Is error "transform_caps returned caps which are not a real subset of the filter caps" a "safe" error?

José Carlos Cazarin Filho jose at
Fri Mar 11 20:33:45 UTC 2022

Hello! I'm writing a custom element that, depending on which element it is connected to, I get the following error:

0:00:00.146996442  2491 0xaaaafa871000 ERROR          basetransform gstbasetransform.c:482:gst_base_transform_transform_caps:<custom_element> transform_caps returned caps video/x-raw, format=(string){ RGB, NV12, I420, YV12, NV21, YUYV, YVYU, UYVY, VYUY, NV16, NV61, YUY2 }, width=(int)[ 1, 2147483647 ], height=(int)[ 1, 2147483647 ], framerate=(fraction)[ 0/1, 2147483647/1 ] which are not a real subset of the filter caps video/x-raw, format=(string)YUY2, width=(int)1920, height=(int)1080

** (gst-launch-1.0:2491): WARNING **: 20:26:35.991: customelement0 : transform_caps returned caps which are not a real subset of the filter caps

But the pipeline doesn't break, it works just fine.

For this custom element I wrote the following transform_caps function:

static GstCaps*
transform_caps(GstBaseTransform* trans,
               GstPadDirection   direction,
               GstCaps*          caps,
               GstCaps*          filter)

    if (direction == GST_PAD_SINK) {

        filter = gst_caps_from_string(
            ";" GST_TENSORS_CAP_MAKE("{ static, flexible }"));
    } else if (direction == GST_PAD_SRC) {

        filter = gst_caps_from_string(
            GST_VIDEO_CAPS_MAKE("{ RGB, NV12, I420, YV12, NV21, YUYV, YVYU, "
                                "UYVY, VYUY, NV16, NV61, YUY2 }"));

    return filter;

The element is used in a machine learning pipeline, and it should receive video frames with the caps referred in the code snippet, and it will output tensors, that's why I used  the NNStreamer defines (namely GST_TENSOR_CAP_DEFAULT, GST_TENSORS_CAP_MAKE ) when the direction is GST_PAD_SINK.

I want to know if there's something wrong in that transform_caps, because the error doesn't break the pipeline nor it always happens. Or can I just ignore this error?

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