[v4,09/16] media: doc: pixfmt-yuv: Document subsampling in more details

Message ID	20201206230316.1221-10-laurent.pinchart@ideasonboard.com (mailing list archive)
State	New, archived
Headers	show Return-Path: <linux-media-owner@kernel.org> From: Laurent Pinchart <laurent.pinchart@ideasonboard.com> To: linux-media@vger.kernel.org Cc: Sakari Ailus <sakari.ailus@linux.intel.com>, Hans Verkuil <hverkuil-cisco@xs4all.nl>, Dylan Yip <dylany@xilinx.com>, Vishal Sagar <vsagar@xilinx.com>, Nicolas Dufresne <nicolas@ndufresne.ca> Subject: [PATCH v4 09/16] media: doc: pixfmt-yuv: Document subsampling in more details Date: Mon, 7 Dec 2020 01:03:09 +0200 Message-Id: <20201206230316.1221-10-laurent.pinchart@ideasonboard.com> In-Reply-To: <20201206230316.1221-1-laurent.pinchart@ideasonboard.com> References: <20201206230316.1221-1-laurent.pinchart@ideasonboard.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk
Series	media: Rework RGB and YUV pixel formats documentation \| expand [v4,00/16] media: Rework RGB and YUV pixel formats documentation [v4,01/16] media: videodev2.h: Remove unneeded comment about 4CC value [v4,02/16] media: videodev2.h: Move HI240 format to vendor-specific section [v4,03/16] media: videodev2.h: Move HM12 format to YUV semi-planar section [v4,04/16] media: doc: pixfmt-rgb: Remove layout table for packed RGB formats [v4,05/16] media: doc: pixfmt-rgb: Add title for deprecated formats [v4,06/16] media: doc: pixfmt-rgb: Clarify naming scheme for RGB formats [v4,07/16] media: doc: pixfmt-rgb: Make 8 bits per component table more compact [v4,08/16] media: doc: pixfmt-rgb: Replace '-' with 'X' to denote padding [v4,09/16] media: doc: pixfmt-yuv: Document subsampling in more details [v4,10/16] media: doc: pixfmt-yuv: Move all packed YUV formats to common file [v4,11/16] media: doc: pixfmt-packed-yuv: Fill padding bits with 'X' [v4,12/16] media: doc: pixfmt-packed-yuv: Express 4:4:4 formats in a more compact way [v4,13/16] media: doc: pixfmt-packed-yuv: Clarify naming scheme for 4:4:4 formats [v4,14/16] media: doc: pixfmt-yuv: Move all luma-only YUV formats to common file [v4,15/16] media: doc: pixfmt-yuv: Move all semi-planar YUV formats to common file [v4,16/16] media: doc: pixfmt-yuv: Move all planar YUV formats to common file

diff --git a/Documentation/userspace-api/media/v4l/pixfmt-m420.rst b/Documentation/userspace-api/media/v4l/pixfmt-m420.rst index 13cf36a8cd5c..c01a949e7c11 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-m420.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-m420.rst @@ -67,60 +67,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 1 - - Y - - - - Y - - Y - - - - Y - * - - * - 2 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 3 - - Y - - - - Y - - Y - - - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally and vertically. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-nv12.rst b/Documentation/userspace-api/media/v4l/pixfmt-nv12.rst index dd2f38129fe6..692117bf83ad 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-nv12.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-nv12.rst @@ -72,58 +72,5 @@ Each cell is one byte. **Color Sample Location:** - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 1 - - Y - - - - Y - - Y - - - - Y - * - - * - 2 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 3 - - Y - - - - Y - - Y - - - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally and vertically. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-nv12m.rst b/Documentation/userspace-api/media/v4l/pixfmt-nv12m.rst index 250f8b977605..002b361d5a9b 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-nv12m.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-nv12m.rst @@ -84,61 +84,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 1 - - Y - - - - Y - - Y - - - - Y - * - - * - 2 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - - - C - - - * - 3 - - Y - - - - Y - - Y - - - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally and vertically. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-nv16.rst b/Documentation/userspace-api/media/v4l/pixfmt-nv16.rst index 22295fc0c359..e6307843d848 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-nv16.