Compute responsive sizes attribute based on the width from the boundingClientRect in captured URL Metrics

plugins/image-prioritizer/class-image-prioritizer-img-tag-visitor.php

@@ -146,21 +146,44 @@ private function process_img( OD_HTML_Tag_Processor $processor, OD_Tag_Visitor_C
 			$processor->remove_attribute( 'fetchpriority' );
 		}
 
-		// Ensure that sizes=auto is set properly.
-		$sizes = $processor->get_attribute( 'sizes' );
-		if ( is_string( $sizes ) ) {
+		// Ensure that sizes is set properly when it is a responsive image (it has a srcset attribute).
+		if ( is_string( $processor->get_attribute( 'srcset' ) ) ) {
+			$sizes = $processor->get_attribute( 'sizes' );
+			if ( ! is_string( $sizes ) ) {
+				$sizes = '';
+			}
+
 			$is_lazy  = 'lazy' === $this->get_attribute_value( $processor, 'loading' );
 			$has_auto = $this->sizes_attribute_includes_valid_auto( $sizes );
 
 			if ( $is_lazy && ! $has_auto ) {
-				$processor->set_attribute( 'sizes', "auto, $sizes" );
+				$new_sizes = 'auto';
+				if ( '' !== trim( $sizes, " \t\f\r\n" ) ) {
+					$new_sizes .= ', ';
+				}
+				$sizes = $new_sizes . $sizes;
 			} elseif ( ! $is_lazy && $has_auto ) {
 				// Remove auto from the beginning of the list.
-				$processor->set_attribute(
-					'sizes',
-					(string) preg_replace( '/^[ \t\f\r\n]*auto[ \t\f\r\n]*(,[ \t\f\r\n]*)?/i', '', $sizes )
-				);
+				$sizes = (string) preg_replace( '/^[ \t\f\r\n]*auto[ \t\f\r\n]*(,[ \t\f\r\n]*)?/i', '', $sizes );
+			}
+
+			// Compute more accurate sizes when it isn't lazy-loaded and sizes=auto isn't taking care of it.
+			if ( ! $is_lazy ) {
+				$computed_sizes = $this->compute_sizes( $context );
+				if ( count( $computed_sizes ) > 0 ) {
+					$new_sizes = join( ', ', $computed_sizes );
+
+					// Preserve the original sizes as a fallback when URL Metrics are missing from one or more viewport group.
+					// Note that when all groups are populated, the media features will span all possible viewport widths from
+					// zero to infinity, so there is no need to include the original sizes since they will never match.
+					if ( '' !== $sizes && ! $context->url_metric_group_collection->is_every_group_populated() ) {
+						$new_sizes .= ", $sizes";
+					}
+					$sizes = $new_sizes;
+				}
 			}
+
+			$processor->set_attribute( 'sizes', $sizes );
 		}
 
 		$parent_tag = $this->get_parent_tag_name( $context );
@@ -385,4 +408,38 @@ private function sizes_attribute_includes_valid_auto( string $sizes_attr ): bool
 			return 'auto' === $sizes_attr || str_starts_with( $sizes_attr, 'auto,' );
 		}
 	}
+
+	/**
+	 * Computes responsive sizes for the current element based on its boundingClientRect width captured in URL Metrics.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param OD_Tag_Visitor_Context $context Context.
+	 * @return non-empty-string[] Computed sizes.
+	 */
+	private function compute_sizes( OD_Tag_Visitor_Context $context ): array {
+		$sizes = array();
+
+		$xpath = $context->processor->get_xpath();
+		foreach ( $context->url_metric_group_collection as $group ) {
+			// Obtain the maximum width that the image appears among all URL Metrics collected for this viewport group.
+			$element_max_width = 0;
+			foreach ( $group->get_xpath_elements_map()[ $xpath ] ?? array() as $element ) {
+				$element_max_width = max( $element_max_width, $element->get_bounding_client_rect()['width'] );
+			}
+
+			// Use the maximum width as the size for image in this breakpoint.
+			if ( $element_max_width > 0 ) {
+				$size          = sprintf( '%dpx', $element_max_width );
+				$media_feature = od_generate_media_query( $group->get_minimum_viewport_width(), $group->get_maximum_viewport_width() );
+				if ( null !== $media_feature ) {
+					// Note: The null case only happens when a site has filtered od_breakpoint_max_widths to be an empty array, meaning there is only one viewport group.
+					$size = "$media_feature $size";
+				}
+				$sizes[] = $size;
+			}
+		}
+
+		return $sizes;
+	}
 }

