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Computer Graphics -- Lecture 1 > terms > Flashcards

terms Flashcards

1
Q

Define computer graphics

A

“Production of images on computers for use in any medium”

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2
Q

Define Image Processing

A

The electronic analysis or manipulation of an image, generally to improve it’s quality

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3
Q

Name some common applications of computer graphics

A

Computer art
Video Games
Healthcare
Film

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4
Q

Name the types of computer graphics

A

Raster Graphics (Bitmaps) –> Uses a grid of individual pixels where each pixel can be a different colour or shade (Digital Images)
Vector Graphics –> Uses mathematical relationships between points and lines connecting them to describe an image

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5
Q

What is a Pixel?

A

Fundamental display element on an electronic screen / bitmapped image.
Two types of images:
RGB (Red, Green Blue)
CMYK (Cyan, Magenta, Yellow, Black)

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6
Q

Define Resolution

A

The number of pixels in an image

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7
Q

What is a Megapixel

A

1 Million pixels

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8
Q

RGB Pixels

A

Tries to imitate the actions of the human eye, capturing red, green and blue light.
For an RGB image, the pixel values are stored in 3 separate 2D arrays.
Each value generally ranging from 0 - 255 (one Byte)

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9
Q

What are Monochrome / Gray Scale images

A

Contain gray-level information and no color information.
Typical gray scale image contains 8 bits/ pixel data, which allows for having 256 different gray levels.

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10
Q

How do you define gray-level resolution L?

A

L = 2^k,
where K = number of bits used to represent the grey levels of digital image

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11
Q

How do you determine the gray-scale of an 8-bit image?

A

L = 2^k = 2^8 = 256,
So, gray-scale of an 8 bit image is [0, 255]

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12
Q

How do you determine the storage size of an 8-bit image with 512x512 resolution?

A

512 x 512 x 8 = 2,097,152 bits

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13
Q

What is a histogram with respect to computer graphics?

A

they plot the frequency of each intensity value in an image.
Used to visualize light and dark areas in an image (to improve brightness / contrast)

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14
Q

How are colours represented in digital form?

A

As a collection of numbers, each representing the intensity of the given component of the colour.
These components are called channels.
Colour channels generally describe how much red, green and blue are used in a pixel to form a colour.

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14
Q

How are colours represented in digital form?

A

As a collection of numbers, each representing the intensity of the given component of the colour.
These components are called channels.
Colour channels generally describe how much red, green and blue are used in a pixel to form a colour.

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15
Q

What is the Alpha channel?

A

Specifies how transparent or opaque a colour is.
Alpha channel = 1: opaque
Alpha channel = 0: Transparent

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16
Q

What is the Alpha channel?

A

Specifies how transparent or opaque a colour is.
Alpha channel = 1: opaque
Alpha channel = 0: Transparent

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17
Q

What is Alpha Composition?

A

Technique of combining an image (foreground) with a background image to produce a composite image that has an appearance of transparency.
Ii = αiFi + (1 − αi)Bi
Fi = Foreground image colours
Bi = Background image colours
ai = alpha matte of foreground image

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18
Q

Applications of alpha composition?

A

digital image editing, movie production,

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19
Q

Describe Image Formation

A

Each point on a 3D object emits multiple rays of light outwards.
Due to the barrier in place between the camera sensor and the 3D object, only very limited number of light rays can pass through the aperture of the barrier and enter the the sensor

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20
Q

What is the Pinhole Camera Model?

A

Simple camera model:
The film is called the image or retinal plane
aperture is called the pinhole / center of camera
Distance between image plane and pinhole = focal length

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21
Q

How do you use the Pinhole Camera?

A

P = [x, y, z] (point on 3D object) is mapped onto the image plane P’ = [x’, y’].
The pinhole itself can be projected onto the image plane too, giving the point C’
This [i, j, k] system is called the camera reference system / camera coordinate system.
Line defined between the pinhole and it’s projection C’ is called the optical axis of the camera system

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22
Q

What are some issues of the pinhole camera system?

A

As the aperture size increases, the number of light rays that pass through the barrier increases (blurring the image)

23
Q

What is the purpose of lenses?

A

They mitigate the conflict between crispness and brightness.
Rays of light emitted from the 3D object are refracted by the lens so that they converge to a single point P’ on the image plane.

24
Q

What is the purpose of lenses?

A

They mitigate the conflict between crispness and brightness.
Rays of light emitted from the 3D object are refracted by the lens so that they converge to a single point P’ on the image plane.

25
Q

Define the term “Depth of field”

A

The effective range at which cameras can take clear photos

26
Q

Define the Focal Point

A

Point on the image plane where all rays of light focus on

27
Q

Define the shutter in a camera?

