*Shading*

Volumetric objects always create shade when being hit by a source of light. As a result, in 2D visual arts, the use of light and shade is one of the methods for creating the illusion of depth. Figure 6 shows an example of an e ffect called "the crater illusion" [2] in which the central square seems to be in front of the background (right image) or behind the background (left image) depending on the position of highlighted or shadowed edges. The results of applying the depth-temperature algorithm in this case can be seen in Table 7, for the left side of the figure, and in Table 8 for the right side of the figure.

**Figure 6.** The crater illusion.

**Table 7.** Temperature-depth mapping of right side of Figure 6. The central square seems much nearer than the external square, so they are given both extreme temperatures of 38 and 14 ◦C, respectively.


**Table 8.** Temperature-depth mapping of left side of Figure 6. The external square seems much nearer than the central square so they are given both extreme temperatures of 38 and 14 ◦C, respectively.


### *Size, placement, and perspective*

Vertical placement: we perceive objects that are placed lower in the image as closer to us, and objects that are placed higher as being further away. A really clear example of this will be seen later when we apply the temperature-depth mapping algorithm to the artwork "*Starry Night*" by Vincent Van Gogh.

Diagonal perspective: we perceive diagonal lines as receding into the distance. As shown in Figure 7 and Table 9, the red-colored square seems to recede due to the diagonal perspective.

**Figure 7.** An example of diagonal perspective: Josef Albers, Homage to the Square [18].

**Table 9.** Temperature-depth mapping for Figure 7. The temperature range selected was [26–38 ◦C], since the external square and the central one seem to be quite far from each other, but not that much far away. The range was then divided by four and each temperature assigned to each one of the depth levels. In this case, basically, the user would feel the temperature decreasing as he/she approached the central square.

