Re: [PATCH v5 1/3] [media] exynos-gsc: Use user configured colorspace if provided

From: Thibault Saunier
Date: Wed Feb 22 2017 - 15:09:01 EST

On 02/22/2017 05:03 PM, Nicolas Dufresne wrote:
Le mercredi 22 fÃvrier 2017 Ã 15:57 -0300, Thibault Saunier a Ãcrit :
On 02/22/2017 03:06 PM, Hans Verkuil wrote:
On 02/22/2017 05:05 AM, Thibault Saunier wrote:

On 02/21/2017 11:19 PM, Hans Verkuil wrote:
On 02/21/2017 11:20 AM, Thibault Saunier wrote:
Use colorspace provided by the user as we are only doing
scaling and
color encoding conversion, we won't be able to transform the
itself and the colorspace won't mater in that operation.

Also always use output colorspace on the capture side.

Start using 576p as a threashold to compute the colorspace.
The media documentation says that the
V4L2_COLORSPACE_SMPTE170M colorspace
should be used for SDTV and V4L2_COLORSPACE_REC709 for HDTV.
But drivers
don't agree on the display resolution that should be used as
a threshold.

From EIA CEA 861B about colorimetry for various

- 5.1 480p, 480i, 576p, 576i, 240p, and 288p
The color space used by the 480-line, 576-line, 240-
line, and 288-line
formats will likely be based on SMPTE 170M [1].
- 5.2 1080i, 1080p, and 720p
The color space used by the high definition formats
will likely be
based on ITU-R BT.709-4

This indicates that in the case that userspace does not
specify what
colorspace should be used, we should use 576p as a threshold
to set
Even if it is
only 'likely' and not a requirement it is the best guess we
can make.

The stream should have been encoded with the information and
has to pass it to the driver if it is not the case, otherwise
we won't be
able to handle it properly anyhow.

Also, check for the resolution in G_FMT instead
unconditionally setting
the V4L2_COLORSPACE_REC709 colorspace.

Signed-off-by: Javier Martinez Canillas <javier@xxxxxxxxxxxxx
Signed-off-by: Thibault Saunier <thibault.saunier@xxxxxxxxxxx
Reviewed-by: Andrzej Hajda <a.hajda@xxxxxxxxxxx>


Changes in v5:
- Squash commit to always use output colorspace on the
capture side
inside this one
- Fix typo in commit message

Changes in v4:
- Reword commit message to better back our assumptions on

Changes in v3:
- Do not check values in the g_fmt functions as Andrzej
explained in previous review
- Added 'Reviewed-by: Andrzej Hajda <a.hajda@xxxxxxxxxxx>'

Changes in v2: None

drivers/media/platform/exynos-gsc/gsc-core.c | 20
drivers/media/platform/exynos-gsc/gsc-core.h | 1 +
2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/drivers/media/platform/exynos-gsc/gsc-core.c
index 59a634201830..772599de8c13 100644
--- a/drivers/media/platform/exynos-gsc/gsc-core.c
+++ b/drivers/media/platform/exynos-gsc/gsc-core.c
@@ -454,6 +454,7 @@ int gsc_try_fmt_mplane(struct gsc_ctx
*ctx, struct v4l2_format *f)
} else {
min_w = variant->pix_min->target_rot_dis_w;
min_h = variant->pix_min->target_rot_dis_h;
+ pix_mp->colorspace = ctx->out_colorspace;
pr_debug("mod_x: %d, mod_y: %d, max_w: %d, max_h =
@@ -472,10 +473,15 @@ int gsc_try_fmt_mplane(struct gsc_ctx
*ctx, struct v4l2_format *f)
pix_mp->num_planes = fmt->num_planes;
- if (pix_mp->width >= 1280) /* HD */
- pix_mp->colorspace = V4L2_COLORSPACE_REC709;
- else /* SD */
- pix_mp->colorspace = V4L2_COLORSPACE_SMPTE170M;
+ if (pix_mp->colorspace == V4L2_COLORSPACE_DEFAULT) {
+ if (pix_mp->width > 720 && pix_mp->height > 576) /*
HD */
I'd use || instead of && here. Ditto for the next patch.

