A helper function to calculate the image sources to include in a ‘srcset’ attribute.
Parameters
$size_array
int[]required- An array of width and height values.
0
intThe width in pixels.1
intThe height in pixels.
$image_src
stringrequired- The
'src'
of the image. $image_meta
arrayrequired- The image meta data as returned by ‘wp_get_attachment_metadata() ‘.
$attachment_id
intoptional- The image attachment ID. Default 0.
Source
* @return string|false The 'srcset' attribute value. False on error or when only one source exists.
*/
function wp_calculate_image_srcset( $size_array, $image_src, $image_meta, $attachment_id = 0 ) {
/**
* Pre-filters 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 ( str_contains( $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() && ! str_starts_with( $image_baseurl, 'https' ) ) {
/*
* Since the `Host:` header might contain a port, it should
* be compared against the image URL using the same port.
*/
$parsed = parse_url( $image_baseurl );
$domain = isset( $parsed['host'] ) ? $parsed['host'] : '';
if ( isset( $parsed['port'] ) ) {
$domain .= ':' . $parsed['port'];
}
if ( $_SERVER['HTTP_HOST'] === $domain ) {
$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 && str_contains( $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.
*/
Changelog
Version | Description |
---|---|
4.4.0 | Introduced. |
Note that only image sizes matching the aspect ratio for the original image will be returned by `wp_calculate_image_srcset() ` since the `srcset` attribute is only meant to be used for resolution switching, not changing aspect ratios at different viewport widths (often referred to as the “art direction” use case). See: http://usecases.responsiveimages.org/.