Generation of ground-truths is of great importance for unbiased performance evaluation of document layout analysis methods. This is especially necessary because many methods are claimed to be skew-tolerant. However, experimental evaluation of this fact is often based only on human subjective judgment and restricted to a few experiments. The main obstacle for obtaining human-independent and more automated performance evaluation is that usually there are only ground-truths for upright images, i.e., images with no skew of text lines, because currently available ground-truthing techniques are too time-consuming. In this paper, we propose a new methodology of automatic generation of ground-truths for skewed images by using the ground-truths available for upright images. This methodology is simple and quite fast because processing is done at the level of small square blocks, but not at pixel level.