Re: [PATCH/RFC 06/16] media: doc: pixfmt-rgb: Clarify naming scheme for RGB formats

[Nicolas Dufresne <nicolas@ndufresne.ca>]

Le mercredi 23 septembre 2020 à 05:43 +0300, Laurent Pinchart a écrit :
> The naming scheme for the RGB pixel formats has been developed
> organically, and isn't consistent between formats stored in 1 or 2
> bytes, and formats stored in 3 or 4 bytes. For the latter category, the
> names use a components order convention that is the opposite of the
> first category, and the opposite of DRM pixel formats. This has lead to
> lots of confusion in the past, and would really benefit from being
> explained more precisely. Do so, which also prepares for the addition of
> additional RGB pixels formats.

This makes it looks like if V4L2 byte order naming is special, while in
fact only a subset of DRM format uses the opposite order and outside of
the Linux kernel, DRM is the special case. I agree there is this RGB565
name that is indeed opposite outside Linux too.

I would rephrase this, otherwise the following changes looks good to
me.

> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---
>  .../userspace-api/media/v4l/pixfmt-rgb.rst    | 196 ++++++++++++------
>  include/uapi/linux/videodev2.h                |   4 +-
>  2 files changed, 141 insertions(+), 59 deletions(-)
> 
> diff --git a/Documentation/userspace-api/media/v4l/pixfmt-rgb.rst b/Documentation/userspace-api/media/v4l/pixfmt-rgb.rst
> index 3208ec94db4c..32bfd68af425 100644
> --- a/Documentation/userspace-api/media/v4l/pixfmt-rgb.rst
> +++ b/Documentation/userspace-api/media/v4l/pixfmt-rgb.rst
> @@ -13,13 +13,63 @@
>  RGB Formats
>  ***********
>  
> -Description
> -===========
> +These formats encode each pixel as a triplet of RGB values. They are packed
> +formats, meaning that the RGB values for one pixel are stored consecutively in
> +memory. Multiple pixels are however not packed in the same byte, each pixel
> +consumes an integer number of bytes. When the number of bits required to store
> +a pixel is not aligned to a byte boundary, the data is padded with additional
> +bits to fill the remaining byte.
>  
> -These formats are designed to match the pixel formats of typical PC
> -graphics frame buffers. They occupy 8, 16, 24 or 32 bits per pixel.
> -These are all packed-pixel formats, meaning all the data for a pixel lie
> -next to each other in memory.
> +The formats differ by the number of bits per RGB component (typically but not
> +always the same for all components), the order of components in memory, and the
> +presence of an alpha component or an additional padding byte.
> +
> +The usage and value of the alpha bits in formats that support them (named ARGB
> +or a permutation thereof, collectively referred to as alpha formats) depend on
> +the device type and hardware operation. :ref:`Capture <capture>` devices
> +(including capture queues of mem-to-mem devices) fill the alpha component in
> +memory. When the device captures an alpha channel the alpha component will have
> +a meaningful value. Otherwise, when the device doesn't capture an alpha channel
> +but can set the alpha bit to a user-configurable value, the
> +:ref:`V4L2_CID_ALPHA_COMPONENT <v4l2-alpha-component>` control is used to
> +specify that alpha value, and the alpha component of all pixels will be set to
> +the value specified by that control. Otherwise a corresponding format without
> +an alpha component (XRGB or XBGR) must be used instead of an alpha format.
> +
> +:ref:`Output <output>` devices (including output queues of mem-to-mem devices
> +and :ref:`video output overlay <osd>` devices) read the alpha component from
> +memory. When the device processes the alpha channel the alpha component must be
> +filled with meaningful values by applications. Otherwise a corresponding format
> +without an alpha component (XRGB or XBGR) must be used instead of an alpha
> +format.
> +
> +Formats that contain padding bits are named XRGB (or a permutation thereof).
