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KCL computer vision > w2 w gemini > Flashcards

w2 w gemini Flashcards

1
Q

What determines where a point in the 3D world appears on a 2D image?

A

Camera position and orientation relative to the 3D point.

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2
Q

What determines how bright the image of a point is?

A

Illumination (intensity

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3
Q

Given the RGB values of a pixel, can you determine the colour?

A

rgb values are also effected by illumination, reflectance and texture/geometry so no

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4
Q

Define “focus” in the context of image formation.

A

Focus means that all rays coming from a scene point converge into a single image point.

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5
Q

Define “exposure” in the context of image formation.

A

Exposure is the time needed to allow enough light through to form an image.

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6
Q

Write down the thin lens equation.

A

1/f = 1/|z| + 1/|z’|
where f = focal length
z is distance from object to lense
z’ is distance from image plane to lens

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7
Q

Derive the thin lens equation (briefly describe the steps).

A

Use similar triangles formed by the object

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8
Q

Where should the image plane be placed in a pinhole camera for a focused image if the object is 3m away?

A

The image plane can be placed at any distance. Pinhole cameras have an infinite focal range.

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9
Q

Compare the focusing mechanism of a camera and an eye.

A

Camera: Moves the lens to change the distance to the image plane. Eye: Changes the shape (and focal length) of the lens

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10
Q

Describe the characteristics of the periphery of the retina.

A

Low resolution (acuity)
Many rods
Few cones

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11
Q

Describe the general receptive field structure of a retinal ganglion cell.

A

Centre-surround.

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12
Q

What are the two main classes of retinal ganglion cell receptive fields?

A

On-centre off surround: active if central stimulus is brighter than background
Off center on surround: active if central stimulus is darker than background

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13
Q

Briefly describe how ganglion cell RFs contribute to efficient image coding.

A

They respond strongly to changes in intensity

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14
Q

In the thin lens equation

A

what does ‘f’ represent?

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15
Q

What are the key ingredients of image formation (two sets of parameters)?

A

Radiometric parameters (determine intensity/colour) and Geometric parameters (determine where a point appears).

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16
Q

Give examples of radiometric parameters.

A

Illumination

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17
Q

Give examples of geometric parameters.

A

Camera position/orientation

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18
Q

What is albedo?

A

The fraction of light reflected at a particular wavelength.

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19
Q

Explain additive colour mixing.

A

Mixing light sources.
Illumination from different light sources adds

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20
Q

Explain subtractive colour mixing.

A

Mixing pigments
Reflection from different surfaces subtracts

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21
Q

Why is measuring surface properties an “ill-posed problem”?

A

We record luminance (L) which depends on both illumination (E) and reflectance (R)

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22
Q

Describe the concept of colour constancy.

A

The ability of the visual system to perceive the colour of surfaces as constant despite changes in illumination.

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23
Q

What is the function of optics in image formation?

A

To focus light rays from a scene point onto a single point on the sensor.

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24
Q

Explain how a pinhole camera works.

A

It restricts light flow through a small hole

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25
Q

What is a advantage and disadvantage of a small pinhole in a pinhole camera?

A

Sharp focus but dim image (requires long exposure).

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26
Q

What is a disadvantage of a large pinhole in a pinhole camera?

A

Brighter image (shorter exposure) but blurred.

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27
Q

Briefly explain how a thin lens works.

A

It refracts light to converge rays from a scene point onto a single image point.

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28
Q

What is the focal point (F’) of a thin lens?

A

The point where parallel rays converge after passing through the lens.

29
Q

What is the focal length (f) of a thin lens?

A

The distance between the lens and the focal point.

30
Q

What is the optical center (O) of a thin lens?

A

The center of the lens

31
Q

State three rules for ray tracing through a thin lens.

A
  1. Rays through the optical center are not refracted. 2. Rays parallel to the optical axis refract through the focal point. 3. Rays through the focal point refract parallel to the optical axis.
32
Q

What does perspective geometry describe?

A

The transformation from the 3D world to a 2D image

33
Q

Give examples of distortions caused by perspective geometry.

A

Parallel lines converging at a vanishing point

34
Q

What is a vanishing point?

A

The projection of a point at infinity onto the image plane.

35
Q

What are intrinsic camera parameters?

A

Internal parameters of the camera
Focal length, Sensor dimensions/resolution

36
Q

What are extrinsic camera parameters?

