What is the point of gst_element_link_filtered() ?

[wally bkg]

I'm coming to the conclusion that v4l2 and gstreamer is fatally flawed.
Every time I get a new v4l2 capture device something breaks.

I've a new device (Sensoray 2255S) and it was simple to get it to work, but
not with code that doesn't break when using some other v4l2 device!

At the end of the day I need a 640x480 grayscale buffer coming out of my
appsink.  The problem is how do I define a pipeline to work around the
various conversions I need when I don't get a error until the pipeline tries
to play?

I was expecting  link_ok = gst_element_link_filtered (source, capsfilter,
caps_gray)   to fail on a card that doesn't support grayscale, when the caps
filter is set to caps_gray, but it doesn't.  I don't know anything is wrong
until I set the pipeline to playing but then its too late!

Perhaps I can best illustrate the problem with a few gst-launch examples,
although without the same hardware you likely can't duplicate the problems.


For the device that can't do gray scale directly, this pipeline works:
gst-launch -v v4l2src device=/dev/video6 ! ffmpegcolorspace !
video/x-raw-gray, framerate=\(fraction\)30000/1001, width=640, height=480 !
ffmpegcolorspace ! xvimagesink

It negotiates:
 ffmpegcsp0.GstPad:sink: caps = video/x-raw-yuv, format=(fourcc)I420,
framerate=(fraction)30000/1001, width=(int)640, height=(int)480

ffmpegcsp0.GstPad:src: caps = video/x-raw-gray, width=(int)640,
height=(int)480, framerate=(fraction)30000/1001, bpp=(int)8, depth=(int)8

The second ffmpegcolor space just fixes the fact that xvimagesink can't do
grayscale, but is not relevant to my programmatic appsink pipeline.



For the Sensoray device:
gst-launch -v v4l2src device=/dev/video2 ! ffmpegcolorspace !
video/x-raw-gray, framerate=\(fraction\)30000/1001, width=640, height=480 !
ffmpegcolorspace ! xvimagesink

negotiates:
ffmpegcsp0.GstPad:sink: caps = video/x-raw-yuv, format=(fourcc)I420,
framerate=(fraction)30000/1001, width=(int)640, height=(int)480 and all
would seem well, except the video has a black bar in the middle because of a
bug in either the Sensoray drive, v4l2, or gstreamer the negotiation is
incorrect because I420 is not supported in the driver according to v4l-info:

inputs
    VIDIOC_ENUMINPUT(0)
    index                   : 0
    name                    : "Composite"
    type                    : CAMERA
    audioset                : 0
    tuner                   : 0
    std                     : 0xb0ff
[PAL_B,PAL_B1,PAL_G,PAL_H,PAL_I,PAL_D,PAL_D1,PAL_K,NTSC_M,NTSC_M_JP,?]
    status                  : 0x0 []

video capture
    VIDIOC_ENUM_FMT(0,VIDEO_CAPTURE)
    index                   : 0
    type                    : VIDEO_CAPTURE
    flags                   : 0
    description             : "4:2:2, planar, YUV422P"
    pixelformat             : 0x50323234 [422P]
    VIDIOC_ENUM_FMT(1,VIDEO_CAPTURE)
    index                   : 1
    type                    : VIDEO_CAPTURE
    flags                   : 0
    description             : "4:2:2, packed, YUYV"
    pixelformat             : 0x56595559 [YUYV]
    VIDIOC_ENUM_FMT(2,VIDEO_CAPTURE)
    index                   : 2
    type                    : VIDEO_CAPTURE
    flags                   : 0
    description             : "4:2:2, packed, UYVY"
    pixelformat             : 0x59565955 [UYVY]
    VIDIOC_ENUM_FMT(3,VIDEO_CAPTURE)
    index                   : 3
    type                    : VIDEO_CAPTURE
    flags                   : 0
    description             : "8bpp GREY"
    pixelformat             : 0x59455247 [GREY]
    VIDIOC_ENUM_FMT(4,VIDEO_CAPTURE)
    index                   : 4
    type                    : VIDEO_CAPTURE
    flags                   : 0
    description             : "JPG"
    pixelformat             : 0x4745504a [JPEG]
    VIDIOC_ENUM_FMT(5,VIDEO_CAPTURE)
    index                   : 5
    type                    : VIDEO_CAPTURE
    flags                   : 0
    description             : "MJPG"
    pixelformat             : 0x47504a4d [MJPG]
    VIDIOC_G_FMT(VIDEO_CAPTURE)
    type                    : VIDEO_CAPTURE
    fmt.pix.width           : 640
    fmt.pix.height          : 480
    fmt.pix.pixelformat     : 0x4745504a [JPEG]
    fmt.pix.field           : INTERLACED
    fmt.pix.bytesperline    : 1920
    fmt.pix.sizeimage       : 921600
    fmt.pix.colorspace      : unknown
    fmt.pix.priv            : 0



Now if I simply do:

 gst-launch -v v4l2src device=/dev/video2 ! xvimagesink

it correctly negotiates a 4:2:2 yuv format and works correctly.  Other of my
capture devices don't support support any 4:2:2 formats, so this is not a
"common" format I could force the captures to.

So how can I detect this problem before I run the pipeline and adjust the
pipeline around it?
Or how do I recover from the "can't negotiate" error when setting the
pipeline to playing?
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