Modeling 1: Polygons - An Introduction to Computer Graphics for Artists - page 80

but not all, applications will also allow you to load images into your perspective
viewport. When you do this, be sure that any secondary images, such as a top view
in addition to a front view, are the same scale as the fi rst drawing you have loaded.
If they aren't, you will have to rescale them within the application to match, and
doing this accurately is not always easy.

6.5 Exercise: Alignment Illusion, Part 1

6.5.1 Alignment Illusion

An alignment illusion is an illusion where the alignment of objects in a particular
camera view makes them appear to be related when they are not. M.C. Escher made
several famous examples of these during his career like a drawing of a staircase that
ascended and descended endlessly (Wu et al. 2010 ). This exercise is to build a 3D
model of an alignment illusion. An alignment illusion requires careful attention to
the camera and good observation of details in the source image. For convenience,
several alignment illusions made by the author and one by a student are provided as
examples (Figs. 6.9 , 6.10 , 6.11 , and 6.12 ), but you may use any alignment illusion
you are familiar with, or make your own.

6.5.2 Alignment Illusion Illustrations

The drawing you use must be the fi rst thing in your scene. This may mean making
a plane with the image as a texture map, or your application will have a utility that
allows you to use a background image or image plane . You should put the image in
your perspective viewport. Check to be sure that the image plane has the same
aspect ratio as the drawing by measuring it and then comparing to your background
image or the plane you attached it to.

6.5.3 Aspect Ratio

The formula for determining aspect ratio is simple: H
W

. “H” is the height of
the image, “W” is the width, and AR is the aspect ratio. For example, if an image is
2,000 × 3,000, its aspect ratio is .67 because 2 000
3 000

= AR

,
,

. With this value, you can

= . 67

scale your plane to the right size by multiplying its width by the aspect ratio.
Therefore, if the width of the plane in centimeters is 1,500, then the height should
be 1,000 cm because 1 500 , . 67 = 1 000 ,

.