A

Parameters describing the camera’s position and orientation in the world.

37
Q

What is the purpose of the projection operator in the camera model?

A

To map 3D points in the camera reference frame to 2D points on the image plane.

38
Q

What additional information is needed to recover depth from a 2D image?

A

Extra views (stereo vision) or prior knowledge about the scene structure.

39
Q

What is digital image digitisation?

A

The process of converting a continuous image into a discrete digital representation.

40
Q

What are pixels?

A

Picture elements

41
Q

What does the value of a pixel typically represent in a grayscale image?

A

The measure of light intensity at that point.

42
Q

What is pixelisation?

A

The process of averaging intensity values within each sampling grid location.

43
Q

What is quantization?

A

The process of representing intensity values using a finite number of discrete levels.

44
Q

What are common bit depths for grayscale images?

A

8 bits (256 levels).

45
Q

What is a Bayer filter?

A

GRGB is the most c9mmon colour filter used in digital cameras
There’s twice as many green filters as red and blue
Improves sensitivity to green to which most humans are most sensitive

46
Q

What is demosaicing?

A

The process of interpolating the missing colour values at each pixel location in a (Bayer-filtered) image.
So that we have RGB values at every pixel

47
Q

Describe nearest neighbour interpolation for demosaicing.

A

Copies the value from an adjacent pixel of the same colour channel
V fast
V inaccurate

48
Q

Describe bilinear interpolation for demosaicing.

A

Averages the values of the nearest two or four pixels of the same colour channel

49
Q

Describe edge-directed interpolation for demosaicing.

A

Analyzes the region around a pixel to interpolate along the axis with the lowest change in value.

Calculating G8:

Calculate vertical and horizontal gradients:
Δ H = |G7 - G9|, ΔV = |G3 - G13|
Interpolation:
If ΔH < ΔV, G8 = (G7 + G9)/2
Elif ΔH > ΔV G8 = (G3 + G13)/2
Else
G8 = (G3 + G13 + G7 + G9)/4

50
Q

What is the function of the cornea in the eye?

A

Performs the initial bulk of the refraction (at fixed focus).

51
Q

What is the function of the lens in the eye?

A

Performs further refraction and can change shape to adjust focal length.

52
Q

What is the function of the iris in the eye?

A

Regulates the amount of light entering the eye.

53
Q

What are photoreceptors?

A

Light-sensitive cells in the retina that transduce light into electrical signals.

54
Q

What are the characteristics of rods?

A

High sensitivity (dim light)

55
Q

What are the characteristics of cones?

A

Low sensitivity (bright light)

56
Q

Where are cones primarily concentrated in the retina?

A

In the fovea.

57
Q

Where are rods more abundant in the retina?

A

In the periphery.

58
Q

What is the fovea responsible for?

A

Region of retina responsible for High-resolution

59
Q

What are ganglion cells?

A

Neurons in the retina that receive input from photoreceptors and whose axons form the optic nerve.

60
Q

What is the receptive field of a ganglion cell?

A

The area of visual space from which a neuron receives input.

61
Q

How do centre-surround receptive fields contribute to edge detection?

A

They respond strongly to differences in light intensity between the center and surround

62
Q

Explain how centre-surround RFs contribute to invariance to lighting.

A

They respond to the contrast between the center and surround

63
Q

What are colour-opponent ganglion cells?

A

Ganglion cells that respond to differences in colour by combining input from both rods and cones in a centre surround configuration

Inputs from cones produce colour opponent cells

Eg
Red on centre green off surround responds most strongly to red light falling within its receptive field

64
Q

What is a common mathematical model for centre-surround receptive fields?

A

Difference of Gaussians (DoG) operator.

65
Q

Radiometric parameters

A

Determine intensity/colour of a given location in an image
Illumination
Surface reflective properties (material, orientation)
Sensor properties (sensitivity to different EM frequencies)

66
Q

Geometric parameters

A

Camera position and orientation in space
Camera optics (focal length)
Projection geometry (mapping from 3d to 2d)

67
Q

Albedo

A

Fraction of light reflected at a given particular wavelength

68
Q

Smooth hue interpolation

A

Interpolation of green pixels (use bilinear)
Interpolation of red or blue (use bilinear of the Hue which is ratio of red/blue and green:)

B12 = ( B6/G6 + B8/G8 + B16/G16 + B18/G18 )/4

KCL computer vision (12 decks)

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