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;
}
srcset? Are you sending the full<img>tag to the external sales portal instead of just the image URL?