224 lines
7.1 KiB
Plaintext
224 lines
7.1 KiB
Plaintext
/** \page page_tutorial5 Tutorial - Part 5: Capturing Video Frames
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\ref page_tutorial4 | \ref page_tutorial "Index" | \ref page_tutorial6
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In this tutorial we show how to use a stream to capture a
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stream of video frames.
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Even though we are now working with a different media type and
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we are capturing instead of playback, you will see that this
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example is very similar to \ref page_tutorial4.
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Let's take a look at the code before we break it down:
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\snippet tutorial5.c code
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Save as tutorial5.c and compile with:
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gcc -Wall tutorial5.c -o tutorial5 -lm $(pkg-config --cflags --libs libpipewire-0.3)
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Most of the application is structured like \ref page_tutorial4.
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We create a stream object with different properties to make it a Camera
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Video Capture stream.
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\code{.c}
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props = pw_properties_new(PW_KEY_MEDIA_TYPE, "Video",
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PW_KEY_MEDIA_CATEGORY, "Capture",
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PW_KEY_MEDIA_ROLE, "Camera",
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NULL);
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if (argc > 1)
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pw_properties_set(props, PW_KEY_TARGET_OBJECT, argv[1]);
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data.stream = pw_stream_new_simple(
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pw_main_loop_get_loop(data.loop),
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"video-capture",
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props,
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&stream_events,
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&data);
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\endcode
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We also optionally allow the user to pass the name of the target node where the session
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manager is supposed to connect the node. The user may also give the value of the
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unique target node serial (`PW_KEY_OBJECT_SERIAL`) as the value.
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In addition to the `process` event, we are also going to listen to a new event,
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`param_changed`:
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\code{.c}
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static const struct pw_stream_events stream_events = {
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PW_VERSION_STREAM_EVENTS,
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.param_changed = on_param_changed,
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.process = on_process,
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};
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\endcode
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Because we capture a stream of a wide range of different
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video formats and resolutions, we have to describe our accepted formats in
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a different way:
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\code{.c}
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const struct spa_pod *params[1];
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uint8_t buffer[1024];
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struct spa_pod_builder b = SPA_POD_BUILDER_INIT(buffer, sizeof(buffer));
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params[0] = spa_pod_builder_add_object(&b,
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SPA_TYPE_OBJECT_Format, SPA_PARAM_EnumFormat,
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SPA_FORMAT_mediaType, SPA_POD_Id(SPA_MEDIA_TYPE_video),
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SPA_FORMAT_mediaSubtype, SPA_POD_Id(SPA_MEDIA_SUBTYPE_raw),
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SPA_FORMAT_VIDEO_format, SPA_POD_CHOICE_ENUM_Id(7,
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SPA_VIDEO_FORMAT_RGB,
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SPA_VIDEO_FORMAT_RGB,
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SPA_VIDEO_FORMAT_RGBA,
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SPA_VIDEO_FORMAT_RGBx,
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SPA_VIDEO_FORMAT_BGRx,
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SPA_VIDEO_FORMAT_YUY2,
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SPA_VIDEO_FORMAT_I420),
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SPA_FORMAT_VIDEO_size, SPA_POD_CHOICE_RANGE_Rectangle(
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&SPA_RECTANGLE(320, 240),
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&SPA_RECTANGLE(1, 1),
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&SPA_RECTANGLE(4096, 4096)),
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SPA_FORMAT_VIDEO_framerate, SPA_POD_CHOICE_RANGE_Fraction(
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&SPA_FRACTION(25, 1),
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&SPA_FRACTION(0, 1),
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&SPA_FRACTION(1000, 1)));
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\endcode
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This is using a `struct spa_pod_builder` to make a `struct spa_pod *` object
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in the buffer array on the stack. The parameter is of type `SPA_PARAM_EnumFormat`
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which means that it enumerates the possible formats for this stream.
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In this example we use the builder to create some `CHOICE` entries for
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the format properties.
