How to set different thumbnail sizes per custom post type?

read https://codex.www.ads-software.com/Function_Reference/add_image_size

@keesiemeijer, tx

As i read it every image is being resized, right?

In my case i want the resizing to be related to the custom post type something like

function alter_image(){
	global $post;
	if($post->post_type == 'reviews') {
	add_image_size( 'review-thumbnail', 150, 200, true );
	}
}
add_action('edit','alter_image');

But that didn’t work. Just adding
add_image_size( 'review-thumbnail', 150, 200, true );
works but now every uploaded image has a ‘review-thumbnail’

Any suggestions?

if you have multiple post types, maybe this will help
(not tested)(in the loop)

<?php
$post_type = get_post_type(); // get's the post type of a post in the loop
$args=array(
  'public'   => true,
  '_builtin' => false
);
$output = 'names'; // names or objects
$operator = 'and'; // 'and' or 'or'
$post_types = get_post_types($args,$output,$operator); // gets all post types you registered
$post_types = array_values($post_types); // makes it a numerically indexed array

if(in_array( $post_type, $post_types)){ // post is a custom post type -> show post type size thumbnail
  $custom_post_thumbnail = get_the_post_thumbnail($post->ID, $post_type.'-thumbnail');
  if($custom_post_thumbnail != ''){ // there is a custom post thumbnail size
    echo $custom_post_thumbnail;
  } else { // no custom post thumbnail size -> show normal thumbnail
    echo get_the_post_thumbnail($post->ID);
  }
} else { // show normal thumbnail for posts and pages
  echo get_the_post_thumbnail($post->ID);
}
?>

You still have to add the image sizes manually in your theme’s functions.php
If you have custom post types review and movies:

add_image_size( 'review-thumbnail', 150, 200, true );
add_image_size( 'movies-thumbnail', 400, 9999 );

And for the default size for normal post thumbnails use: https://codex.www.ads-software.com/Function_Reference/set_post_thumbnail_size

if ( function_exists( 'add_theme_support' ) ) {
	add_theme_support( 'post-thumbnails' );
        set_post_thumbnail_size( 150, 150 ); // default Post Thumbnail dimensions
}

if ( function_exists( 'add_image_size' ) ) {
	add_image_size( 'review-thumbnail', 150, 200, true );
        add_image_size( 'movies-thumbnail', 400, 9999 );
}

Hello keesiemeijeer!

I guess that your snippet does not prevent to be generated images in this custom sizes for each image uploaded.

There is the point.