> +The padding bits contain undefined values and must be ignored by applications,
> +devices and drivers, for both :ref:`capture` and :ref:`output` devices.
> +
> +.. note::
> +
> +   - In all the tables that follow, bit 7 is the most significant bit in a byte.
> +   - 'r', 'g' and 'b' denote bits of the red, green and blue components
> +     respectively. 'a' denotes bits of the the alpha component (if supported by
> +     the format), and '-' denotes padding bits.
> +
> +
> +8 or 16 Bits Per Pixel
> +======================
> +
> +These formats store an RGB triplet in one or two bytes. They are named based on
> +the order of the RGB components as seen in a 8- or 16-bit word, which is then
> +stored in memory in little endian byte order (unless otherwise noted by the
> +presence of bit 31 in the 4CC value), and on the number of bits for each
> +component. For instance, the RGB565 format stores a pixel in a 16-bit word
> +[15:0] laid out at as [R\ :sub:`4` R\ :sub:`3` R\ :sub:`2` R\ :sub:`1`
> +R\ :sub:`0` G\ :sub:`5` G\ :sub:`4` G\ :sub:`3` G\ :sub:`2` G\ :sub:`1`
> +G\ :sub:`0` B\ :sub:`4` B\ :sub:`3` B\ :sub:`2` B\ :sub:`1` B\ :sub:`0`], and
> +stored in memory in two bytes, [R\ :sub:`4` R\ :sub:`3` R\ :sub:`2` R\ :sub:`1`
> +R\ :sub:`0` G\ :sub:`5` G\ :sub:`4` G\ :sub:`3`] followed by [G\ :sub:`2`
> +G\ :sub:`1` G\ :sub:`0` B\ :sub:`4` B\ :sub:`3` B\ :sub:`2` B\ :sub:`1`
> +B\ :sub:`0`].
>  
>  .. raw:: latex
>  
> @@ -27,10 +77,10 @@ next to each other in memory.
>      \tiny
>      \setlength{\tabcolsep}{2pt}
>  
> -.. tabularcolumns:: |p{2.8cm}|p{2.0cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|
> +.. tabularcolumns:: |p{2.8cm}|p{2.0cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|
>  
>  
> -.. flat-table:: RGB Image Formats
> +.. flat-table:: RGB Formats With 8 or 16 Bits Per Pixel
>      :header-rows:  2
>      :stub-columns: 0
>  
> @@ -38,8 +88,6 @@ next to each other in memory.
>        - Code
>        - :cspan:`7` Byte 0 in memory
>        - :cspan:`7` Byte 1
> -      - :cspan:`7` Byte 2
> -      - :cspan:`7` Byte 3
>      * -
>        -
>        - 7
> @@ -59,24 +107,6 @@ next to each other in memory.
>        - 2
>        - 1
>        - 0
> -
> -      - 7
> -      - 6
> -      - 5
> -      - 4
> -      - 3
> -      - 2
> -      - 1
> -      - 0
> -
> -      - 7
> -      - 6
> -      - 5
> -      - 4
> -      - 3
> -      - 2
> -      - 1
> -      - 0
>      * .. _V4L2-PIX-FMT-RGB332:
>  
>        - ``V4L2_PIX_FMT_RGB332``
> @@ -551,6 +581,82 @@ next to each other in memory.
>        - b\ :sub:`1`
>        - b\ :sub:`0`
>        -
> +
> +.. raw:: latex
> +
> +    \endgroup
> +
> +
> +24 or 32 Bits Per Pixel
> +=======================
> +
> +These formats store an RGB triplet in three or four bytes. They are named based
> +on the order of the RGB components as stored in memory, and on the total number
> +of bits per pixel (with an exception for the BGR666 format). For instance,
> +RGB24 format stores a pixel with [R\ :sub:`7` R\ :sub:`6` R\ :sub:`5`
> +R\ :sub:`4` R\ :sub:`3` R\ :sub:`2` R\ :sub:`1` R\ :sub:`0`] in the first byte,
> +[G\ :sub:`7` G\ :sub:`6` G\ :sub:`5` G\ :sub:`4` G\ :sub:`3` G\ :sub:`2`
> +G\ :sub:`1` G\ :sub:`0`] in the second byte and [B\ :sub:`7` B\ :sub:`6`
> +B\ :sub:`5` B\ :sub:`4` B\ :sub:`3` B\ :sub:`2` B\ :sub:`1` B\ :sub:`0`] in the
> +third byte. This differs from the DRM format nomenclature that instead use the
> +order of components as seen in a 24- or 32-bit little endian word.
> +
> +.. raw:: latex
> +
> +    \begingroup
> +    \tiny
> +    \setlength{\tabcolsep}{2pt}
> +
> +.. tabularcolumns:: |p{2.8cm}|p{2.0cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|p{0.22cm}|
> +
> +
> +.. flat-table:: RGB Formats With 24 or 32 Bits Per Pixel
> +    :header-rows:  2
> +    :stub-columns: 0
> +
> +    * - Identifier
> +      - Code
> +      - :cspan:`7` Byte 0 in memory
> +      - :cspan:`7` Byte 1
> +      - :cspan:`7` Byte 2
> +      - :cspan:`7` Byte 3
> +    * -
> +      -
> +      - 7
> +      - 6
> +      - 5
