How to write code that takes photo using your webcam?

[Радомир Хаџић]

Hello.

I have a small problem. I don't know how to write a program that takes a photo using a webcam. I know how to do that using gst-launch-1.0, though.

gst-launch-1.0 -e v4l2src device=/dev/video0 num-buffers=1 ! videoconvert ! pngenc ! filesink location=sample.png sync=true

This works. A photo of me is successfully saved as sample.png.

What I'd like to do is to write GStreamer code equivalent to the command above. Here is my best try.

#include <gst/gst.h>

int main(int argc, char **argv)
{
    // initialize return value
    int status;

    // initialize GStreamer
    gst_init(&argc, &argv);

    // create pipeline and other elements to use
    GstElement *pipeline = gst_pipeline_new("pipeline");
    GstElement *source = gst_element_factory_make("v4l2src", "source");
    GstElement *converter = gst_element_factory_make("videoconvert", "converter");
    GstElement *encoder = gst_element_factory_make("pngenc", "encoder");
    GstElement *file = gst_element_factory_make("filesink", "file");

    // add all elements to the pipeline and connect them
    gst_bin_add_many(GST_BIN(pipeline), source, converter, encoder, file, NULL);
    gst_element_link_many(source, converter, encoder, file, NULL);

    // set the properties
    g_object_set(source, "device", "/dev/video0", "num-buffers", 1, NULL);
    g_object_set(file, "location", "sample.png", "sync", 1, NULL);

    // play
    gst_element_set_state(pipeline, GST_STATE_PLAYING);

    // wait for EOS
    GstBus *bus = gst_element_get_bus(pipeline);
    GstMessage *msg = gst_bus_timed_pop(bus, GST_CLOCK_TIME_NONE);

    // determine return value using msg
    switch (GST_MESSAGE_TYPE(msg))
    {
        case GST_MESSAGE_ERROR:
            status = -1;
            break;
        case GST_MESSAGE_EOS:
            status = 0;
            break;
        default:
            status = 1;
            break;
    }

    // clean up
    gst_element_set_state(pipeline, GST_STATE_NULL);
    if (msg != NULL)
        gst_message_unref(msg);
    gst_object_unref(bus);
    gst_object_unref(pipeline);

    // exit from the program and return status
    return status;
}

The problem with this code is that really creates sample.png, but the file is corrupted and cannot be opened. Why the code above doesn't work as intended and what do I need to change to make it work the way I want?

Thank you for reading this. Have a nice day.

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[Olivier Crête-3]

Hi,

You're missing a loop around the bus message fetching, the first message is not the one you want.

Olivier

On October 20, 2018 11:57:30 a.m. EDT, "Радомир Хаџић" <[hidden email]> wrote:

>Hello.
>
>I have a small problem. I don't know how to write a program that takes
>a
>photo using a webcam. I know how to do that using *gst-launch-1.0*,
>though.
>*gst-launch-1.0 -e v4l2src device=/dev/video0 num-buffers=1 !
>videoconvert
>! pngenc ! filesink location=sample.png sync=true*
>This works. A photo of me is successfully saved as *sample.png*.
>
>What I'd like to do is to write GStreamer code equivalent to the
>command
>above. Here is my best try.
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>*#include <gst/gst.h>int main(int argc, char **argv){    // initialize
>return value    int status;    // initialize GStreamer  
>gst_init(&argc,
>&argv);    // create pipeline and other elements to use    GstElement
>*pipeline = gst_pipeline_new("pipeline");    GstElement *source =
>gst_element_factory_make("v4l2src", "source");    GstElement *converter
>=
>gst_element_factory_make("videoconvert", "converter");    GstElement
>*encoder = gst_element_factory_make("pngenc", "encoder");    GstElement
>*file = gst_element_factory_make("filesink", "file");    // add all
>elements to the pipeline and connect them
>gst_bin_add_many(GST_BIN(pipeline), source, converter, encoder, file,
>NULL);    gst_element_link_many(source, converter, encoder, file,
>NULL);
>// set the properties    g_object_set(source, "device", "/dev/video0",
>"num-buffers", 1, NULL);    g_object_set(file, "location",
>"sample.png",
>"sync", 1, NULL);    // play    gst_element_set_state(pipeline,
>GST_STATE_PLAYING);    // wait for EOS    GstBus *bus =
>gst_element_get_bus(pipeline);    GstMessage *msg =
>gst_bus_timed_pop(bus,
>GST_CLOCK_TIME_NONE);    // determine return value using msg    switch
>(GST_MESSAGE_TYPE(msg))    {        case GST_MESSAGE_ERROR:
>status = -1;            break;        case GST_MESSAGE_EOS:
>status = 0;            break;        default:            status = 1;
>    break;    }    // clean up    gst_element_set_state(pipeline,
>GST_STATE_NULL);    if (msg != NULL)        gst_message_unref(msg);
>gst_object_unref(bus);    gst_object_unref(pipeline);    // exit from
>the
>program and return status    return status;}*
>The problem with this code is that really creates *sample.png*, but the
>file is corrupted and cannot be opened. Why the code above doesn't work
>as
>intended and what do I need to change to make it work the way I want?
>
>Thank you for reading this. Have a nice day.

--
Olivier Crête
[hidden email]
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