Regenerates Thumbnail PDFs to ensure that PDFs without a background are filled with white and not black.

Wordrpess PDF Thumbnail Fix

add_filter( 'wp_generate_attachment_metadata', 'tdp_create_pdf_thumbnail', 10, 2 );
function tdp_create_pdf_thumbnail( $metadata, $attachment_id ) {

    //Get the attachment/post object
    $attachment_obj = get_post( $attachment_id );

    //Check for mime type pdf
    if( 'application/pdf' == get_post_mime_type( $attachment_obj ) ) {
 
         //Get attachment URL http://yousite.org/wp-content/uploads/yourfile.pdf
        $attachment_url = wp_get_attachment_url($attachment_id);
        
        //Get attachment path /some/folder/on/server/wp-content/uploads/yourfile.pdf
        $attachment_path = get_attached_file($attachment_id );
        $attachment_filename = basename ( get_attached_file($attachment_id) );

        //By adding [0] the first page gets selected, important because otherwise multi paged files wont't work
        $pdf_source = $attachment_path.'[0]';
        
        //Thumbnail format
        $tn_format = 'jpg';
             
        // Get All Wordpress Image Sizes
        global $_wp_additional_image_sizes;
        
        $media_sizes = get_intermediate_image_sizes();
        
        // Setup to overwrite the Full Size thumbnail
        array_push($media_sizes, "full");
        
        // Loop through all Images Sizes
        foreach ( $media_sizes as $_size ) {
          if ( in_array( $_size, array('thumbnail', 'medium', 'medium_large', 'large') ) ) {
            
            $width  = get_option($_size . '_size_w');
            $height = get_option($_size . '_size_h');
            $crop   = (bool) get_option( $_size . '_crop' );
          	
          } else if(in_array( $_size, array('full') )){

            $width  = 0;
          	$height = 1080;
          	$crop   = 0;
          	
          } elseif ( isset( $_wp_additional_image_sizes[ $_size ] ) ) {

            $width  = $_wp_additional_image_sizes[ $_size ]['width'];
          	$height = $_wp_additional_image_sizes[ $_size ]['height'];
          	$crop   = $_wp_additional_image_sizes[ $_size ]['crop'];
          	
          }
                
          $img = new Imagick();
          $img->setResolution(200,200);
          $img->readImage($pdf_source);
          $img->setImageBackgroundColor('white');
          $img->setImageAlphaChannel(11); // Imagick::ALPHACHANNEL_REMOVE
          $img = $img->mergeImageLayers(Imagick::LAYERMETHOD_FLATTEN);
          
          if( $height != 0 && $width != 0){
            $img->scaleImage(0,$height);
            
          } else if( $height != 0 || $width == 0){
            $img->scaleImage(0,$height);
           
          } else if( $height == 0 || $width !== 0){
            $img->scaleImage($width,0);
             
          } else {
            $img->scaleImage(1000,0);
          }
          
          $imgWidth  = $img->getImageWidth();
          $imgHeight = $img->getImageHeight();
          
          $img->setImageFormat('jpg');
          
          
          if($_size === 'full'){
            // Thumbnail output as path + size + format
            $thumb_out = substr($attachment_path, 0, -4).'-pdf.'.$tn_format;
            $thumb_filename = substr($attachment_filename, 0, -4).'-pdf.'.$tn_format;
          } else {
            // Thumbnail output as path + size + format
            $thumb_out = substr($attachment_path, 0, -4).'-pdf-'.$imgWidth.'x'.$imgHeight.'.'.$tn_format;
            $thumb_filename = substr($attachment_filename, 0, -4).'-pdf-'.$imgWidth.'x'.$imgHeight.'.'.$tn_format;
          }
          
          // Write new image
          $img->writeImages($thumb_out, false);
      
      
          //Add thumbnail URL as metadata of pdf attachment
          $metadata['sizes'][$_size]['file'] = $thumb_filename;
          $metadata['sizes'][$_size]['width'] = $imgWidth;
          $metadata['sizes'][$_size]['height'] = $imgHeight;
          $metadata['sizes'][$_size]['mime-type'] = 'image/jpeg';
    
        }
        

    }

    return $metadata;

}