Image Post Format: URL Auto Discovery

I’ve been geeking out with the new post formats feature in WordPress 3.1 for the past few weeks and I’ve decided to make a theme that supports all of the different formats. I’m just about have through them and I would like to share a neat trick that I used to enable users to post images a bit faster.

The Image Post Format

The WordPress Codex suggests that an image format is:

A single image. The first <img > tag in the post should be considered the image. Alternatively, if the post consists only of a URL, that will be the image URL and the title of the post (post_title) will be the title attribute for the image.

This is pretty straight forward, and I was intrigued by the possibility of using the content to hold only a url to an image. If you could just copy and paste the url to an image into the post content, it would really speed up the workflow.

I won’t go into too many details but will say that this has passed all of my tests. If you’ve got a minute to test it out, please let me know your thoughts!

<?php
/*
 * Filter content for posts having an image format.
 *
 * This filter will allow authors to post only a url to an image
 * in the post content and have the image display properly in the
 * theme as an img tag.
 *
 * An "alt" attribute will always be generated for each image. An
 * author can specify the value of the alt attribute by creating
 * a custom field with a key of "mfields_alt". If no custom field
 * is available, an empty string will be used.
 *
 * The title attribute of the img tag will be populated by the value
 * produced by get_the_title(). In the event that there is no post
 * title available no title attribute will be printed.
 *
 * Child themes may customize the attributes generated for each image
 * by registering a custom function for the
 * 'mfields-image-format-attributes' filter. An example follows:
 *
 * This filter will add a class of "mysite" to all images hosted at
 * the same domain as WordPress is installed on.
 *
 * <code>
 * function mytheme_filter_image_format_attributes( $atts, $src ) {
 *     if ( false !== strstr( $src, site_url( '', 'http' ) ) ) {
 *         if ( isset( $atts['class'] ) ) {
 *             $atts['class'] = (array) $atts['class'];
 *         }
 *         $atts['class'][] = 'mysite';
 *     }
 *     return $atts;
 * }
 * add_filter( 'mfields-image-format-attributes', 'mytheme_filter_image_format_attributes', 10, 2 );
 * </code>
 *
 * @param     string    Post content.
 * @return    string    HTML img tag if all conditions are met, unfiltered content otherwise.
 *
 * @since     1.0
 */
function mfields_filter_content_for_image_format( $content ) {
	if ( 'image' == get_post_format() ) {
		$src = esc_url( $content );
		$gis = @getimagesize( $src );
		if ( ! empty( $src ) && is_array( $gis ) ) {
			$attributes = array();
			$post_id = get_the_ID();
			$title = get_the_title();
			$defaults = array(
				'alt'    => get_post_meta( $post_id, 'mfields_alt', true ),
				'width'  => $gis[0],
				'height' => $gis[1]
				);
			if ( ! empty( $title ) ) {
				$defaults['title'] = $title;
			}
			$filtered = apply_filters( 'mfields-image-format-attributes', $defaults, $src, $gis );
			$atts = array_merge( $defaults, $filtered );
			foreach( $atts as $name => $value ) {
				if ( in_array( $name, array( 'id', 'class', 'alt', 'title', 'height', 'width', 'longdesc', 'style' ) ) ) {
					if ( 'class' == $name && is_array( $value ) ) {
						$value = implode( ' ', $value );
					}
					$attributes[] = $name . '="' . esc_attr( $value ) . '"';
				}
			}
			$content = '<img src="' . $src . '" ' . implode( ' ', $attributes ) . '>';
		}
	}
	return $content;
}
add_filter( 'the_content', 'mfields_filter_content_for_image_format', 0 );
?>