Setup Audio Resampler with swr_alloc_set_opts2

This article explains how to configure and initialize an audio resampling context in FFmpeg using the modern swr_alloc_set_opts2 function. It covers setting input and output parameters, allocating the context, and properly initializing the resampler to handle sample rate, sample format, and channel layout conversions.

Understanding `swr_alloc_set_opts2`

In modern versions of FFmpeg (version 5.1 and later), swr_alloc_set_opts2 replaces the deprecated swr_alloc_set_opts function. The primary difference is the migration from legacy channel layout bitmasks to the modern AVChannelLayout structure.

The function signature is defined as follows:

int swr_alloc_set_opts2(struct SwrContext **ps,
                        const AVChannelLayout *out_ch_layout,
                        enum AVSampleFormat out_sample_fmt, int out_sample_rate,
                        const AVChannelLayout *in_ch_layout,
                        enum AVSampleFormat in_sample_fmt, int in_sample_rate,
                        int log_offset, void *log_ctx);

Step-by-Step Implementation

Setting up the resampler involves declaring channel layouts, calling the allocation function, and initializing the context.

1. Define Input and Output Parameters

Set up the target formats, sample rates, and channel layouts. AVChannelLayout structures must be initialized properly.

#include <libswresample/swresample.h>
#include <libavutil/channel_layout.h>
#include <libavutil/pixfmt.h>

// Define input parameters
AVChannelLayout in_ch_layout = AV_CHANNEL_LAYOUT_STEREO;
enum AVSampleFormat in_sample_fmt = AV_SAMPLE_FMT_FLTP; // Float planar
int in_sample_rate = 48000;

// Define output parameters (e.g., converting to Mono, S16 interleaved)
AVChannelLayout out_ch_layout = AV_CHANNEL_LAYOUT_MONO;
enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16; // Signed 16-bit packed
int out_sample_rate = 44100;

2. Allocate and Configure the Context

Initialize a pointer to SwrContext to NULL. Pass its address to swr_alloc_set_opts2 along with your configuration parameters.

SwrContext *swr_ctx = NULL;

int ret = swr_alloc_set_opts2(&swr_ctx,
                             &out_ch_layout, out_sample_fmt, out_sample_rate,
                             &in_ch_layout, in_sample_fmt, in_sample_rate,
                             0, NULL);

if (ret < 0) {
    // Handle allocation error
    fprintf(stderr, "Failed to allocate SwrContext\n");
    return ret;
}

3. Initialize the Context

After allocation, you must explicitly initialize the context using swr_init before performing any resampling operations.

ret = swr_init(swr_ctx);
if (ret < 0) {
    fprintf(stderr, "Failed to initialize the resampling context\n");
    swr_free(&swr_ctx);
    return ret;
}

4. Clean Up Resources

When you are finished with the resampler, free the allocated context using swr_free to prevent memory leaks.

swr_free(&swr_ctx);

Complete Code Example

Below is a complete, minimal code block demonstrating the entire setup and teardown process.

#include <stdio.h>
#include <libswresample/swresample.h>
#include <libavutil/channel_layout.h>

int main(void) {
    SwrContext *swr_ctx = NULL;
    int ret;

    // Define layouts
    AVChannelLayout in_layout = AV_CHANNEL_LAYOUT_STEREO;
    AVChannelLayout out_layout = AV_CHANNEL_LAYOUT_MONO;

    // Allocate the context
    ret = swr_alloc_set_opts2(&swr_ctx,
                             &out_layout, AV_SAMPLE_FMT_S16, 44100,
                             &in_layout, AV_SAMPLE_FMT_FLTP, 48000,
                             0, NULL);
    if (ret < 0) {
        fprintf(stderr, "Could not allocate resampler context\n");
        return ret;
    }

    // Initialize the context
    ret = swr_init(swr_ctx);
    if (ret < 0) {
        fprintf(stderr, "Could not initialize resampler context\n");
        swr_free(&swr_ctx);
        return ret;
    }

    printf("Audio resampler context successfully initialized.\n");

    // Free the context when done
    swr_free(&swr_ctx);
    return 0;
}