How can i use wp_get_image_editor for image resizing

Question

A possible solution:

Goal: Don’t use add_image_size() to show a custom sized image.

If you calculate the image without saving it, all requests will sum up to a huge page load due to repeated unnecessary processing time. So caching or saving the results should be an additional goal.

Add a image-processor.php in your template/plugin:

//Load your WordPress enviroment
$path = dirname(__FILE__);
$path = substr($path, 0, stripos($path, "wp-content") );
require( $path . "wp-load.php");

//TODO: filter and handle your $_GET params
$post_id = absint($_GET['post_id']);

$testing_img_url = wp_get_attachment_image_url(
    get_post_thumbnail_id($post->ID), 'full'
);

if (!$testing_img_url)
    $testing_img_url="http://domain.tld/path/to/defaul-image.jpg";

$img = wp_get_image_editor( $testing_img_url );

if ( ! is_wp_error( $img ) ) {
    header('Content-type:'.finfo_file ($testing_img_url));
    $img->resize( 500, NULL, false );
    $img->set_quality( 100 );
    $img->stream();
}

You can display the image in a loop now like this:

Template:

<img src="https://wordpress.stackexchange.com/questions/191522/<?= get_template_directory_uri()?>/image-processor.php?post_id=<?= get_the_ID()?>">

Plugin:

<img src="https://wordpress.stackexchange.com/questions/191522/<?= plugin_dir_path(__FILE__)?>image-processor.php?post_id=<?= get_the_ID()?>">

Possible improvements:

Caching/saving the processed images.
Better error handling.