From 1dba638698cb283f52fe6d59848951e7951136fd Mon Sep 17 00:00:00 2001
From: Sukhendu Sekhar Guria <sukhenduguria2017@gmail.com>
Date: Tue, 14 Jul 2026 15:36:44 +0530
Subject: [PATCH] fix(media): prevent Imagick pre-sampling upscale

---
 .../class-wp-image-editor-imagick.php         | 19 +++-
 tests/phpunit/tests/image/editorImagick.php   | 93 +++++++++++++++++++
 2 files changed, 107 insertions(+), 5 deletions(-)

diff --git a/src/wp-includes/class-wp-image-editor-imagick.php b/src/wp-includes/class-wp-image-editor-imagick.php
index 2cb3a694c567b..73cadbba3bc0b 100644
--- a/src/wp-includes/class-wp-image-editor-imagick.php
+++ b/src/wp-includes/class-wp-image-editor-imagick.php
@@ -492,15 +492,24 @@ protected function thumbnail_image( $dst_w, $dst_h, $filter_name = 'FILTER_TRIAN
 
 			/*
 			 * To be more efficient, resample large images to 5x the destination size before resizing
-			 * whenever the output size is less that 1/3 of the original image size (1/3^2 ~= .111),
-			 * unless we would be resampling to a scale smaller than 128x128.
+			 * whenever the output size is less than 1/3 of the original image size (1/3^2 ~= 0.111),
+			 * but only when resampling reduces both source dimensions and keeps both dimensions above 128px.
 			 */
 			if ( is_callable( array( $this->image, 'sampleImage' ) ) ) {
-				$resize_ratio  = ( $dst_w / $this->size['width'] ) * ( $dst_h / $this->size['height'] );
+				$source_size   = $this->image->getImageGeometry();
+				$resize_ratio  = ( $dst_w / $source_size['width'] ) * ( $dst_h / $source_size['height'] );
 				$sample_factor = 5;
+				$sample_width  = $dst_w * $sample_factor;
+				$sample_height = $dst_h * $sample_factor;
 
-				if ( $resize_ratio < .111 && ( $dst_w * $sample_factor > 128 && $dst_h * $sample_factor > 128 ) ) {
-					$this->image->sampleImage( $dst_w * $sample_factor, $dst_h * $sample_factor );
+				if (
+					$resize_ratio < 0.111
+					&& $sample_width > 128
+					&& $sample_height > 128
+					&& $sample_width < $source_size['width']
+					&& $sample_height < $source_size['height']
+				) {
+					$this->image->sampleImage( $sample_width, $sample_height );
 				}
 			}
 
diff --git a/tests/phpunit/tests/image/editorImagick.php b/tests/phpunit/tests/image/editorImagick.php
index e120c32502ad5..1009ef1d11fb4 100644
--- a/tests/phpunit/tests/image/editorImagick.php
+++ b/tests/phpunit/tests/image/editorImagick.php
@@ -65,6 +65,99 @@ public function test_resize() {
 		);
 	}
 
+	/**
+	 * Tests that pre-sampling does not upscale the source image.
+	 *
+	 * @ticket 48842
+	 */
+	public function test_thumbnail_image_presampling_does_not_upscale() {
+		$image = new class() {
+			public $source_size;
+			public $sample_size;
+
+			// phpcs:disable WordPress.NamingConventions.ValidFunctionName.MethodNameInvalid
+			public function getImageGeometry() {
+				return $this->source_size;
+			}
+
+			public function sampleImage( $width, $height ) {
+				$this->sample_size = array( $width, $height );
+			}
+
+			public function scaleImage() {
+			}
+			// phpcs:enable
+		};
+
+		$editor = new class( null ) extends WP_Image_Editor_Imagick {
+			public function call_thumbnail_image( $image, $source_size, $editor_size, $destination_size ) {
+				$image->source_size = array(
+					'width'  => $source_size[0],
+					'height' => $source_size[1],
+				);
+				$image->sample_size = null;
+				$this->image        = $image;
+				$this->size         = array(
+					'width'  => $editor_size[0],
+					'height' => $editor_size[1],
+				);
+
+				$result = $this->thumbnail_image( $destination_size[0], $destination_size[1], 'FILTER_TRIANGLE', false );
+
+				return is_wp_error( $result ) ? $result : $image->sample_size;
+			}
+		};
+
+		// The ticket's source and destination dimensions must not trigger an upscale.
+		$this->assertNull(
+			$editor->call_thumbnail_image(
+				$image,
+				array( 10315, 7049 ),
+				array( 10315, 7049 ),
+				array( 2560, 1750 )
+			)
+		);
+
+		// Use current geometry because cropping can make the editor's stored size stale.
+		$this->assertNull(
+			$editor->call_thumbnail_image(
+				$image,
+				array( 200, 1000 ),
+				array( 1000, 1000 ),
+				array( 50, 100 )
+			)
+		);
+
+		// Skip pre-sampling unless both intermediate dimensions are smaller than the source.
+		$this->assertNull(
+			$editor->call_thumbnail_image(
+				$image,
+				array( 1500, 1000 ),
+				array( 1500, 1000 ),
+				array( 300, 150 )
+			)
+		);
+		$this->assertNull(
+			$editor->call_thumbnail_image(
+				$image,
+				array( 2000, 750 ),
+				array( 2000, 750 ),
+				array( 300, 150 )
+			)
+		);
+
+		// Keep pre-sampling when both intermediate dimensions are smaller than the source.
+		$this->assertSame(
+			array( 1500, 750 ),
+			$editor->call_thumbnail_image(
+				$image,
+				array( 4800, 2400 ),
+				array( 4800, 2400 ),
+				array( 300, 150 )
+			)
+		);
+	}
+
 	/**
 	 * Tests multi_resize() with single image resize and no crop.
 	 */