- * Allocate SwrContext.
- * @see swr_init(),swr_free()
- * @return NULL on error
+ * Allocate SwrContext if needed and set/reset common parameters.
+ *
+ * This function does not require s to be allocated with swr_alloc(). On the
+ * other hand, swr_alloc() can use swr_alloc_set_opts() to set the parameters
+ * on the allocated context.
+ *
+ * @param s Swr context, can be NULL
+ * @param out_ch_layout output channel layout (AV_CH_LAYOUT_*)
+ * @param out_sample_fmt output sample format (AV_SAMPLE_FMT_*).
+ * @param out_sample_rate output sample rate (frequency in Hz)
+ * @param in_ch_layout input channel layout (AV_CH_LAYOUT_*)
+ * @param in_sample_fmt input sample format (AV_SAMPLE_FMT_*).
+ * @param in_sample_rate input sample rate (frequency in Hz)
+ * @param log_offset logging level offset
+ * @param log_ctx parent logging context, can be NULL
+ *
+ * @see swr_init(), swr_free()
+ * @return NULL on error, allocated context otherwise