Filter PNG image quality in WordPress

png_quality_filter.php

<?php

// well-known filter to change JPG quality:
add_filter( 'jpeg_quality', function( $arg ){ return 100; } );

// lesser-known filter to change quality for any image type:
add_filter( 'wp_editor_set_quality', 'any_image_quality', 10, 2 );
add_filter( 'jpeg_quality', 'any_image_quality' );

function any_image_quality( $default_quality, $mime_type = NULL ) {
  // you could do if ( 'image/png' == $mime_type ) here if you want to be specific
  return 100;
}