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 >>> >>> Advanced Driver Information Technology GmbH >>> Engineering Software Multimedia 1 (ADITG/ESM1) >>> Robert-Bosch-Str. 200 >>> 31139 Hildesheim >>> Germany >>> >>> Tel. +49 5121 49 6936 >>> Fax +49 5121 49 6999 >>> mrodin@xxxxxxxxxxxxxx >>> Web: www.adit-jv.com >>> >>> ADIT is a joint venture company of Robert Bosch GmbH/Robert Bosch Car Multimedia GmbH and DENSO Corporation >>> Sitz: Hildesheim, Registergericht: Amtsgericht Hildesheim HRB 3438 >>> Geschaeftsfuehrung: Wilhelm Grabow, Ken Yaguchi >>> >