plugins/image-prioritizer/readme.txt

@@ -7,7 +7,7 @@ License:      GPLv2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         performance, optimization, image, lcp, lazy-load
 
-Prioritizes the loading of images and videos based on how visible they are to actual visitors; adds fetchpriority and applies lazy loading.
+Prioritizes the loading of images and videos based on how they appear to actual visitors: adds fetchpriority, preloads, lazy-loads, and sets sizes.
 
 == Description ==
 
@@ -27,7 +27,9 @@ The current optimizations include:
    1. Apply lazy loading to `IMG` tags based on whether they appear in any breakpoint’s initial viewport.
    2. Implement lazy loading of CSS background images added via inline `style` attributes.
    3. Lazy-load `VIDEO` tags by setting the appropriate attributes based on whether they appear in the initial viewport. If a `VIDEO` is the LCP element, it gets `preload=auto`; if it is in an initial viewport, the `preload=metadata` default is left; if it is not in an initial viewport, it gets `preload=none`. Lazy-loaded videos also get initial `preload`, `autoplay`, and `poster` attributes restored when the `VIDEO` is going to enter the viewport.
-5. Ensure that [`sizes=auto`](https://make.wordpress.org/core/2024/10/18/auto-sizes-for-lazy-loaded-images-in-wordpress-6-7/) is added to all lazy-loaded `IMG` elements.
+5. Responsive image sizes:
+   1. Ensure [`sizes=auto`](https://make.wordpress.org/core/2024/10/18/auto-sizes-for-lazy-loaded-images-in-wordpress-6-7/) is added to all lazy-loaded `IMG` elements.
+   2. Compute the `sizes` attribute using the widths of an image collected from URL Metrics for each breakpoint (when not lazy-loaded since then handled by `sizes=auto`).
 6. Reduce the size of the `poster` image of a `VIDEO` from full size to the size appropriate for the maximum width of the video (on desktop).
 
 **This plugin requires the [Optimization Detective](https://wordpress.org/plugins/optimization-detective/) plugin as a dependency.** Please refer to that plugin for additional background on how this plugin works as well as additional developer options.

plugins/image-prioritizer/tests/test-cases/background-image-outside-viewport-on-all-breakpoints-but-not-desktop-with-fully-populated-sample-data/set-up.php

@@ -10,13 +10,6 @@ static function () use ( $breakpoint_max_widths ) {
 		}
 	);
 
-	$outside_viewport_rect = array_merge(
-		$test_case->get_sample_dom_rect(),
-		array(
-			'top' => 100000,
-		)
-	);
-
 	foreach ( $breakpoint_max_widths as $non_desktop_viewport_width ) {
 		for ( $i = 0; $i < $sample_size; $i++ ) {
 			OD_URL_Metrics_Post_Type::store_url_metric(
@@ -29,8 +22,7 @@ static function () use ( $breakpoint_max_widths ) {
 								'xpath'              => '/HTML/BODY/DIV[@id=\'page\']/*[2][self::DIV]',
 								'isLCP'              => false,
 								'intersectionRatio'  => 0.0,
-								'intersectionRect'   => $outside_viewport_rect,
-								'boundingClientRect' => $outside_viewport_rect,
+								'boundingClientRect' => array( 'top' => 100000 ),
 							),
 						),
 					)

plugins/image-prioritizer/tests/test-cases/background-image-outside-viewport-with-desktop-metrics-missing/set-up.php

