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Source code

function image_get_intermediate_size($post_id, $size='thumbnail') {

	if ( !is_array( $imagedata = wp_get_attachment_metadata( $post_id ) ) )

		return false;



	// get the best one for a specified set of dimensions

	if ( is_array($size) && !empty($imagedata['sizes']) ) {

		foreach ( $imagedata['sizes'] as $_size => $data ) {

			// already cropped to width or height; so use this size

			if ( ( $data['width'] == $size[0] && $data['height'] <= $size[1] ) || ( $data['height'] == $size[1] && $data['width'] <= $size[0] ) ) {

				$file = $data['file'];

				list($width, $height) = image_constrain_size_for_editor( $data['width'], $data['height'], $size );

				return compact( 'file', 'width', 'height' );

			}

			// add to lookup table: area => size

			$areas[$data['width'] * $data['height']] = $_size;

		}

		if ( !$size || !empty($areas) ) {

			// find for the smallest image not smaller than the desired size

			ksort($areas);

			foreach ( $areas as $_size ) {

				$data = $imagedata['sizes'][$_size];

				if ( $data['width'] >= $size[0] || $data['height'] >= $size[1] ) {

					// Skip images with unexpectedly divergent aspect ratios (crops)

					// First, we calculate what size the original image would be if constrained to a box the size of the current image in the loop

					$maybe_cropped = image_resize_dimensions($imagedata['width'], $imagedata['height'], $data['width'], $data['height'], false );

					// If the size doesn't match within one pixel, then it is of a different aspect ratio, so we skip it, unless it's the thumbnail size

					if ( 'thumbnail' != $_size && ( !$maybe_cropped || ( $maybe_cropped[4] != $data['width'] && $maybe_cropped[4] + 1 != $data['width'] ) || ( $maybe_cropped[5] != $data['height'] && $maybe_cropped[5] + 1 != $data['height'] ) ) )

						continue;

					// If we're still here, then we're going to use this size

					$file = $data['file'];

					list($width, $height) = image_constrain_size_for_editor( $data['width'], $data['height'], $size );

					return compact( 'file', 'width', 'height' );

				}

			}

		}

	}



	if ( is_array($size) || empty($size) || empty($imagedata['sizes'][$size]) )

		return false;



	$data = $imagedata['sizes'][$size];

	// include the full filesystem path of the intermediate file

	if ( empty($data['path']) && !empty($data['file']) ) {

		$file_url = wp_get_attachment_url($post_id);

		$data['path'] = path_join( dirname($imagedata['file']), $data['file'] );

		$data['url'] = path_join( dirname($file_url), $data['file'] );

	}

	return $data;

}