+ pix_mp->colorspace = V4L2_COLORSPACE_REC709;
+ else /* SD */
+ pix_mp->colorspace = V4L2_COLORSPACE_SMPTE170M;
+ }
Are you sure this is in fact how it is used? If the source of
the video
is the sensor (camera), then the colorspace is typically SRGB.
I'm not
really sure you should guess here. All a mem2mem driver should
do is to
pass the colorspace etc. information from the output to the
capture side,
and not invent things. That simply makes no sense.
This is not a camera device but a colorspace conversion device,
in many
Not really, this is a color encoding conversion device. I.e. it
only affects
the Y'CbCr encoding and quantization range. The colorspace (aka
and transfer function remain unchanged.
Well, right, sorry I am talking in GStreamer terminlogy where what
you call
colorspace is called colorimetry, and colorspace is what I am
about here.

In fact, I suspect (correct me if I am wrong) that it only converts
RGB, Y'CbCr 601 and Y'CbCr 709 encodings, where RGB is full range
and the Y'CbCr
formats are limited range.
That sounds correct.

If you pass in limited range RGB it will probably do the wrong
thing as I don't
seen any quantization checks in the code.

So the colorspace and xfer_func fields remain unchanged by this

If you want to do this really correctly, then you need to do more.
I don't have
time right now to go into this in-depth, but I will try to get back
to this on
Monday. I am thinking of documenting this as part of the V4L2
documentation. This stuff is complex and if you don't know the
rules then it
can be hard to implement correctly.
Here I am just making sure that we set the colorspace (v4l2
if the case the user is passing DEFAULT (meaning he does not know
it should be and lets the driver set it, this will happen in the
where the
information was not contain in the source, which means the value has
to be
guessed, and here I am simply doing an educated guess relying on what
previously named standard suggests.

I would be happy to read more information about that subject and will
try to fix remaining suggested points once I have a better
of the whole concepts and problems in that driver, but I still think
that this
patch is correct for what it is aiming at fixing.


Thibault Saunier

cases the info will not be passed by the userspace, basically
because the info
is not encoded in the media stream (this often happens). I am not
inventing here,
just making sure we use the most likely value in case none was
provided (and if none
was provided it should always be correct given that the encoded
stream was not broken).

In the case the source is a camera and then we use the colorspace
converter then the info
should copied from the camera to the transcoding node (by
userspace) so there should be
no problem.

What I am doing here is what is done in other drivers.
Most (all?) other mem2mem drivers do not do color encoding
conversion, they just copy
all the colorspace info from output to capture.

If there are other m2m drivers that do this, then I doubt they do
the right thing.
They are probably right for most cases, but not all.


I think Hans is right about the colorspace here. Unlike the field for
interlace modes, there it is no mandatory requirement in the spec for
the driver to change DEFAULT into something else. If you don't know the
input colorspace, then you don't known the output. Even in GStreamer,
this would be best, since the only guessing will happen at the display,
or in some conversion to RGB. Though, if you don't support a specific
value, then the driver should change it.

Well the documentation, even if not really precise states:

V4L2_COLORSPACE_DEFAULT: The default colorspace. This can be used by applications to let the driver fill in the colorspace.

Which is why I supposed the driver should fill it in, also this is what other drivers do too.


Where I don't agree though, is that an m2m color converter will always
copy blindly that colorspace. Even though the support is limited,
sometime it may change, this need to be sorted case by case. I have
rarely seen such a driver keep the limited range when turning buffers
into RGB data.

But changing the colorspace is complicated, since it's slightly vague
concept. It will in fact imply a specific set of v4l2_xfer_func,
v4l2_ycbcr_encoding and v4l2_quantization (ignoring hsv, as it does not
apply). If you have a limited driver, that only target one of these 3
items, you will likely keep the colorspace as-is, and override the
specific functions.