Components of a PP Experiment

Question 1

What does a Psychophysical experiment contain?

Answer 1

• Stimulus
• Measure
• Task
• Procedure
• Method
• Analysis

Question 2

Describe psychophysics, stimuli, display technology?

Answer 2

• PP is heavily stimulus driven
• Study of vision is usually broken down into sub-processes such as motion, depth or size
• Pixel:
  o Picture element – v basic aspect of any digitally created image
  o Usually so small that they cannot be resolved individually by the bare eyes, so they blend together to form continuous toned image
  o Pixels have different brightness values: 1 = black, 256 = white
  o Image is just a distribution of intensity values across a 2D surface made up of pixels
  o Any image on screen – just bunch of numbers representing grey levels or colours
  o Pixelated image – image not clear and can see the square pixels  happens when image contains limited number of pixels compared to the size of the image
• Retinal “Pixel” Array: Photoreceptor Mosaic:
  o Centra Fovea (~1500 cones)
  o Photoreceptors in retina are like a pixelized structure
  o When image falls on retina, each photoreceptor signals the retinal illuminance in wavelength of the respective parts of the image
  o Photoreceptors can be thought of as dynamic pixel signalling intensity and colour of image

Question 3

What is the pixel mosaic of a digital display?

Answer 3

• Inspired by biological retina, modern day digital displays adopted similar concept to produce image on screen
• Under flat panel surface, layer of pixels composed of red, green and blue elements.
  o Each element can be independently controlled by electric signals to produce desired colour and luminance

Question 4

Describe Display Resolution 1: Pixel Resolution?

Answer 4

• Absolute number of pixels display has
• Typically quoted as width x height with unison pixels
  o 1024 x 768 – 1024 pixels across, and 768 pixels vertically
• Pixel resolution of digital display has increased

Question 5

Describe Display Resolution 2: Pixel Density (PD)?

Answer 5

• More pixels have, better picture quality get
  o Only true when consider number of pixels relative to the physical display size
• What really determines the display resolution is pixel density measured in pixels per inch (PPI) – how many pixels are squeezed in a unit inch (horizontally or vertically)
  o Typically, only care about one of the dimensions (the width) assuming that a pixel is perfectly square

Question 6

Describe pixel density: Pixels Per Inch (PPI)?

Answer 6

• Find out current pixel setting of display (e.g. 1600x1000 – 1600 horizontally & 1000 vertically)
• Pixel density will change according to resolution setting
• Monitor size does not change
• E.g. if width of monitor is 20” – PPI across the display when resolution is 1600x1000, PPI of this monitor is 1600/20” = 80 PPI
• For psychophysical experiments on digital displays, record size of monitor and display resolution you ran the experiment – as it changes the pixel density and thus affects the size dimensions of visual stimulus that is displayed on the screen

Question 7

How do you work out the pixel density in visual angle?

Answer 7

Use SOH CAH TOA & do inverse of them to get the variable on its own

Question 8

How to calculate Pixels Per Degree (PPD) of a monitor?

Answer 8

• Horizontal pixel resolution = 1600 pixels
• Viewing distance (d) = 100cm
• Width of monitor (w) = 50cm

Equation: α = 2tan^-1(h/2d)

tan^-1(w/2d) in radians
multiply above by 180/pi to get degrees
α = 2tan^-1 (h/2d) => x degrees

50cm of monitor subtends x degrees from 1 metre (100cm) away

Question 9

How much of visual angle at arms length does the width of thumbnail subtend on retina?

Answer 9

Width of thumbnail approx. subtends 2° of visual angle on retina at arm’s length