@@ -9,13 +9,6 @@ static function () use ( $breakpoint_max_widths ) {
 		}
 	);
 
-	$outside_viewport_rect = array_merge(
-		$test_case->get_sample_dom_rect(),
-		array(
-			'top' => 100000,
-		)
-	);
-
 	foreach ( $breakpoint_max_widths as $non_desktop_viewport_width ) {
 		OD_URL_Metrics_Post_Type::store_url_metric(
 			od_get_url_metrics_slug( od_get_normalized_query_vars() ),
@@ -27,8 +20,7 @@ static function () use ( $breakpoint_max_widths ) {
 							'xpath'              => '/HTML/BODY/DIV[@id=\'page\']/*[2][self::DIV]',
 							'isLCP'              => false,
 							'intersectionRatio'  => 0.0,
-							'intersectionRect'   => $outside_viewport_rect,
-							'boundingClientRect' => $outside_viewport_rect,
+							'boundingClientRect' => array( 'top' => 100000 ),
 						),
 					),
 				)

plugins/image-prioritizer/tests/test-cases/common-lcp-background-image-and-lazy-loaded-background-image-outside-viewport-with-fully-populated-sample-data/set-up.php

@@ -1,12 +1,5 @@
 <?php
 return static function ( Test_Image_Prioritizer_Helper $test_case ): void {
-	$outside_viewport_rect = array_merge(
-		$test_case->get_sample_dom_rect(),
-		array(
-			'top' => 100000,
-		)
-	);
-
 	$test_case->populate_url_metrics(
 		array(
 			array(
@@ -17,15 +10,13 @@
 				'xpath'              => '/HTML/BODY/DIV[@id=\'page\']/*[3][self::DIV]',
 				'isLCP'              => false,
 				'intersectionRatio'  => 0.0,
-				'intersectionRect'   => $outside_viewport_rect,
-				'boundingClientRect' => $outside_viewport_rect,
+				'boundingClientRect' => array( 'top' => 100000 ),
 			),
 			array(
 				'xpath'              => '/HTML/BODY/DIV[@id=\'page\']/*[4][self::DIV]',
 				'isLCP'              => false,
 				'intersectionRatio'  => 0.0,
-				'intersectionRect'   => $outside_viewport_rect,
-				'boundingClientRect' => $outside_viewport_rect,
+				'boundingClientRect' => array( 'top' => 100000 ),
 			),
 		)
 	);

plugins/image-prioritizer/tests/test-cases/common-lcp-image-and-lazy-loaded-image-outside-viewport-with-fully-populated-sample-data/buffer.html

@@ -11,9 +11,9 @@
 				<img src="https://example.com/foo3.jpg" alt="Foo" width="1200" height="800" fetchpriority="high" srcset="https://example.com/foo3-480w.jpg 480w, https://example.com/foo3-800w.jpg 800w" sizes="(max-width: 600px) 480px, 800px">
 			</div>
 			<p>Pretend this is a super long paragraph that pushes the next image mostly out of the initial viewport.</p>
-			<img src="https://example.com/bar.jpg" alt="Bar" width="10" height="10" fetchpriority="high" loading="lazy">
+			<img src="https://example.com/bar.jpg" alt="Bar" width="1024" height="768" fetchpriority="high" loading="lazy" srcset="https://example.com/bar-300w.jpg 300w, https://example.com/bar-480w.jpg 480w, https://example.com/bar-800w.jpg 800w, https://example.com/bar-900w.jpg 900w, https://example.com/bar-1000w.jpg 1000w" sizes="(max-width: 1024px) 100vw, 1024px">
 			<p>Now the following image is definitely outside the initial viewport.</p>
-			<img src="https://example.com/baz.jpg" alt="Baz" width="10" height="10" fetchpriority="high">
+			<img src="https://example.com/baz.jpg" alt="Baz" width="3000" height="1500" fetchpriority="high" srcset="https://example.com/baz-300w.jpg 300w, https://example.com/baz-480w.jpg 480w, https://example.com/baz-800w.jpg 800w, https://example.com/baz-900w.jpg 900w, https://example.com/baz-1000w.jpg 1000w, https://example.com/baz-1500w.jpg 1500w, https://example.com/baz-2000w.jpg 2000w, https://example.com/baz-2500w.jpg 2500w" sizes="(max-width: 3000px) 100vw, 3000px">
 			<img src="https://example.com/qux.jpg" alt="Qux" width="10" height="10" fetchpriority="high" loading="eager">
 			<img src="https://example.com/quux.jpg" alt="Quux" width="10" height="10" loading="LAZY"><!-- This one is all good. -->
 		</div>

