Lecture 3/4 Display tech / Image formation and filtering

Question 1

Why is a barrier needed between an object and film in a pin-hole camera?

Answer 1

To control the rays of light from the object
Only allowing light to enter through a pin-hole aperture.

Question 2

Dimensionality Reduction Machine (?? to ??)
What does this machine accomplish?

Answer 2

Convert the 3D world to a 2D image

Question 3

How does a large aperture affect an image?

Answer 3

Blurry and bright image

Question 4

How will having a tiny aperture affect an image?

Answer 4

Too small an aperture will create a blurry and dark image

Question 5

What does a lens do to an image?

Answer 5

A lens focuses parallel rays of light onto film
- Objects focus at a distance of f
- Other points project to a “circle of confusion” in the image
- Changing the shape of the lens changes the focal distance

Question 6

How does focal distance relate to object and image distance?

Answer 6

f: focal distance
u: object distance
v: image distance

Question 7

What shape does a lens take?

Answer 7

Lenses are typically spherical - this is easier to produce

Question 8

What is D?

Answer 8

The aperture of diameter D restricts the range of rays
- aperture may be on either side of the lens

Question 9

What is f?

Answer 9

The focal point is at a distance f beyond the plane of the lens
- f is a function of the shape and index of refraction of the lens

Question 10

What are the effects of changing the aperture size?

Answer 10

Changing the aperture size affects the depth of field
- A smaller aperture increases the range in which the object is approximately in focus

Question 11

Compare a camera to the human eye

Answer 11

Iris - colored annulus with radial muscles
Pupil - the hole (aperture) whose size is controlled by the iris

Question 12

What’s the “film” equivalent in an eye?

Answer 12

photoreceptor cells (rods and cones) in the retina

Question 13

What does this image (from a sensor array) show?

Answer 13

A continuous image projected onto a sensor array

Question 14

What does this image (from a sensor array) show?

Answer 14

Result of image sampling and quantisation

Question 15

How does a sensor array record light?

Answer 15

▪ turn continuous light into pixels
▪ cells detect photons (as electrons)
over an array of pixels

Question 16

What is a CCD?

Answer 16

Stands for Charge Coupled Device. A type of image sensor.
They are serial devices where pixels
are read out one at a time.

Question 17

What is a CMOS?

Answer 17

A type of image sensor.
each pixel contains an
amplifier, so read-out can be faster (than a CCD)

Question 18

Why are there different image sensor types, what are they?

Answer 18

Different sensor types use different
approaches to read out electrons.
CCD - pixels read out 1 by 1
CMOS - each pixel contains an amplifier… faster read-out

Question 19

What are the active image display technologies?

Answer 19

LCD - Liquid Crystal Display
Digital Mirror Displays
OLED
Plasma Displays
Cathode Ray Tube

Question 20

What are passive image display technologies?

Answer 20

PRINTERS:
Laser printers
ink-jet printers
Commercial printers

Question 21

What active image display is most common today?

Answer 21

LCD - Liquid Crystal Display

Question 22

What active image display was standard for most of the 20th century?

Answer 22

Cathode Ray Tube

Question 23

What are OLEDs used for?

Answer 23

Expensive high end devices.
Emit RGB no need for a back light (like LCDs)

Question 24

What is a negative of plasma displays?

Answer 24

They’re power-hungry.
They were briefly popular.

Question 25

How do LCDs work?

Answer 25

liquid crystals twist the polarisation of light - voltage across the liquid crystal controls how much light passes: – voltage on: no light (crystals align the electric field) – Voltage off: maximum light (crystals align to each other, smoothly twisting light)

Question 26

How is light controlled when voltage is ON in an LCD display?

Answer 26

ON = No light crystals align with the electric field

Question 27

How is light controlled when voltage is OFF in an LCD display?

Answer 27

OFF = Maximum light crystals align to each other, smoothly twisting light

Question 28

How does the effect of quantisation vary as the bit number is increased?

Answer 28

More continuous and detailed image with more bits

Question 29

How many bits (colour depth) are needed in a qunatised image?

Answer 29

Question 30

What does error diffusion achieve

Answer 30

Error diffusion achieves visually more pleasing results by distributing quantisation errors across neighbouring pixels

Question 31

How would you increase the brightness of an image?

Answer 31

Increase the value of each pixel by a fixed amount such that all pixel values are equal to or less than 255 (max)

Question 32

In per pixel processing what is the image inversion equation?

Answer 32

Inverted Image

Question 33

In per pixel processing what is the contrast enhancing equation?

Answer 33

Question 34

What are these point-based filter maps showing?

Answer 34

Darkening and lightning

Question 35

What are these point-based filter maps showing?

Answer 35

High and low contrast

Question 36

What are these point-based filter maps showing?

Answer 36

Compressed to darks and compressed to lights

Question 37

What are these point-based filter maps showing?

Answer 37

Emphasise shadows and lights

Question 38

What are point-based filters?

Answer 38

These filters operate on a per-pixel basis. They involve applying a function to each pixel of an image individually, based on the value of that pixel. Common point-based filters: linear, median, and convolutional filters.

Question 39

What are the limitations of a point-based filter?

Answer 39

Limited ability to capture structure and context: causes degraded image quality or loss of important image features. Sensitivity to noise: result in the amplification of noise and degraded image quality. Limited control: they apply a fixed function to each pixel. This can make it difficult to fine-tune the processing. Over- or under-processing: unnatural or distorted appearance.

Question 40

How to overcome noise in point-based filters?

Answer 40

Use neighbourhood filters

Question 41

What filter can be applied to an image to remove salt and pepper noise?

Answer 41

Median filter: this will replace each pixel with the median value of all pixels in its neighborhood. NOT A CONVOLUTION FILTER

Question 42

What is the difference between correlation and convolution?

Answer 42

Convolution is correlation with the kernel rotated 180°

Question 43

In a morphological filter, we can obtain erosion of a pixel using the following formula

Answer 43

Question 44

In a morphological filter, we can obtain the dilation of a pixel using the following formula

Answer 44

Question 45

In a morphological filter describe Opening

Answer 45

DILATION AFTER EROSION

Question 46

In a morphological filter describe Closing

Answer 46

EROSION AFTER DILATION

Question 47

What are examples of sobel filters?

Answer 47

Horizontal gradients Vertical gradients Diagonal gradients

Question 48

What does a sobel filter kernel look like for: Horizontal gradients

Answer 48

Question 49

What does a sobel filter kernel look like for: Vertical gradients

Answer 49

Question 50

What does a sobel filter kernel look like for: Diagonal gradients

Answer 50

Question 51

Show equation for a gaussian (low pass) filter

Answer 51

Question 52

What are the fundamental differences between a box filter and a gaussian filter?

Answer 52

Box filter gives equal weight to all pixels Gaussian trusts the center pixel more

Question 53

Visualise a kernel for A motion blur convolution filter

Answer 53

Question 54

Visualise the convolution kernel: Sharpening using unsharp masking

Answer 54