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-nv16.rst @@ -80,74 +80,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 1 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - - * - 2 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 3 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-nv16m.rst b/Documentation/userspace-api/media/v4l/pixfmt-nv16m.rst index 812bf2ccabf0..58e97205d41f 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-nv16m.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-nv16m.rst @@ -84,74 +84,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 1 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - - * - 2 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - - * - 3 - - Y - - - - Y - - Y - - - - Y - * - - - - - C - - - - - - C - - +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-uyvy.rst b/Documentation/userspace-api/media/v4l/pixfmt-uyvy.rst index bae975fb14f6..1abc3066b712 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-uyvy.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-uyvy.rst @@ -66,45 +66,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - C - - Y - - Y - - C - - Y - * - 1 - - Y - - C - - Y - - Y - - C - - Y - * - 2 - - Y - - C - - Y - - Y - - C - - Y - * - 3 - - Y - - C - - Y - - Y - - C - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-vyuy.rst b/Documentation/userspace-api/media/v4l/pixfmt-vyuy.rst index aff8588b67a9..4fd0630a83a4 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-vyuy.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-vyuy.rst @@ -66,43 +66,5 @@ Each cell is one byte. **Color Sample Location:** - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - - - 2 - - 3 - * - 0 - - Y - - C - - Y - - Y - - C - - Y - * - 1 - - Y - - C - - Y - - Y - - C - - Y - * - 2 - - Y - - C - - Y - - Y - - C - - Y - * - 3 - - Y - - C - - Y - - Y - - C - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-y41p.rst b/Documentation/userspace-api/media/v4l/pixfmt-y41p.rst index d14cedf8f317..bb83eb6db3e5 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-y41p.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-y41p.rst @@ -89,63 +89,5 @@ Each cell is one byte. **Color Sample Location:** - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - 1 - - - - 2 - - 3 - - 4 - - 5 - - - - 6 - - 7 - * - 0 - - Y - - Y - - C - - Y - - Y - - Y - - Y - - C - - Y - - Y - * - 1 - - Y - - Y - - C - - Y - - Y - - Y - - Y - - C - - Y - - Y - * - 2 - - Y - - Y - - C - - Y - - Y - - Y - - Y - - C - - Y - - Y - * - 3 - - Y - - Y - - C - - Y - - Y - - Y - - Y - - C - - Y - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuv410.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuv410.rst index de2e519adc60..d79cd46d5055 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yuv410.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuv410.rst @@ -68,60 +68,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - - - 2 - - - - 3 - * - 0 - - Y - - - - Y - - - - Y - - - - Y - * - - * - 1 - - Y - - - - Y - - - - Y - - - - Y - * - - - - - - - - - C - - - - - - - * - 2 - - Y - - - - Y - - - - Y - - - - Y - * - - * - 3 - - Y - - - - Y - - - - Y - - - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally and vertically. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuv411p.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuv411p.rst index 998aa9b1328f..8a70eeca8d83 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yuv411p.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuv411p.rst @@ -76,40 +76,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - 1 - - - - 2 - - 3 - * - 0 - - Y - - Y - - C - - Y - - Y - * - 1 - - Y - - Y - - C - - Y - - Y - * - 2 - - Y - - Y - - C - - Y - - Y - * - 3 - - Y - - Y - - C - - Y - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuv420.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuv420.rst index f1c7baf32685..a36ec8d6d8a4 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yuv420.