WordPress srcset is a great feature but there are occasions when it’s important to control exactly which image sizes are included in the srcset code.
For example, on one of our sites we generate a custom image size which is ONLY intended for external use – this means an external sales portal collects this image automatically for display on their site. It is NOT intended for display anywhere on our site.
However, the WordPress srcset functionality currently includes ALL image sizes. For example:
<img width="350" height="426" src="https://xxx/wp-content/uploads/omega-pink-gold-1235-1-350x426.jpg" class="attachment-shop_catalog size-shop_catalog" alt="xxx" srcset="https://xxx/.../omega-pink-gold-1235-1-350x426.jpg 350w, https://xxx/.../omega-pink-gold-1235-1-246x300.jpg 246w, https://xxx/.../omega-pink-gold-1235-1-768x935.jpg 768w, https://xxx/.../omega-pink-gold-1235-1-841x1024.jpg 841w, https://xxx/.../omega-pink-gold-1235-1-1051x1280.jpg 1051w, https://xxx/.../omega-pink-gold-1235-1-534x650.jpg 534w, https://xxx/.../omega-pink-gold-1235-1-104x127.jpg 104w, https://xxx/.../omega-pink-gold-1235-1-595x725.jpg 595w, https://xxx/.../omega-pink-gold-1235-1-680x828.jpg 680w" sizes="(max-width: 350px) 100vw, 350px" title="xxx">
How can we control this better?
Thanks in advance for any help!
2 Answers
WordPress allows you to hook into wp_calculate_image_srcset
See Core file on Trac
This filter gives you 5 arguments:
- $sources One or more arrays of source data to include in the ‘srcset’
- $size_array Array of width and height values in pixels (in that order).
- $image_src The ‘src’ of the image.
- $image_meta The image meta data as returned by ‘wp_get_attachment_metadata()
- $attachment_id the post_id from the image
Here is a sample code you can use within your themes function.php or plugin files to get started:
add_filter( 'wp_calculate_image_srcset', 'my_custom_image_srcset', 10, 5);
function my_custom_image_srcset($sources, $size_array, $image_src, $image_meta, $attachment_id) {
// The following code is an adaption from wp-includes/media.php:1061-1180
$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];
$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 '1600'.
* @param array $size_array Array of width and height values in pixels (in that order).
*/
$max_srcset_image_width = apply_filters( 'max_srcset_image_width', 1600, $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 $identifier => $image ) {
// Continue if identifier is unwanted
if ($identifier === 'unwanted-identifier') {
continue;
}
$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 = $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;
}
}
}
return $sources;
}