> +      - 4
> +      - 3
> +      - 2
> +      - 1
> +      - 0
> +
> +      - 7
> +      - 6
> +      - 5
> +      - 4
> +      - 3
> +      - 2
> +      - 1
> +      - 0
> +
> +      - 7
> +      - 6
> +      - 5
> +      - 4
> +      - 3
> +      - 2
> +      - 1
> +      - 0
> +
> +      - 7
> +      - 6
> +      - 5
> +      - 4
> +      - 3
> +      - 2
> +      - 1
> +      - 0
>      * .. _V4L2-PIX-FMT-BGR24:
>  
>        - ``V4L2_PIX_FMT_BGR24``
> @@ -980,40 +1086,14 @@ next to each other in memory.
>  
>      \endgroup
>  
> -.. note:: Bit 7 is the most significant bit.
> -
> -The usage and value of the alpha bits (a) in the ARGB and ABGR formats
> -(collectively referred to as alpha formats) depend on the device type
> -and hardware operation. :ref:`Capture <capture>` devices (including
> -capture queues of mem-to-mem devices) fill the alpha component in
> -memory. When the device outputs an alpha channel the alpha component
> -will have a meaningful value. Otherwise, when the device doesn't output
> -an alpha channel but can set the alpha bit to a user-configurable value,
> -the :ref:`V4L2_CID_ALPHA_COMPONENT <v4l2-alpha-component>` control
> -is used to specify that alpha value, and the alpha component of all
> -pixels will be set to the value specified by that control. Otherwise a
> -corresponding format without an alpha component (XRGB or XBGR) must be
> -used instead of an alpha format.
> -
> -:ref:`Output <output>` devices (including output queues of mem-to-mem
> -devices and :ref:`video output overlay <osd>` devices) read the alpha
> -component from memory. When the device processes the alpha channel the
> -alpha component must be filled with meaningful values by applications.
> -Otherwise a corresponding format without an alpha component (XRGB or
> -XBGR) must be used instead of an alpha format.
> -
> -The XRGB and XBGR formats contain undefined bits (-). Applications,
> -devices and drivers must ignore those bits, for both
> -:ref:`capture` and :ref:`output` devices.
> -
>  
>  Deprecated RGB Formats
>  ======================
>  
> -Formats defined in :ref:`pixfmt-rgb-deprecated` are deprecated and
> -must not be used by new drivers. They are documented here for reference.
> -The meaning of their alpha bits ``(a)`` are ill-defined and interpreted as in
> -either the corresponding ARGB or XRGB format, depending on the driver.
> +Formats defined in :ref:`pixfmt-rgb-deprecated` are deprecated and must not be
> +used by new drivers. They are documented here for reference. The meaning of
> +their alpha bits ``(a)`` is ill-defined and they are interpreted as in either
> +the corresponding ARGB or XRGB format, depending on the driver.
>  
>  .. raw:: latex
>  
> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> index f4b66bfb859f..3f5f1cf8d1c0 100644
> --- a/include/uapi/linux/videodev2.h
> +++ b/include/uapi/linux/videodev2.h
> @@ -520,7 +520,7 @@ struct v4l2_pix_format {
>  
>  /*      Pixel format         FOURCC                          depth  Description  */
>  
> -/* RGB formats */
> +/* RGB formats (1 or 2 bytes per pixel) */
>  #define V4L2_PIX_FMT_RGB332  v4l2_fourcc('R', 'G', 'B', '1') /*  8  RGB-3-3-2     */
>  #define V4L2_PIX_FMT_RGB444  v4l2_fourcc('R', '4', '4', '4') /* 16  xxxxrrrr ggggbbbb */
>  #define V4L2_PIX_FMT_ARGB444 v4l2_fourcc('A', 'R', '1', '2') /* 16  aaaarrrr ggggbbbb */
> @@ -545,6 +545,8 @@ struct v4l2_pix_format {
>  #define V4L2_PIX_FMT_ARGB555X v4l2_fourcc_be('A', 'R', '1', '5') /* 16  ARGB-5-5-5 BE */
>  #define V4L2_PIX_FMT_XRGB555X v4l2_fourcc_be('X', 'R', '1', '5') /* 16  XRGB-5-5-5 BE */
>  #define V4L2_PIX_FMT_RGB565X v4l2_fourcc('R', 'G', 'B', 'R') /* 16  RGB-5-6-5 BE  */
> +
> +/* RGB formats (3 or 4 bytes per pixel) */
>  #define V4L2_PIX_FMT_BGR666  v4l2_fourcc('B', 'G', 'R', 'H') /* 18  BGR-6-6-6	  */
>  #define V4L2_PIX_FMT_BGR24   v4l2_fourcc('B', 'G', 'R', '3') /* 24  BGR-8-8-8     */
>  #define V4L2_PIX_FMT_RGB24   v4l2_fourcc('R', 'G', 'B', '3') /* 24  RGB-8-8-8     */