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuv420.rst @@ -77,67 +77,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - - - 2 - - - - 3 - * - 0 - - Y - - - - Y - - - - Y - - - - Y - * - - - - - C - - - - - - - - C - - - * - 1 - - Y - - - - Y - - - - Y - - - - Y - * - - * - 2 - - Y - - - - Y - - - - Y - - - - Y - * - - - - - C - - - - - - - - C - - - * - 3 - - Y - - - - Y - - - - Y - - - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally and vertically. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuv420m.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuv420m.rst index cd20a57e0621..c2cf17b9743c 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yuv420m.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuv420m.rst @@ -86,67 +86,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - - - 2 - - - - 3 - * - 0 - - Y - - - - Y - - - - Y - - - - Y - * - - - - - C - - - - - - - - C - - - * - 1 - - Y - - - - Y - - - - Y - - - - Y - * - - * - 2 - - Y - - - - Y - - - - Y - - - - Y - * - - - - - C - - - - - - - - C - - - * - 3 - - Y - - - - Y - - - - Y - - - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally and vertically. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuv422m.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuv422m.rst index 32bf15e1426e..f92b54853fc9 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yuv422m.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuv422m.rst @@ -97,45 +97,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - C - - Y - - Y - - C - - Y - * - 1 - - Y - - C - - Y - - Y - - C - - Y - * - 2 - - Y - - C - - Y - - Y - - C - - Y - * - 3 - - Y - - C - - Y - - Y - - C - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuv422p.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuv422p.rst index b178be558361..33e6b923e325 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yuv422p.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuv422p.rst @@ -85,45 +85,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - C - - Y - - Y - - C - - Y - * - 1 - - Y - - C - - Y - - Y - - C - - Y - * - 2 - - Y - - C - - Y - - Y - - C - - Y - * - 3 - - Y - - C - - Y - - Y - - C - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuv444m.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuv444m.rst index 90bdee2e2b0d..7c03cc5b07d9 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yuv444m.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuv444m.rst @@ -107,35 +107,4 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - 1 - - 2 - - 3 - * - 0 - - YC - - YC - - YC - - YC - * - 1 - - YC - - YC - - YC - - YC - * - 2 - - YC - - YC - - YC - - YC - * - 3 - - YC - - YC - - YC - - YC +Chroma samples are :ref:`co-sited<yuv-chroma-cosited>`. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuyv.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuyv.rst index ca073a5098a9..4d5773922ba1 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yuyv.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuyv.rst @@ -69,50 +69,5 @@ Each cell is one byte. **Color Sample Location:** - - - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - - - 2 - - - - 3 - * - 0 - - Y - - C - - Y - - - - Y - - C - - Y - * - 1 - - Y - - C - - Y - - - - Y - - C - - Y - * - 2 - - Y - - C - - Y - - - - Y - - C - - Y - * - 3 - - Y - - C - - Y - - - - Y - - C - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yvyu.rst b/Documentation/userspace-api/media/v4l/pixfmt-yvyu.rst index 81ebec525ae5..14b56b611349 100644 --- a/Documentation/userspace-api/media/v4l/pixfmt-yvyu.rst +++ b/Documentation/userspace-api/media/v4l/pixfmt-yvyu.rst @@ -66,43 +66,5 @@ Each cell is one byte. **Color Sample Location:** - -.. flat-table:: - :header-rows: 0 - :stub-columns: 0 - - * - - - 0 - - - - 1 - - 2 - - - - 3 - * - 0 - - Y - - C - - Y - - Y - - C - - Y - * - 1 - - Y - - C - - Y - - Y - - C - - Y - * - 2 - - Y - - C - - Y - - Y - - C - - Y - * - 3 - - Y - - C - - Y - - Y - - C - - Y +Chroma samples are :ref:`interstitially sited<yuv-chroma-centered>` +horizontally. diff --git a/Documentation/userspace-api/media/v4l/yuv-formats.rst b/Documentation/userspace-api/media/v4l/yuv-formats.rst index 