Change srcset when rtAmazon-s3 plugin is active. Use at your own RISK.

functions.php

<?php

/**
 * Add this function into theme's functions.php
 *
 * This function will replace domain with amazon s3 bucket url for image's srcset. 
 */
function rtawss3_wp_calculate_image_srcset_latin( $sources, $size_array, $image_src, $image_meta, $attachment_id ) {

	if ( ! class_exists( 'RTAWSS3_Class' ) ) {
		return;
	}

	// Get keys from rtAmazon setting.
	$rtawss3 = RTAWSS3_Class::instance();
	$rtawss3_default_option = $rtawss3->rtawss3_get_access_keys();

	// If keys are not added, then no need to proceed.
	if ( empty( $rtawss3_default_option ) ) {
		return;
	}

	// Where clause for query.
	$where = array(
		'parent_id' => intval( $attachment_id ),
	);


	// Get cached data.
	$rows = wp_cache_get( 'rt_amazon_s3_srcset_query_' . $attachment_id );

	// Get thumbs.
	if( false === $rows ) {
		$rows = $rtawss3->rtawss3_db( 'get_results', array(), 'status', 'moved', false, array(), $where );
		wp_cache_set( 'rt_amazon_s3_srcset_query_' . $attachment_id, $rows, '', WEEK_IN_SECONDS );
	}

	// total image sizes.
	$image_sizes = $image_meta['sizes'];

	// Get cached data.
	$sources_new = wp_cache_get( 'rt_amazon_s3_source_' . $attachment_id );

	// Get new srcset.
	if ( is_array( $image_sizes ) && ! empty( $image_sizes ) && false === $sources_new ) {
		foreach ( $image_sizes as $key => $value ) {
			foreach ( $rows as $row ) {
				if ( $key === $row->media_size && ! empty( $row->s3_url ) ) {
					$sources_new[ $value['width'] ] = array(
						'url' => $row->s3_url,
						'descriptor' => 'x',
						'value' => $value['width'],
					);
				}
			}
		}

		// Set data into cache.
		wp_cache_set( 'rt_amazon_s3_source_' . $attachment_id, $sources_new, '', WEEK_IN_SECONDS );
	}

	if ( empty( $sources_new ) ) {
		return $sources;
	}

	return $sources_new;
}

add_filter( 'wp_calculate_image_srcset', 'rtawss3_wp_calculate_image_srcset_latin', 99, 5 );