PNG quality not handled with GD

[Iulia Cazan]

The quality of the PNG images is not handled correctly with GD (in fact at all). To make this work it would be required two small changes:

1. wp-includes/class-wp-image-editor-gd.php @ line 433 - pass the quality argument for the function used to make the image

   <?php
   if ( ! $this->make_image( $filename, 'imagepng', array( $image, $filename, $this->get_quality() ) ) ) {
           return new WP_Error( 'image_save_error', __( 'Image Editor Save Failed' ) );
   }
   

2. wp-includes/class-wp-image-editor.php @ line 261 - append the quality verification in case the image is PNG (the quality range is 1 -> 9, not 1 -> 100 like for JPEG )

   <?php
   if ( 'image/png' == $this->mime_type ) {
           if ( $quality > 9 ) {
                   $quality = ceil( $quality / 10 );
                   $quality = ( 10 >= $quality ) ? 9 : $quality;
           }
   }
   

[birgire]

Thank you for the ticket and proposed patch @iulia-cazan

According to the PHP docs there's also the 0 for no compression and -1 for default zlib compression level (6). Also mentioned there the inverse scale mapping.

There's another older ticket #24380 about this topic, so please join the conversation there.

So I'm marking this one as a duplicate.