plugins/image-prioritizer/tests/test-cases/common-lcp-image-and-lazy-loaded-image-outside-viewport-with-fully-populated-sample-data/set-up.php

@@ -1,13 +1,7 @@
 <?php
 return static function ( Test_Image_Prioritizer_Helper $test_case ): void {
-	$slug                  = od_get_url_metrics_slug( od_get_normalized_query_vars() );
-	$sample_size           = od_get_url_metrics_breakpoint_sample_size();
-	$outside_viewport_rect = array_merge(
-		$test_case->get_sample_dom_rect(),
-		array(
-			'top' => 100000,
-		)
-	);
+	$slug        = od_get_url_metrics_slug( od_get_normalized_query_vars() );
+	$sample_size = od_get_url_metrics_breakpoint_sample_size();
 	foreach ( array_merge( od_get_breakpoint_max_widths(), array( 1000 ) ) as $viewport_width ) {
 		for ( $i = 0; $i < $sample_size; $i++ ) {
 			OD_URL_Metrics_Post_Type::store_url_metric(
@@ -31,31 +25,31 @@
 								'intersectionRatio' => 0.0, // Subsequent carousel slide.
 							),
 							array(
-								'xpath'             => '/HTML/BODY/DIV[@id=\'page\']/*[3][self::IMG]',
-								'isLCP'             => false,
-								'intersectionRatio' => 0 === $i ? 0.5 : 0.0, // Make sure that the _max_ intersection ratio is considered.
+								'xpath'              => '/HTML/BODY/DIV[@id=\'page\']/*[3][self::IMG]',
+								'isLCP'              => false,
+								'intersectionRatio'  => 0 === $i ? 0.5 : 0.0, // Make sure that the _max_ intersection ratio is considered.
+								'boundingClientRect' => array(
+									'width' => $viewport_width - 10,
+								),
 							),
 							// All are outside all initial viewports.
 							array(
 								'xpath'              => '/HTML/BODY/DIV[@id=\'page\']/*[5][self::IMG]',
 								'isLCP'              => false,
 								'intersectionRatio'  => 0.0,
-								'intersectionRect'   => $outside_viewport_rect,
-								'boundingClientRect' => $outside_viewport_rect,
+								'boundingClientRect' => array( 'top' => 100000 ),
 							),
 							array(
 								'xpath'              => '/HTML/BODY/DIV[@id=\'page\']/*[6][self::IMG]',
 								'isLCP'              => false,
 								'intersectionRatio'  => 0.0,
-								'intersectionRect'   => $outside_viewport_rect,
-								'boundingClientRect' => $outside_viewport_rect,
+								'boundingClientRect' => array( 'top' => 100000 ),
 							),
 							array(
 								'xpath'              => '/HTML/BODY/DIV[@id=\'page\']/*[7][self::IMG]',
 								'isLCP'              => false,
 								'intersectionRatio'  => 0.0,
-								'intersectionRect'   => $outside_viewport_rect,
-								'boundingClientRect' => $outside_viewport_rect,
+								'boundingClientRect' => array( 'top' => 100000 ),
 							),
 						),
 					)