 8174]: https://datatracker.ietf.org/doc/html/rfc8174 [rfc 8174]: https://datatracker.ietf.org/doc/html/rfc8174