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Tài liệu tham khảo bằng tiếng Anh về hội họa - Perspective Drawing Handbook - Phần 4 | Chapter 8 MORE ON LOOKING UP DOWN AND STRAIGHT AHEAD Glance again at pages 40 and 42 where the cube was viewed by looking down and looking up and note that the vertical lines do not actually remain vertical in the picture but instead appear to converge downwards and upwards respectively. Many books state arbitrarily that such lines should always appear vertical. Although contrary to the truth of seeing this rule is laid down in order to simplify matters. But such simplification is helpful only in mechanical T-square and triangle perspective where converging verticals means complicated drafting to establish and work with distant vanishing points and complicated procedures to determine vertical measurements. Therefore when working freehand without drafting considerations let the visual truth dictate. If you have difficulty accepting this truth the following will help. Try thia again from both viewpoints with the book held almost on a level with the central visual ray. The principle is now more dramatically demonstrated because convergence and foreshortening are almost at a maximum. Take a book and hold it horizontally in this manner left . What you see right are lines converging to a central vanishing point at eye level. Being standard perspective drawing this is readily accepted. Now hold the book vertically above your head in this manner left and view it at approximately the same angle. What you see right is exactly the same as before only now the convergence is upward instead of horizontal. Therefore the convergence and hence the picture is identical from both viewpoints. THE REASON IS SIMPLY THAT THE RELATIONSHIP BETWEEN EYES SIGHT LINES AND SUBJECT BOOK IS IDENTICAL IN BOTH CASES. Note angle -0-. 54 Things Seen By Looking Straight Out And Things Seen by Looking Up But again why is upward and downward convergence so rarely used The reason is that we usually see things by looking more or less horizontally. Not only is this attitude more natural to the anatomical .
