Source

File: wp-includes/class-wp-image-editor-imagick.php

public function set_quality( $quality = null ) {
		$quality_result = parent::set_quality( $quality );
		if ( is_wp_error( $quality_result ) ) {
			return $quality_result;
		} else {
			$quality = $this->get_quality();
		}

		try {
			if ( 'image/jpeg' === $this->mime_type ) {
				$this->image->setImageCompressionQuality( $quality );
				$this->image->setImageCompression( imagick::COMPRESSION_JPEG );
			} else {
				$this->image->setImageCompressionQuality( $quality );
			}
		} catch ( Exception $e ) {
			return new WP_Error( 'image_quality_error', $e->getMessage() );
		}

		return true;
	}