scep2

Hi Michael

Thank you very much. This works like a charm. If someone has the same need, here is the complete code.

CSS

span.unearned_achievement {
	opacity: .4;
}

PHP (WordPress-Template)

<?php
$posts = get_field('badges'); // This is a custom field generated with the Advanced Custom Fields plugin

if( $posts ): ?>
	<?php $earned_achievements = badgeos_get_user_earned_achievement_ids( get_current_user_id() ); ?>
	<?php foreach( $posts as $post): // variable must be called $post (IMPORTANT) ?>
		<?php setup_postdata($post); ?>
		<a href="<?php echo get_permalink( $post->ID ) ?>" target="_new">
			<?php
			if (in_array($post->ID, $earned_achievements)) {
				echo badgeos_get_achievement_post_thumbnail( $post->ID );
			} else {
				echo "<span class='unearned_achievement'>"; // This allows for changing opacity with CSS
				echo badgeos_get_achievement_post_thumbnail( $post->ID );
				echo "</span>";
			}
			?>
		</a>
		<?php wp_reset_postdata(); // IMPORTANT - reset the $post object so the rest of the page works correctly ?>
	<?php endforeach; ?>
<?php endif; ?>