====GRAY8 vs RTP====
**problem**: \\
single plane raw formats are not part of [[https://tools.ietf.org/html/rfc4175|rfc4175]], and as such 2nd class citizens to gstreamer, RTSP or [[https://www.kernel.org/doc/html/v4.10/media/uapi/v4l/pixfmt-grey.html|anywhere]] else. 

**solution**: \\
an image format is **just a convention**, internally they are byte(octet) streams. we can re-package a datastream to another format, as long as we mind our strides. 

view the image format as just a stream of octets. here is our stream in Y8 format
       +--+--+--+--+ +--+--+--+--+
       |Y1|xx|xx|xx| |Y1|xx|xx|xx| ...
       +--+--+--+--+ +--+--+--+--+
and again, data quartered and represented in 8 bit depth and with stride of 4
       +--+--+--+--+ +--+--+--+--+
       |R0|G0|B0|A0| |R1|G1|B1|A1| ...
       +--+--+--+--+ +--+--+--+--+

note the above represents two sequential pixels. 

the trick is to use gstreamer built in [[https://gstreamer.freedesktop.org/documentation/additional/design/mediatype-video-raw.html?gi-language=c#formats|format spec]] and let that handle the conventions. 

the [[https://www.collabora.com/news-and-blog/blog/2016/02/16/a-programmers-view-on-digital-images-the-essentials/|hammer]]

{{drawio>gst:Y8toRGBA}}


as easy as \\
<code dot>
gst-launch-1.0 -q \
  videotestsrc pattern=white num-buffers=1 \
! video/x-raw, format=GRAY8, width=4, height=4 \
! identity dump=1 \
! rawvideoparse format=rgba width=1 height=4 \
! identity dump=1 \
! fakesink
00000000 (0x7f369c00a8c0): eb eb eb eb eb eb eb eb eb eb eb eb eb eb eb eb  ................
00000000 (0x556228700560): eb eb eb eb eb eb eb eb eb eb eb eb eb eb eb eb  ................
</code>
**note**: why white pixel is not FF=(255)?\\
gstreamer uses YUV colorimetry when generating a GRAY8 test image \\
and YUV doesnt use the entire 0-255 space. [[https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/738|issue]]

**note2**: this example is running on linux/debian, \\
to adapt to windows using powershell replace ''\'' with ''`'' and \\
place quotes around  ''video/x-raw, format=GRAY8, width=4, height=4''


====FakeRawPay====
gray8 raw video transported as RTP payload via a "fake" rgba parsing step\\
conforming to [[https://tools.ietf.org/html/rfc4175|RFC4157]]\\
pay
<code dot>
gst-launch-1.0 -v \
  videotestsrc pattern=white is-live=1 \
  ! video/x-raw, format=GRAY8, width=4, height=4, framerate=30/1 \
  ! rawvideoparse width=1 height=4 format=rgba \
  ! rtpvrawpay \
  ! queue \
  ! udpsink host=localhost port=5000
</code> 
depay and back to gray8
<code dot>
gst-launch-1.0 -v \
  udpsrc port=5000 \
  ! "application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)RAW, sampling=(string)RGBA, depth=(string)8, width=(string)1, height=(string)4, colorimetry=(string)SMPTE240M, payload=(int)96" \
   ! rtpvrawdepay \
   ! queue \
   ! rawvideoparse width=4 height=4 format=gray8 \
   ! videoconvert \
   ! fpsdisplaysink
</code>
  


====strides and offsets====

there are caveats to this approach (yuv). get the hammer and study the [[https://gstreamer.freedesktop.org/documentation/additional/design/mediatype-video-raw.html?gi-language=c#formats|design document]]. 


{{ :pasted:20200328-193549.png?700 }}

<blockquote>**pstride** only exist for simple pixel formats. It represent the** distance
in bytes between two pixels**. This value, when it exists, is constant for
a specific format.

**rtstride** (row stride), is **the size, in bytes, of one [image] line**. In general,
it must be larger or equal to pstride * width, when pstride exist for
your format. This design document outline the calculation of the
default strides and **offsets [channels\planes]** within the libgstvideo library. Notice the
use of RU4 or RU2, which are round up, these are used to ensure the
each row starts on an align pointer. This simplify memory access in
software processing.

