# Examining User Perception of the Size of Multiple Objects in Virtual Reality
## 2 Materials and Methods  
The experiment is conducted entirely with an HTC Vive Pro HMD, and the participant is presented two cylinders that are approximately 500 mm in front of them and 300 mm below eye level, see Figure 3. The users were able to move their head to achieve motion parallax by bending at the waist, but they were not to walk from the prescribed standing position. The cylinder on the left is the target (standard) and is of red colour, see Figure 4. The cylinder on the right is the cylinder the participant is adjusting (comparison). The goal of the experiment is to determine the point of subjective equality and just-noticeable difference for users observing two cylinders that approximate buttons, dials and levers on a control panel. Two colours--red (255,0,0) and yellow (255,255,0)--were employed to highlight a user's ability to understand the size differences when controls are of different materials. User estimations varied significantly between the colour pairs blue-yellow and blue-green [37,38]. Their results indicated that users had more accuracy in the depth matching task with yellow and green virtual objects than with blue ones. In the CIELAB colour space ([[http://colorizer.org/](http://colorizer.org/)]([http://colorizer.org/](http://colorizer.org/))), the luminosity of yellow (255, 255, 0) is 97 and the luminosity of red (255, 0, 0) is 53. Therefore, yellow virtual cylinders seem brighter than red. The principle being that among similarly sized and distant objects, the brighter appear closer than dimmer objects. [PERSON] and [PERSON] [39] demonstrated the perceived size of products depends on the saturation of their colour. They performed six experiments, using physical objects and products with different shapes and hues, and studies determined that increasing colour saturation increases size perceptions. The authors indicate the influence may be explained by the inclination for saturated colours to capture a user's attention. There are a vast number of options for the choice of these two colours. The space can be broken down into a YUV (Y'CbCr) colour space, and the luminosity could have been control. The choice of red and yellow was for saturated colours that are commonly employed as physical controls. The experiment examines both adjusting the height and width of the cylinder separately. The ten conditions of the standard cylinder are as follows. * With a fixed diameter of 30 mm:
* height 10 mm,
* height 30 mm,
* height 50 mm,
* height 70 mm and
* height 90 mm. * With a fixed height 50 mm:
* diameter 10 mm,
* diameter 20 mm,
* diameter 30 mm,
* diameter 40 mm and
* diameter 50 mm. With a fixed diameter task, the participant adjusted the height of the comparison object. In the case of a fixed height, the participant adjusted the width of the comparison object. The starting height or diameter of the comparison cylinder was set to be noticeably different than the standard cylinder. The comparison height or diameter was set from a random value of a scale factor of the target size. The starting scale factor for the grow tasks is between 0.75 to 0.9, and the scale factor for the shrink tasks is between 1.1 to 1.25. These scale factors provided very noticeable starting points from the target size without requiring the participants to make extremely large adjustments. Each participant performed the following. * 10 tasks per trial,
* 2 trials per condition (two grow and two shrink),
* 10 conditions,
* 5 heights fixed diameter, and
* 5 diameters fixed height. The experiment was conducted with 10 participants. There were eight males and two females, ages 23 to 35. The number of participants was based on _local standards_[40]. All participants were computer science students and experienced users of virtual reality with the HTC VIVE technology. The selection criteria required the participants must be between the ages of eighteen and forty, right handed, normal or corrected to normal vision, and in good health. There was no reported simulator sickness during the experiment.