4a05a105a9e6..79a4fda566c6 100644 --- a/Documentation/userspace-api/media/v4l/yuv-formats.rst +++ b/Documentation/userspace-api/media/v4l/yuv-formats.rst @@ -14,11 +14,251 @@ reconstructed by subtracting from the brightness component. See :ref:`colorspaces` for conversion examples. YUV was chosen because early television would only transmit brightness information. To add color in a way compatible with existing receivers a new signal carrier -was added to transmit the color difference signals. Secondary in the YUV -format the U and V components usually have lower resolution than the Y -component. This is an analog video compression technique taking -advantage of a property of the human visual system, being more sensitive -to brightness information. +was added to transmit the color difference signals. + + +Subsampling +=========== + +YUV formats commonly encode images with a lower resolution for the chroma +components than for the luma component. This compression technique, taking +advantage of the human eye being more sensitive to luminance than color +differences, is called chroma subsampling. + +While many combinations of subsampling factors in the horizontal and vertical +direction are possible, common factors are 1 (no subsampling), 2 and 4, with +horizontal subsampling always larger than or equal to vertical subsampling. +Common combinations are named as follows. + +- `4:4:4`: No subsampling +- `4:2:2`: Horizontal subsampling by 2, no vertical subsampling +- `4:2:0`: Horizontal subsampling by 2, vertical subsampling by 2 +- `4:1:1`: Horizontal subsampling by 4, no vertical subsampling +- `4:1:0`: Horizontal subsampling by 4, vertical subsampling by 4 + +Subsampling the chroma component effectively creates chroma values that can be +located in different spatial locations: + +- .. _yuv-chroma-centered: + + The subsampled chroma value may be calculated by simply averaging the chroma + value of two consecutive pixels. It effectively models the chroma of a pixel + sited between the two original pixels. This is referred to as centered or + interstitially sited chroma. + +- .. _yuv-chroma-cosited: + + The other option is to subsample chroma values in a way that place them in + the same spatial sites as the pixels. This may be performed by skipping every + other chroma sample (creating aliasing artifacts), or with filters using an + odd number of taps. This is referred to as co-sited chroma. + +The following examples show different combination of chroma siting in a 4x4 +image. + +.. flat-table:: 4:2:2 subsampling, interstitially sited + :header-rows: 1 + :stub-columns: 1 + + * - + - 0 + - + - 1 + - + - 2 + - + - 3 + * - 0 + - Y + - C + - Y + - + - Y + - C + - Y + * - 1 + - Y + - C + - Y + - + - Y + - C + - Y + * - 2 + - Y + - C + - Y + - + - Y + - C + - Y + * - 3 + - Y + - C + - Y + - + - Y + - C + - Y + +.. flat-table:: 4:2:2 subsampling, co-sited + :header-rows: 1 + :stub-columns: 1 + + * - + - 0 + - + - 1 + - + - 2 + - + - 3 + * - 0 + - Y/C + - + - Y + - + - Y/C + - + - Y + * - 1 + - Y/C + - + - Y + - + - Y/C + - + - Y + * - 2 + - Y/C + - + - Y + - + - Y/C + - + - Y + * - 3 + - Y/C + - + - Y + - + - Y/C + - + - Y + +.. flat-table:: 4:2:0 subsampling, horizontally interstitially sited, vertically co-sited + :header-rows: 1 + :stub-columns: 1 + + * - + - 0 + - + - 1 + - + - 2 + - + - 3 + * - 0 + - Y + - C + - Y + - + - Y + - C + - Y + * - 1 + - Y + - + - Y + - + - Y + - + - Y + * - 2 + - Y + - C + - Y + - + - Y + - C + - Y + * - 3 + - Y + - + - Y + - + - Y + - + - Y + +.. flat-table:: 4:1:0 subsampling, horizontally and vertically interstitially sited + :header-rows: 1 + :stub-columns: 1 + + * - + - 0 + - + - 1 + - + - 2 + - + - 3 + * - 0 + - Y + - + - Y + - + - Y + - + - Y + * - + - + - + - + - + - + - + - + * - 1 + - Y + - + - Y + - + - Y + - + - Y + * - + - + - + - + - C + - + - + - + * - 2 + - Y + - + - Y + - + - Y + - + - Y + * - + - + - + - + - + - + - + - + * - 3 + - Y + - + - Y + - + - Y + - + - Y .. toctree::

[v4,09/16] media: doc: pixfmt-yuv: Document subsampling in more details

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