Re: Questions regarding Documentation/media/uapi/v4l/field-order.rst

[Hans Verkuil]

On 5/1/19 2:09 AM, Steve Longerbeam wrote:
> Hi Hans,
> 
> Thanks for the clarifications.
> 
> I have one additional comment.
> 
> The distinction between which field is _older_ (e.g. the field that was 
> recorded or captured first by the video camera) and which field is 
> _transmitted_ first needs to be more clear IMHO.
> 
> Even if there is no distinction, that is, all video equipment transmit 
> fields in the same order they were recorded, I think this should be 
> clarified better. The doc is using both terminology (captured first vs. 
> transmitted first) so it would be helpful to clarify that they mean the 
> same thing.

Thank you for the suggestion. This can be improved. I'll add it as a TODO,
but it's not high on my prio list, I'm afraid. If you don't want to wait
for me, then feel free to post a patch.

Regards,

	Hans

> 
> Steve
> 
> 
> On 4/23/19 7:05 AM, Hans Verkuil wrote:
>> On 4/16/19 3:54 PM, Rodin, Michael (Ferchau; ADITG/ESM1) wrote:
>>> Hi,
>>>
>>> I would like to ask several questions regarding the documentation of the enum "v4l2_field" [1].
>>> These questions came up during my investigations of issues with interaction between
>>> the gstreamer plugin v4l2src and the rcar video input driver [2].
>>>
>>> The documentation [1] specifies that:
>>> "All video capture and output devices must report the current field order.
>>> Some drivers may permit the selection of a different order, to this end
>>> applications initialize the field field of struct v4l2_pix_format before
>>> calling the VIDIOC_S_FMT ioctl. If this is not desired it should have
>>> the value V4L2_FIELD_ANY (0)."
>>>
>>> If I have understood these lines correctly, this means that if userspace sets "field" member of
>>> the struct "v4l2_pix_format" to V4L2_FIELD_ANY and uses this as parameter for the VIDIOC_S_FMT ioctl,
>>> then a driver should select/report the field order, which was previously set by media-ctl utility
>>> in the next subdevice, which is connected to the /dev/videoX node
>>> (From my understanding this would be equivalent to the "current field order").
>>>
>>> If the described behavior is correct, then the description in the table row for V4L2_FIELD_ANY in [1] is incomplete:
>>> "Applications request this field order when any one of the V4L2_FIELD_NONE, V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM, or V4L2_FIELD_INTERLACED formats is acceptable."
>>> What if V4L2_FIELD_ALTERNATE or V4L2_FIELD_SEQ_TB or V4L2_FIELD_SEQ_BT are also acceptable for the application?
>>> I think that the specification is either unprecise or my understanding of the specification is wrong.
>> The spec is a bit out of date: those missing field values were probably added after this
>> text was written. I'll make a patch fixing this.
>>
>>> Another potential issue, which I found in this documentation is that it does not distinguish between
>>> multiple contexts in which enum v4l2_field can be used. I can think of at least two different contexts:
>>> - When used to select the field order with VIDIOC_S_FMT ioctl.
>>> - When used to report the field order in a buffer: for example application sets V4L2_FIELD_ALTERNATE in VIDIOC_S_FMT ioctl and then gets buffers, which have V4L2_FIELD_TOP/BOTTOM set.
>> IMHO the text is reasonably clear on that. But if you have suggestions to
>> improve it, then make a proposal.
>>
>>> Now with this in mind, when I read the description of V4L2_FIELD_NONE:
>>> "The driver may also indicate this order when it cannot distinguish between V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM."
>> Whoops, that makes no sense. There are no drivers that do this. I'll remove this
>> line. If a driver can't tell the difference, then it should just pick FIELD_TOP
>> or BOTTOM.
>>
>>> I see two possible meanings/interpretations:
>>> - If application sets V4L2_FIELD_ALTERNATE in VIDIOC_S_FMT ioctl, report V4L2_FIELD_NONE back
>>>     so the application knows that the driver can not provide any TOP/BOTTOM metadata in the buffers
>>>     (which may be necessary for the application for example for deinterlacing) before it has got any buffer.
>>> - If application sets V4L2_FIELD_ALTERNATE in VIDIOC_S_FMT ioctl, driver reports V4L2_FIELD_ALTERNATE back,
>>>     even if it can not distinguish between TOP/BOTTOM. But when the application starts to read buffers,
>>>     they have V4L2_FIELD_NONE set if it's not possible to distinguish between TOP/BOTTOM.
>> Actually, drivers cannot ever return NONE for a top or bottom field. FIELD_NONE
>> indicates that a full frame has arrived, and doing something else would break
>> userspace.
>>
>>> Also there is another ambiguity in the description of V4L2_FIELD_NONE:
>>> "Images are in progressive format, not interlaced."
>>> What does "interlaced" mean in this case? Does it mean the other possible enum values or just the V4L2_FIELD_INTERLACED?
>> It means that the source video transmitted full frames, not top and bottom
>> fields. I clarified the text a bit.
>>
>>> If this just means V4L2_FIELD_INTERLACED, then it would imply that for example V4L2_FIELD_SEQ_TB
>>> and V4L2_FIELD_ALTERNATE are progressive formats, which is obviously not true.
>>> And also generally, in which of described contexts should be V4L2_FIELD_NONE set or reported (buffer or VIDIOC_S_FMT ioctl)?
>> For video capture (that's what we are talking about here) it is returned by the
>> driver in v4l2_buffer, never by userspace. Userspace can try to request a specific
>> field value when calling S_FMT, but the driver can overwrite it.
>>
>> The possible field values that a driver can support are dependent on the video
>> source (i.e. sensors are always FIELD_NONE) and the hardware capabilities.
>>
>>> Another point is that V4L2_FIELD_INTERLACED is also used by v4l2src to tell rcar-vin driver to combine the fields before giving them to application,
>>> so basically it requests progressive signal. So the meanings of V4L2_FIELD_INTERLACED and V4L2_FIELD_NONE are basically the same in this case.
>> Certainly not. FIELD_INTERLACED combines two fields into a single buffer, but the
>> odd and even lines in the frame were captured at different times. Whereas for
>> FIELD_NONE all lines were captured at the same time.
>>
>> So a FIELD_INTERLACED buffer may need to undergo additional deinterlacing.
>>
>> If the hardware already does high-quality deinterlacing then that might be
>> a reason for the driver to return FIELD_NONE to avoid additional deinterlacing
>> in userspace.
>>
>> In practice there are three main categories in the way the field is handled:
>>
>> 1) The video source is a webcam: field is always FIELD_NONE, set by the driver.
>>
>> 2) The video source is HDMI: if the video is progressive, then the field is always
>>     FIELD_NONE. If the video is interlaced, then the field is always FIELD_ALTERNATE
>>     in v4l2_format and alternating FIELD_TOP and BOTTOM in v4l2_buffer.
>>
>> 3) The video source is SDTV (i.e. S-Video or composite): the video is always
>>     interlaced, and it depends on the hardware which field values are supported,
>>     except for FIELD_NONE, which is never returned as far as I am aware.
>>
>> Regards,
>>
>> 	Hans
>>
>>> Thank you in advance and sorry for the long mail!
>>>
>>> [1] Documentation/media/uapi/v4l/field-order.rst
>>> [2] drivers/media/platform/rcar-vin
>>>
>>> Best regards
>>>
>>> Michael Rodin
>>>
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