How does WP media uploader create the 3 different sized images, and how can I duplicate it

I have finally!! got this thing I’ve tried about 12 times to make and 12 different ways, but finally got it to work,… sort of.

I made a custom metabox for uploading and attaching images to posts, and it doesn’t require you to use the horrible thickbox media uploader built into WP. I hate that thing. No, what I’ve done is just a group of inputs (title, description, file), which you can also duplicate, to add additional attachments if you want. So you fill in the fields, select an image to upload, and save draft or publish the post. Once there are attachments added to a post, the metabox will display the input fields, along with a preview image of the attached image for each attachment you added. The title and description fields are used to generate the file meta data, nothing is saved as the post_meta, that I know of. That’s currently all that I have gotten working so far.

I need to make it so that when you save/publish a post, inturn upload/create the attachment files, it will create the three image sizes as the default wp uploader would, thumbnail, medium, large, and keeping the full size image too. If that’s possible somehow. If not, I would like to otherwise use add_image_size() to create/define new custom sizes, and generate them that way instead, upon uploading.

I’m not sure which function is the most ideal to use in this case, maybe the image_make_intermediate_size() function would be better, or wp_create_thumbnail() or wp_crop_image()… who knows!!

I cannot figure out how to go about doing that, if I need to run the wp_handle_upload() function for each one, or maybe something involving the wp_generate_attachment_metadata() function. It’s confusing to me since the 3 image sizes are to be associated as variants of the same attachment, and how to go about doing that.

I have scoured the web, read the source of every wp media/upload/image related file, and played with just about every function there is for the media upload stuff and cannot find how WP creates the 3 image sizes anywhere, or how to do it myself.

In wp-includes/media.php the image_resize() function looks like it would be best since it’s exactly what it should be. I just can’t figure out for the life of me what the hell I’m missing or have tried doing but did wrong to make the image thumbnails.

Here’s my working function that does the wp_handle_upload() stuff and things, but it also needs to create the 100px thumb, and make a resize version of the image that is max-width like 500px, and saved as new files of the uploaded one.

function update_attachment(){
  global $post;

  wp_update_attachment_metadata( $post->ID, $_POST['a_image'] );

  if( !empty( $_FILES['a_image']['name'] )) { //New upload
    require_once( ABSPATH . 'wp-admin/includes/file.php' );

    $override['action'] = 'editpost';
    $url = wp_handle_upload( $_FILES['a_image'], $override );

    // $medium = image_make_intermediate_size( $uploaded_file['url'], 500, 400, true );
    // $thumb = = image_make_intermediate_size( $uploaded_file['url'], 100, 100, true );

    if ( isset( $file['error'] )) {
      return new WP_Error( 'upload_error', $file['error'] );
    }

    $array_type = wp_check_filetype
    $allowed_file_types = array('image/jpg','image/jpeg','image/gif','image/png');

    $name_parts = pathinfo( $name );
    $name = trim( substr( $name, 0, - ( 1 + strlen( $name_parts['extension'] )) ));

    $type = $file['type'];
    $file = $file['file'];
    $title = $_POST['a_title'] ? $_POST['a_title'] : $name;
    $content = $_POST['a_desc']

    $post_id = $post->ID;
    $attachment = array(
      'post_title' => $title,
      'post_type' => 'attachment',
      'post_content' => $content,
      'post_parent' => $post_id,
      'post_mime_type' => $type,
      'guid' => $url['url']
    );


    // Save the data
    $id = wp_insert_attachment( $attachment, $_FILES['a_image'][ 'file' ]/*, $post_id - for post_thumbnails*/);

    if ( !is_wp_error( $id )) {
      $attach_meta = wp_generate_attachment_metadata( $id, $uploaded_file['url'] );
      wp_update_attachment_metadata( $attach_id, $attach_meta );
    }
    update_post_meta( $post->ID, 'a_image', $uploaded_file['url'] );
  }
}

Anyone able to help me finally fix this so it works proper would be loved. I’ve spent so many ridiculous countless hours numerous different times trying to develop this thing and the documentation sucks, and there’s not really any good posts anywhere on how to do it.

Thanks