When you use VAAPI, you are using some HW accelerators. These
accelerators have wider memory alignment requirement, hence the end
results is that strides will be larger, and sometimes there will also
be couple of padding rows.
<cite>[[http://gstreamer-devel.966125.n4.nabble.com/A-question-regarding-stride-in-YUV-buffers-tp4691294p4691307.html|question]]
</cite>
</blockquote> 

**GRAY8**\\
8-bit grayscale "Y800" same as "GRAY8" 
^ Component ^ color ^ depth ^ pstride ^ offset ^ rstride ^  size  ^
|  0  | Y | 8 | 1 | 0 | width |  rstride*height  |

<code dot>
+--+--+--+--+ +--+--+--+--+
|Y0|x |x |x | |Y1|x |x |x | ... (two pixel representation, in octet))
+--+--+--+--+ +--+--+--+--+
</code>

**RGBA** \\
rgb with alpha channel last (big-endian)
^ Component ^ color ^ depth ^ pstride ^ offset ^ rtsride ^  size  ^
|  0  | R | 8 | 4 | 0 | width*4 |  rstride * height  |
|  1  | G | 8 | 4 | 1 | ::: |  :::  |
|  2  | B | 8 | 4 | 2 | ::: |  :::  |
|  3  | A | 8 | 4 | 3 | ::: |  :::  |
<code dot>
+--+--+--+--+ +--+--+--+--+
|R0|G0|B0|A0| |R1|G1|B1|A1| ...(two pixel representation, in octet)
+--+--+--+--+ +--+--+--+--+
</code>


<hidden extended format view>
<code dot >
"GRAY8" 8-bit grayscale "Y800" same as "GRAY8"
        Component 0: Y
          depth:           8
          offset:          0
          pstride:         1
          default rstride: RU4 (width)
          default size:    rstride (component0) * height
          
        Image
          default size:    size (component0)
          
"RGBA" rgb with alpha channel last
       +--+--+--+--+ +--+--+--+--+
       |R0|G0|B0|A0| |R1|G1|B1|A1| ...
       +--+--+--+--+ +--+--+--+--+

        Component 0: R
          depth:           8
          pstride:         4
          offset:          0

        Component 1: G
          depth:           8
          pstride:         4
          offset:          1

        Component 2: B
          depth            8
          pstride:         4
          offset:          2

        Component 3: A
          depth            8
          pstride:         4
          offset:          3

        Image
          default rstride: width * 4
          default size:    rstride (image) * height
</code>

</hidden>

<hidden more digging>
<file cpp /gst-plugins-base/gst-libs/gst/video/video-format.c>

#define PACK_GRAY8 GST_VIDEO_FORMAT_AYUV, unpack_GRAY8, 1, pack_GRAY8
static void
unpack_GRAY8 (const GstVideoFormatInfo * info, GstVideoPackFlags flags,
    gpointer dest, const gpointer data[GST_VIDEO_MAX_PLANES],
    const gint stride[GST_VIDEO_MAX_PLANES], gint x, gint y, gint width)
{
  const guint8 *restrict s = GET_LINE (y);

  s += x;

  video_orc_unpack_GRAY8 (dest, s, width);
}

static void
pack_GRAY8 (const GstVideoFormatInfo * info, GstVideoPackFlags flags,
    const gpointer src, gint sstride, gpointer data[GST_VIDEO_MAX_PLANES],
    const gint stride[GST_VIDEO_MAX_PLANES], GstVideoChromaSite chroma_site,
    gint y, gint width)
{
  guint8 *restrict d = GET_LINE (y);

  video_orc_pack_GRAY8 (d, src, width);
}

#define DPTH8 8, 1, { 0, 0, 0, 0 }, { 8, 0, 0, 0 }
#define PSTR1 { 1, 0, 0, 0 }
#define PLANE0 1, { 0, 0, 0, 0 }
#define OFFS0 { 0, 0, 0, 0 }
#define SUB4 { 0, 0, 0, 0 }, { 0, 0, 0, 0 }
#define PACK_GRAY8 GST_VIDEO_FORMAT_AYUV, unpack_GRAY8, 1, pack_GRAY8
MAKE_GRAY_FORMAT (GRAY8, "raw video", DPTH8, PSTR1, PLANE0, OFFS0, SUB4,
      PACK_GRAY8),
</file> 
[[https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/blob/master/gst-libs/gst/video/video-format.c|video-format.c]]
</hidden>

[[https://github.com/torvalds/linux/blob/master/include/uapi/drm/drm_fourcc.h#L174|fourcc]] on linux\\
[[https://docs.microsoft.com/en-us/windows/win32/medfound/image-stride?redirectedfrom=MSDN|MSDN on stride]]