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?