diff --git a/plugins/webp-uploads/hooks.php b/plugins/webp-uploads/hooks.php
index 98a27aa485..fd8c6833af 100644
--- a/plugins/webp-uploads/hooks.php
+++ b/plugins/webp-uploads/hooks.php
@@ -803,3 +803,77 @@ function webp_uploads_enable_additional_mime_type_support_for_all_sizes( array $
return $allowed_sizes;
}
add_filter( 'webp_uploads_image_sizes_with_additional_mime_type_support', 'webp_uploads_enable_additional_mime_type_support_for_all_sizes' );
+
+/**
+ * Converts palette PNG images to truecolor PNG images.
+ *
+ * GD cannot convert palette-based PNG to WebP/AVIF formats, causing conversion failures.
+ * This function detects and converts palette PNG to truecolor during upload.
+ *
+ * @since n.e.x.t
+ *
+ * @param array<string, mixed>|mixed $file The uploaded file data.
+ * @return array<string, mixed> The modified file data.
+ */
+function webp_uploads_convert_palette_png_to_truecolor( $file ): array {
+ if ( ! is_array( $file ) ) {
+ $file = array();
+ }
+ if ( ! isset( $file['tmp_name'], $file['name'] ) ) {
+ return $file;
+ }
+ if ( isset( $file['type'] ) && is_string( $file['type'] ) ) {
+ if ( 'image/png' !== strtolower( $file['type'] ) ) {
+ return $file;
+ }
+ } elseif ( 'image/png' !== wp_check_filetype( $file['name'] )['type'] ) {
+ return $file;
+ }
+
+ $editor = wp_get_image_editor( $file['tmp_name'] );
+
+ if ( is_wp_error( $editor ) || ! $editor instanceof WP_Image_Editor_GD ) {
+ return $file;
+ }
+
+ // Check if required GD functions exist.
+ if (
+ ! function_exists( 'imagecreatefrompng' ) ||
+ ! function_exists( 'imageistruecolor' ) ||
+ ! function_exists( 'imagealphablending' ) ||
+ ! function_exists( 'imagesavealpha' ) ||
+ ! function_exists( 'imagepalettetotruecolor' ) ||
+ ! function_exists( 'imagepng' ) ||
+ ! function_exists( 'imagedestroy' )
+ ) {
+ return $file;
+ }
+
+ $image = imagecreatefrompng( $file['tmp_name'] );
+
+ // Check if the image was created successfully.
+ if ( false === $image ) {
+ return $file;
+ }
+
+ // Check if the image is already truecolor.
+ if ( imageistruecolor( $image ) ) {
+ imagedestroy( $image );
+ return $file;
+ }
+
+ // Preserve transparency.
+ imagealphablending( $image, false );
+ imagesavealpha( $image, true );
+
+ // Convert the palette to truecolor.
+ if ( imagepalettetotruecolor( $image ) ) {
+ // Overwrite the upload with the new truecolor PNG.
+ imagepng( $image, $file['tmp_name'] );
+ }
+ imagedestroy( $image );
+
+ return $file;
+}
+add_filter( 'wp_handle_upload_prefilter', 'webp_uploads_convert_palette_png_to_truecolor' );
+add_filter( 'wp_handle_sideload_prefilter', 'webp_uploads_convert_palette_png_to_truecolor' );
diff --git a/plugins/webp-uploads/tests/data/images/dice-palette.png b/plugins/webp-uploads/tests/data/images/dice-palette.png
new file mode 100644
index 0000000000..e269d0996a
Binary files /dev/null and b/plugins/webp-uploads/tests/data/images/dice-palette.png differ
diff --git a/plugins/webp-uploads/tests/test-load.php b/plugins/webp-uploads/tests/test-load.php
index b78048f78b..0459060bb8 100644
--- a/plugins/webp-uploads/tests/test-load.php
+++ b/plugins/webp-uploads/tests/test-load.php
@@ -1116,4 +1116,152 @@ public function test_that_it_should_convert_webp_to_avif_on_upload(): void {
}
wp_delete_attachment( $attachment_id );
}
+
+ /**
+ * Tests that the `webp_uploads_convert_palette_png_to_truecolor` function is hooked to the upload filters.
+ */
+ public function test_webp_uploads_convert_palette_png_to_truecolor_hooks(): void {
+ $this->assertSame( 10, has_filter( 'wp_handle_upload_prefilter', 'webp_uploads_convert_palette_png_to_truecolor' ) );
+ $this->assertSame( 10, has_filter( 'wp_handle_sideload_prefilter', 'webp_uploads_convert_palette_png_to_truecolor' ) );
+ }
+
+ /**
+ * Tests converting a palette PNG to a truecolor PNG.
+ *
+ * @dataProvider data_to_test_webp_uploads_convert_palette_png_to_truecolor
+ *
+ * @covers ::webp_uploads_convert_palette_png_to_truecolor
+ *
+ * @param string|null $image_path The path to the image file to test.
+ * @param bool $expect_changed Whether the png should be converted to truecolor.
+ */
+ public function test_webp_uploads_convert_palette_png_to_truecolor( ?string $image_path, bool $expect_changed ): void {
+ add_filter(
+ 'wp_image_editors',
+ static function () {
+ return array( 'WP_Image_Editor_GD' );
+ }
+ );
+
+ // Temp file will be copied and unlinked by WordPress core during sideload processing.
