Source

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

public function resize( $max_w, $max_h, $crop = false ) {
		if ( ( $this->size['width'] == $max_w ) && ( $this->size['height'] == $max_h ) ) {
			return true;
		}

		$dims = image_resize_dimensions( $this->size['width'], $this->size['height'], $max_w, $max_h, $crop );
		if ( ! $dims ) {
			return new WP_Error( 'error_getting_dimensions', __( 'Could not calculate resized image dimensions' ) );
		}

		list( $dst_x, $dst_y, $src_x, $src_y, $dst_w, $dst_h, $src_w, $src_h ) = $dims;

		if ( $crop ) {
			return $this->crop( $src_x, $src_y, $src_w, $src_h, $dst_w, $dst_h );
		}

		// Execute the resize.
		$thumb_result = $this->thumbnail_image( $dst_w, $dst_h );
		if ( is_wp_error( $thumb_result ) ) {
			return $thumb_result;
		}

		return $this->update_size( $dst_w, $dst_h );
	}