Source

File: wp-includes/media.php

function wp_calculate_image_srcset( $size_array, $image_src, $image_meta, $attachment_id = 0 ) {
	/**
	 * Let plugins pre-filter the image meta to be able to fix inconsistencies in the stored data.
	 *
	 * @since 4.5.0
	 *
	 * @param array  $image_meta    The image meta data as returned by 'wp_get_attachment_metadata()'.
	 * @param int[]  $size_array    {
	 *     An array of requested width and height values.
	 *
	 *     @type int $0 The width in pixels.
	 *     @type int $1 The height in pixels.
	 * }
	 * @param string $image_src     The 'src' of the image.
	 * @param int    $attachment_id The image attachment ID or 0 if not supplied.
	 */
	$image_meta = apply_filters( 'wp_calculate_image_srcset_meta', $image_meta, $size_array, $image_src, $attachment_id );

	if ( empty( $image_meta['sizes'] ) || ! isset( $image_meta['file'] ) || strlen( $image_meta['file'] ) < 4 ) {
		return false;
	}

	$image_sizes = $image_meta['sizes'];

	// Get the width and height of the image.
	$image_width  = (int) $size_array[0];
	$image_height = (int) $size_array[1];

	// Bail early if error/no width.
	if ( $image_width < 1 ) {
		return false;
	}

	$image_basename = wp_basename( $image_meta['file'] );

	/*
	 * WordPress flattens animated GIFs into one frame when generating intermediate sizes.
	 * To avoid hiding animation in user content, if src is a full size GIF, a srcset attribute is not generated.
	 * If src is an intermediate size GIF, the full size is excluded from srcset to keep a flattened GIF from becoming animated.
	 */
	if ( ! isset( $image_sizes['thumbnail']['mime-type'] ) || 'image/gif' !== $image_sizes['thumbnail']['mime-type'] ) {
		$image_sizes[] = array(
			'width'  => $image_meta['width'],
			'height' => $image_meta['height'],
			'file'   => $image_basename,
		);
	} elseif ( strpos( $image_src, $image_meta['file'] ) ) {
		return false;
	}

	// Retrieve the uploads sub-directory from the full size image.
	$dirname = _wp_get_attachment_relative_path( $image_meta['file'] );

	if ( $dirname ) {
		$dirname = trailingslashit( $dirname );
	}

	$upload_dir    = wp_get_upload_dir();
	$image_baseurl = trailingslashit( $upload_dir['baseurl'] ) . $dirname;

	/*
	 * If currently on HTTPS, prefer HTTPS URLs when we know they're supported by the domain
	 * (which is to say, when they share the domain name of the current request).
	 */
	if ( is_ssl() && 'https' !== substr( $image_baseurl, 0, 5 ) && parse_url( $image_baseurl, PHP_URL_HOST ) === $_SERVER['HTTP_HOST'] ) {
		$image_baseurl = set_url_scheme( $image_baseurl, 'https' );
	}

	/*
	 * Images that have been edited in WordPress after being uploaded will
	 * contain a unique hash. Look for that hash and use it later to filter
	 * out images that are leftovers from previous versions.
	 */
	$image_edited = preg_match( '/-e[0-9]{13}/', wp_basename( $image_src ), $image_edit_hash );

	/**
	 * Filters the maximum image width to be included in a 'srcset' attribute.
	 *
	 * @since 4.4.0
	 *
	 * @param int   $max_width  The maximum image width to be included in the 'srcset'. Default '2048'.
	 * @param int[] $size_array {
	 *     An array of requested width and height values.
	 *
	 *     @type int $0 The width in pixels.
	 *     @type int $1 The height in pixels.
	 * }
	 */
	$max_srcset_image_width = apply_filters( 'max_srcset_image_width', 2048, $size_array );

	// Array to hold URL candidates.
	$sources = array();

	/**
	 * To make sure the ID matches our image src, we will check to see if any sizes in our attachment
	 * meta match our $image_src. If no matches are found we don't return a srcset to avoid serving
	 * an incorrect image. See #35045.
	 */
	$src_matched = false;

	/*
	 * Loop through available images. Only use images that are resized
	 * versions of the same edit.
	 */
	foreach ( $image_sizes as $image ) {
		$is_src = false;

		// Check if image meta isn't corrupted.
		if ( ! is_array( $image ) ) {
			continue;
		}

		// If the file name is part of the `src`, we've confirmed a match.
		if ( ! $src_matched && false !== strpos( $image_src, $dirname . $image['file'] ) ) {
			$src_matched = true;
			$is_src      = true;
		}

		// Filter out images that are from previous edits.
		if ( $image_edited && ! strpos( $image['file'], $image_edit_hash[0] ) ) {
			continue;
		}

		/*
		 * Filters out images that are wider than '$max_srcset_image_width' unless
		 * that file is in the 'src' attribute.
		 */
		if ( $max_srcset_image_width && $image['width'] > $max_srcset_image_width && ! $is_src ) {
			continue;
		}

		// If the image dimensions are within 1px of the expected size, use it.
		if ( wp_image_matches_ratio( $image_width, $image_height, $image['width'], $image['height'] ) ) {
			// Add the URL, descriptor, and value to the sources array to be returned.
			$source = array(
				'url'        => $image_baseurl . $image['file'],
				'descriptor' => 'w',
				'value'      => $image['width'],
			);

			// The 'src' image has to be the first in the 'srcset', because of a bug in iOS8. See #35030.
			if ( $is_src ) {
				$sources = array( $image['width'] => $source ) + $sources;
			} else {
				$sources[ $image['width'] ] = $source;
			}
		}
	}

	/**
	 * Filters an image's 'srcset' sources.
	 *
	 * @since 4.4.0
	 *
	 * @param array  $sources {
	 *     One or more arrays of source data to include in the 'srcset'.
	 *
	 *     @type array $width {
	 *         @type string $url        The URL of an image source.
	 *         @type string $descriptor The descriptor type used in the image candidate string,
	 *                                  either 'w' or 'x'.
	 *         @type int    $value      The source width if paired with a 'w' descriptor, or a
	 *                                  pixel density value if paired with an 'x' descriptor.
	 *     }
	 * }
	 * @param array $size_array     {
	 *     An array of requested width and height values.
	 *
	 *     @type int $0 The width in pixels.
	 *     @type int $1 The height in pixels.
	 * }
	 * @param string $image_src     The 'src' of the image.
	 * @param array  $image_meta    The image meta data as returned by 'wp_get_attachment_metadata()'.
	 * @param int    $attachment_id Image attachment ID or 0.
	 */
	$sources = apply_filters( 'wp_calculate_image_srcset', $sources, $size_array, $image_src, $image_meta, $attachment_id );

	// Only return a 'srcset' value if there is more than one source.
	if ( ! $src_matched || ! is_array( $sources ) || count( $sources ) < 2 ) {
		return false;
	}

	$srcset = '';

	foreach ( $sources as $source ) {
		$srcset .= str_replace( ' ', '%20', $source['url'] ) . ' ' . $source['value'] . $source['descriptor'] . ', ';
	}

	return rtrim( $srcset, ', ' );
}