+ $tmp_file = wp_tempnam();
+ copy( $image_path, $tmp_file );
+ $file = array(
+ 'name' => basename( $image_path ),
+ 'tmp_name' => $tmp_file,
+ 'type' => wp_check_filetype( $image_path )['type'],
+ 'size' => filesize( $tmp_file ),
+ 'error' => UPLOAD_ERR_OK,
+ );
+
+ // Store the original file hash and the original file size for later comparison.
+ $original_file_hash = isset( $file['tmp_name'] ) ? md5_file( $file['tmp_name'] ) : '';
+ $original_file_size = (int) filesize( $file['tmp_name'] );
+
+ // This will trigger the `wp_handle_sideload_prefilter` filter.
+ $attachment_id = media_handle_sideload( $file );
+
+ try {
+ $this->assertIsNumeric( $attachment_id );
+
+ // For getting a original image path for computation of the file hash.
+ $meta = wp_get_attachment_metadata( $attachment_id );
+ $upload_dir = wp_get_upload_dir();
+ $path = null;
+ if ( isset( $meta['original_image'], $meta['file'] ) ) {
+ $path = path_join(
+ $upload_dir['basedir'],
+ dirname( $meta['file'] ) . '/' . $meta['original_image']
+ );
+ }
+ $this->assertNotNull( $path );
+ $this->assertFileExists( $path );
+
+ // Hash will be modified if the image was converted to truecolor.
+ $modified_file_hash = md5_file( $path );
+
+ if ( ! $expect_changed ) {
+ $this->assertSame( $original_file_hash, $modified_file_hash );
+ } else {
+ $this->assertNotSame( $original_file_hash, $modified_file_hash );
+ $img = imagecreatefrompng( $path );
+ $this->assertTrue( imageistruecolor( $img ) );
+ imagedestroy( $img );
+
+ // Make sure the image converted to modern image format is not 0 bytes.
+ $modern_image_format_path = get_attached_file( $attachment_id );
+ $this->assertNotFalse( $modern_image_format_path );
+ $this->assertFileExists( $modern_image_format_path );
+ $modern_image_format_filesize = (int) filesize( $modern_image_format_path );
+ $this->assertGreaterThan( 0, $modern_image_format_filesize );
+
+ // Ensure the file size of the converted image is less than or equal to the original indexed PNG file size.
+ $this->assertLessThanOrEqual( $original_file_size, $modern_image_format_filesize );
+ }
+ } finally {
+ wp_delete_attachment( $attachment_id );
+ }
+ }
+
+ /**
+ * Data provider for `test_webp_uploads_convert_palette_png_to_truecolor`.
+ *
+ * @return array<string, mixed> Returns an array of test cases.
+ */
+ public function data_to_test_webp_uploads_convert_palette_png_to_truecolor(): array {
+ $non_palette_png = TESTS_PLUGIN_DIR . '/tests/data/images/dice.png';
+ $palette_png = TESTS_PLUGIN_DIR . '/tests/data/images/dice-palette.png';
+ $test_jpg = TESTS_PLUGIN_DIR . '/tests/data/images/leaves.jpg';
+
+ return array(
+ 'wrong_extension' => array(
+ 'image_path' => $test_jpg,
+ 'expected_changed' => false,
+ ),
+ 'non_palette_png' => array(
+ 'image_path' => $non_palette_png,
+ 'expected_changed' => false,
+ ),
+ 'palette_png' => array(
+ 'image_path' => $palette_png,
+ 'expected_changed' => true,
+ ),
+ );
+ }
+
+ /**
+ * Tests the webp_uploads_convert_palette_png_to_truecolor function with various conditions.
+ *
+ * @covers ::webp_uploads_convert_palette_png_to_truecolor
+ */
+ public function test_webp_uploads_convert_palette_png_to_truecolor_conditions(): void {
+ $this->assertSameSets( array(), webp_uploads_convert_palette_png_to_truecolor( 'test' ) );
+ $this->assertSameSets( array(), webp_uploads_convert_palette_png_to_truecolor( array() ) );
+
+ $file = array(
+ 'tmp_name' => TESTS_PLUGIN_DIR . '/tests/data/images/leaves.jpg',
+ 'name' => 'leaves.jpg',
+ 'type' => 'image/jpeg',
+ );
+ $this->assertSameSets( $file, webp_uploads_convert_palette_png_to_truecolor( $file ) );
+
+ $file = array(
+ 'tmp_name' => TESTS_PLUGIN_DIR . '/tests/data/images/leaves.jpg',
+ 'name' => 'leaves.jpg',
+ );
+ $this->assertSameSets( $file, webp_uploads_convert_palette_png_to_truecolor( $file ) );
+
+ add_filter(
+ 'wp_image_editors',
+ static function () {
+ return array();
+ }
+ );
+ $file = array(
+ 'tmp_name' => TESTS_PLUGIN_DIR . '/tests/data/images/dice-palette.png',
+ 'name' => 'dice-palette.png',
+ 'type' => 'image/png',
+ );
+ $this->assertSameSets( $file, webp_uploads_convert_palette_png_to_truecolor( $file ) );
+ }
}