A

blocks the light coming into the camera through the lens from entering the camera’s image sensor.

28
Q

Define the Image Sensor of the camera

A

Sensor that detects light and measures the intensity of it to create the image.

29
Q

What is the field of view?

A

Focal length of the camera determines how much of a scene is captured in an image.

30
Q

How do you add two vectors?

A

Sum the elements at their corresponding positions

31
Q

How do you get the magnitude of a vector?

A

Use Pythagoras Theorem
This can also be generalized to an n-dimensional vector

32
Q

What is a normalized vector?

A
33
Q

What are the 5 parameters of the intrinsic matrix?

A

Focal Length
Pixel Scaling
Skew
Principle Point Coordinates
Distortion

34
Q

What is the Intrinsics matrix of a camera?

A

A 3x3 matrix that describes the internal parameters of a camera.
Represented as :K =
fx s cx
0 fy cy
0 0 1

Where fx and fy = Focal Length
s = Skew between x and y axis (sometimes used)
cx and cy = coordinates of the Principle point (center point)

35
Q

What is the Extrinsic Matrix?

A

Define the location and orientation of the camera with respect to the world frame

36
Q

What are the parameters of the Extrinsic Matrix?

A

Rotation relative to the world frame.
Translation relative to the world frame (translation vector)

Rotation —> angle at which the camera is facing the 3D object.
Translation –> Position on a 3D grid where the camera is located.

37
Q

What is the Identity Matrix?

A

A special type of square matrix that has the same number of rows as columns.
It is a diagonal matrix with ones (1) on the main diagonal and zero’s everywhere else.

38
Q

How do you inverse the Identity Matrix?

A

the rotation matrix inverse is just the transpose of the rotation matrix.
e.g. [ cos(theta) -sin(theta) 0 ]
[ sin(theta) cos(theta) 0 ]
[ 0 0 1 ]

the inverse of this would be:
[ cos(theta) sin(theta) 0 ]
[ -sin(theta) cos(theta) 0 ]
[ 0 0 1 ]

To prove this, multiply the original matrix by the inverse and you should get the identity matrix.

39
Q

What is a protective transformation?

A

Point P in 3D space getting projected to point P’ in a 2D space (image plane).
Issues: Points in digital images are generally in a difference reference system to points in the image plane.

40
Q

Camera matrix model?

A

Describes the set of parameters that affect how world point P is projected onto image coordinate P’
(created by multiplying the intrinsic matrix by the extrinsic matrix)

40
Q

Camera matrix model?

A

Describes the set of parameters that affect how world point P is projected onto image coordinate P’
(created by multiplying the intrinsic matrix by the extrinsic matrix)

41
Q

What is the Perspective Camera model?

A

Given a matrix M, it allows us to map the 3D point P in the world reference system to 2D point P’ on the image plance

42
Q

How do you describe the mapping from the world to individual pixels?

A

Using the intrinsic and extrinsic matrices of the camera matrix model

43
Q

When is a camera uncalibrated?

A

When the intrinsics of the camera are unknown.
Calibrated otherwise

44
Q

What is a metric camera?

A

When the intrinsics of the camera are known and do not change

45
Q

When does distortion occur?

A

Distortion occurs when the size of each point on the image plane is magnified in a different way.

46
Q

What are the two types of lens distortion?

A

Optical distortion and Perspective distortion

47
Q

Optical Distortion

A

Three types:
1. Barrel distortion
2. Pincushion distortion
3. Mustache distortion

48
Q

Barrel Distortion?

A

A type of optical distortion:
Lines that are straight in real life appear to curve inwards.
Occurs when using wide-angle lenses (outside of a ball)

49
Q

Pincushion Distortion

A

A type of optical distortion:
Causes straight lines in real life to appear to curve outwards (inside of a ball).

50
Q

Mustache distortion

A

Type of optical distortion:
Combination of barrel and pincushion distortion.
Image center resembles barrel while edges of image resemble pincushion

51
Q

Perspective Distortion

A

Has to do with the positioning of a subject in relation to the camera and the angle at which it points at the subject. (Nothing to do with lenses).
Two types of perspective distrotion:
Wide-angle distortion
compression distortion

52
Q

Hue in HSV color space

A

colour portion of the HSV model, expressed from 0 - 360 degrees:
Red –> 0 - 60 degrees
Yellow –> 61 - 120 degrees
Green –> 121 - 180 degrees
Cyan –> 181 - 240 degrees
Blue –> 241 - 300 Degrees
Magenta –> 301 - 360 degrees

53
Q

Saturation

A

Describes the amount of gray in a particular colour
0 = grey
1 = primary colour

Computer Graphics -- Lecture 1 (14 decks)

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