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We have an enumeration of formats, we need to first give the amount of enumerations
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that follow, then the default (preferred) value, followed by alternatives in order
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of preference:
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\code{.c}
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SPA_FORMAT_VIDEO_format, SPA_POD_CHOICE_ENUM_Id(7,
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SPA_VIDEO_FORMAT_RGB, /* default */
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SPA_VIDEO_FORMAT_RGB, /* alternative 1 */
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SPA_VIDEO_FORMAT_RGBA, /* alternative 2 */
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SPA_VIDEO_FORMAT_RGBx, /* .. etc.. */
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SPA_VIDEO_FORMAT_BGRx,
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SPA_VIDEO_FORMAT_YUY2,
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SPA_VIDEO_FORMAT_I420),
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\endcode
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We also have a `RANGE` of values for the size. We need to give a default (preferred)
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size and then a min and max value:
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\code{.c}
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SPA_FORMAT_VIDEO_size, SPA_POD_CHOICE_RANGE_Rectangle(
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&SPA_RECTANGLE(320, 240), /* default */
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&SPA_RECTANGLE(1, 1), /* min */
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&SPA_RECTANGLE(4096, 4096)), /* max */
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\endcode
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We have something similar for the framerate.
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Note that there are other video parameters that we don't specify here. This
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means that we don't have any restrictions for their values.
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See \ref page_spa_pod for more information about how to make these
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POD objects.
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Now we're ready to connect the stream and run the main loop:
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\code{.c}
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pw_stream_connect(data.stream,
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PW_DIRECTION_INPUT,
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PW_ID_ANY,
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PW_STREAM_FLAG_AUTOCONNECT |
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PW_STREAM_FLAG_MAP_BUFFERS,
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params, 1);
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pw_main_loop_run(data.loop);
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\endcode
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To connect we specify that we have a `PW_DIRECTION_INPUT` stream. The third
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argument is always `PW_ID_ANY`.
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We're setting the `PW_STREAM_FLAG_AUTOCONNECT` flag to make an automatic
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connection to a suitable camera and `PW_STREAM_FLAG_MAP_BUFFERS` to let the
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stream mmap the data for us.
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And last we pass the extra parameters for our stream. Here we only have the
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allowed formats (`SPA_PARAM_EnumFormat`).
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Running the mainloop will start the connection and negotiation process.
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First our `param_changed` event will be called with the format that was
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negotiated between our stream and the camera. This is always something that
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is compatible with what we enumerated in the EnumFormat param when we
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connected.
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Let's take a look at how we can parse the format in the `param_changed`
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event:
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\code{.c}
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static void on_param_changed(void *userdata, uint32_t id, const struct spa_pod *param)
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{
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struct data *data = userdata;
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if (param == NULL || id != SPA_PARAM_Format)
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return;
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\endcode
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First check if there is a param. A NULL param means that it is cleared. The ID
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of the param tells you what param it is. We are only interested in Format
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param (`SPA_PARAM_Format`).
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We can parse the media type and subtype as below and ensure that it is
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of the right type. In our example this will always be true but when your
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EnumFormat contains different media types or subtypes, this is how you can
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parse them:
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\code{.c}
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if (spa_format_parse(param,
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&data->format.media_type,
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&data->format.media_subtype) < 0)
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return;
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if (data->format.media_type != SPA_MEDIA_TYPE_video ||
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data->format.media_subtype != SPA_MEDIA_SUBTYPE_raw)
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return;
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\endcode
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For the `video/raw` media type/subtype there is a utility function to
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parse out the values into a `struct spa_video_info`. This makes it easier
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to deal with.
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\code{.c}
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if (spa_format_video_raw_parse(param, &data->format.info.raw) < 0)
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return;
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printf("got video format:\n");
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printf(" format: %d (%s)\n", data->format.info.raw.format,
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spa_debug_type_find_name(spa_type_video_format,
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data->format.info.raw.format));
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printf(" size: %dx%d\n", data->format.info.raw.size.width,
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data->format.info.raw.size.height);
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printf(" framerate: %d/%d\n", data->format.info.raw.framerate.num,
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data->format.info.raw.framerate.denom);
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/** prepare to render video of this size */
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}
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\endcode
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In this example we dump the video size and parameters but in a real playback
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or capture application you might want to set up the screen or encoder to
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deal with the format.
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After negotiation, the process function is called for each new frame. Check out
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\ref page_tutorial4 for another example.
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\snippet tutorial5.c on_process
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In a real playback application, one would do something with the data, like
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copy it to the screen or encode it into a file.
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\ref page_tutorial4 | \ref page_tutorial "Index" | \ref page_tutorial6
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*/
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