doc: add DSP filter tutorial

Add CLAUDE generated tutorial7 based on the audio-dsp-filter
example.

doc/examples/tutorial7.c

@@ -0,0 +1,152 @@
+/*
+  [title]
+  \ref page_tutorial7
+  [title]
+ */
+/* [code] */
+#include <stdio.h>
+#include <errno.h>
+#include <math.h>
+#include <signal.h>
+
+#include <spa/pod/builder.h>
+#include <spa/param/latency-utils.h>
+
+#include <pipewire/pipewire.h>
+#include <pipewire/filter.h>
+
+struct data;
+
+struct port {
+	struct data *data;
+};
+
+struct data {
+	struct pw_main_loop *loop;
+	struct pw_filter *filter;
+	struct port *in_port;
+	struct port *out_port;
+};
+
+/* [on_process] */
+static void on_process(void *userdata, struct spa_io_position *position)
+{
+	struct data *data = userdata;
+	float *in, *out;
+	uint32_t n_samples = position->clock.duration;
+
+	pw_log_trace("do process %d", n_samples);
+
+	in = pw_filter_get_dsp_buffer(data->in_port, n_samples);
+	out = pw_filter_get_dsp_buffer(data->out_port, n_samples);
+
+	if (in == NULL || out == NULL)
+		return;
+
+	/* Simple passthrough - copy input to output.
+	 * Here you could implement any audio processing:
+	 * - Filters (lowpass, highpass, bandpass)
+	 * - Effects (reverb, delay, distortion)
+	 * - Dynamic processing (compressor, limiter)
+	 * - Equalization
+	 * - etc.
+	 */
+	memcpy(out, in, n_samples * sizeof(float));
+}
+/* [on_process] */
+
+static const struct pw_filter_events filter_events = {
+	PW_VERSION_FILTER_EVENTS,
+	.process = on_process,
+};
+
+static void do_quit(void *userdata, int signal_number)
+{
+	struct data *data = userdata;
+	pw_main_loop_quit(data->loop);
+}
+
+int main(int argc, char *argv[])
+{
+	struct data data = { 0, };
+	const struct spa_pod *params[1];
+	uint32_t n_params = 0;
+	uint8_t buffer[1024];
+	struct spa_pod_builder b = SPA_POD_BUILDER_INIT(buffer, sizeof(buffer));
+
+	pw_init(&argc, &argv);
+
+	/* make a main loop. If you already have another main loop, you can add
+	 * the fd of this pipewire mainloop to it. */
+	data.loop = pw_main_loop_new(NULL);
+
+	pw_loop_add_signal(pw_main_loop_get_loop(data.loop), SIGINT, do_quit, &data);
+	pw_loop_add_signal(pw_main_loop_get_loop(data.loop), SIGTERM, do_quit, &data);
+
+	/* Create a simple filter, the simple filter manages the core and remote
+	 * objects for you if you don't need to deal with them.
+	 *
+	 * Pass your events and a user_data pointer as the last arguments. This
+	 * will inform you about the filter state. The most important event
+	 * you need to listen to is the process event where you need to process
+	 * the data.
+	 */
+	data.filter = pw_filter_new_simple(
+			pw_main_loop_get_loop(data.loop),
+			"audio-filter",
+			pw_properties_new(
+				PW_KEY_MEDIA_TYPE, "Audio",
+				PW_KEY_MEDIA_CATEGORY, "Filter",
+				PW_KEY_MEDIA_ROLE, "DSP",
+				NULL),
+			&filter_events,
+			&data);
+
+	/* make an audio DSP input port */
+	data.in_port = pw_filter_add_port(data.filter,
+			PW_DIRECTION_INPUT,
+			PW_FILTER_PORT_FLAG_MAP_BUFFERS,
+			sizeof(struct port),
+			pw_properties_new(
+				PW_KEY_FORMAT_DSP, "32 bit float mono audio",
+				PW_KEY_PORT_NAME, "input",
+				NULL),
+			NULL, 0);
+
+	/* make an audio DSP output port */
+	data.out_port = pw_filter_add_port(data.filter,
+			PW_DIRECTION_OUTPUT,
+			PW_FILTER_PORT_FLAG_MAP_BUFFERS,
+			sizeof(struct port),
+			pw_properties_new(
+				PW_KEY_FORMAT_DSP, "32 bit float mono audio",
+				PW_KEY_PORT_NAME, "output",
+				NULL),
+			NULL, 0);
+
+	/* Set processing latency information */
+	params[n_params++] = spa_process_latency_build(&b,
+			SPA_PARAM_ProcessLatency,
+			&SPA_PROCESS_LATENCY_INFO_INIT(
+				.ns = 10 * SPA_NSEC_PER_MSEC
+			));
+
+	/* Now connect this filter. We ask that our process function is
+	 * called in a realtime thread. */
+	if (pw_filter_connect(data.filter,
+				PW_FILTER_FLAG_RT_PROCESS,
+				params, n_params) < 0) {
+		fprintf(stderr, "can't connect\n");
+		return -1;
+	}
+
+	/* and wait while we let things run */
+	pw_main_loop_run(data.loop);
+
+	pw_filter_destroy(data.filter);
+	pw_main_loop_destroy(data.loop);
+	pw_deinit();
+
+	return 0;
+}
+/* [code] */