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Intro to multimedia > Week 2 - Image formats > Flashcards

Week 2 - Image formats Flashcards

1
Q

Why do we use image formats?

A

To be able to save image data for future manipulation or display in a consistent manner
Formats enable standardisation and the use of graphical data by more than one application

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2
Q

What are the two types of graphical information?

A

Bitmaps (raster or pixel maps)and vectors

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3
Q

What are bitmaps?

A

Images made up of pixels in a grid

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4
Q

Advantages of bitmaps

A

Works with sampled images produced by real world applications
Good at representing complex variation in colour, shades and shapes

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5
Q

What type of compression does GIF use?

A

Lossless

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6
Q

What is the disadvantage of a GIF?

A

Only a 256 colour palette

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7
Q

What is GIF?

A

One of the two widely used image formats.

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8
Q

What is GIF best for?

A

line-art images - icons, graphs, line-art logos

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9
Q

Why are GIFs useful?

A

Allow for transparency by allowing you to pick one colour from the colour map to be transparent

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10
Q

What are animated GIFs?

A

Produced by displaying many images sequentially - Not good for longer/larger animations but works well with simple ones

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11
Q

What is the GIF screen descriptor?

A

Describes the overall parameters for GIF images in the file
Placement of image in the space of the GIF
Flags to define colour map and whether there is transparency

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12
Q

GIF Screen descriptor - What is M?

A

1 if global colour map follows descriptor

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13
Q

GIF Screen descriptor - What is CR?

A

Bits per pixel in imge

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14
Q

GIF Screen descriptor - What is S?

A

Whether global colour table is sorted

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15
Q

GIF Screen descriptor - What is Size?

A

Size of the global colour table

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16
Q

GIF Screen descriptor - What is Background?

A

The index of background colour in the global colour table

17
Q

GIF Screen descriptor - What is Aspect Ratio?

A

Aspect ratio of the pixels in GIF

18
Q

What is the GIF global colour map?

A

A colour table of bytes representing values of red, green, blue which are used for the GIF

19
Q

What are the two types of compression?

A

Lossy and lossless

20
Q

What is an example of lossless compression?

A

Run length code - Stores numbers of alternating values of bits

21
Q

What is lossless compression?

A

Compression by organising data better or storing it more appropriately
Rarely gives compression better than 2:1
No loss of data

22
Q

What is Lossy compression?

A

Removes data that is not noticeable by humans - uses the properties of human sensory system - vision and hearing e.g. the eye is poor at differentiating differences of chrominance

23
Q

When is compression most effective?

A

When lossy and lossless compression is applied

24
Q

What are JPEGs?

A

High quality true colour images - full-colour unlike GIFs

25
Q

What is the compression rates of JPEGs?

A

30:1 with true colour images

26
Q

What is the first stage of JPEG image compression?

A

Convert RGB to Brightness and chrominance - Y, Cb, Cr
Eye is less sensitive to changes in colour than changes in brightness
Lossless compression

27
Q

What is the second stage of JPEG compression?

A

Colour/Chroma info is subsampled by a factor of 2

28
Q

What is the third stage of JPEG compression?

A

Image is split into 8x8 blocks and Discrete Cosine Transform
(DCT) is applied - Converts spatial data into frequency components i.e. intensity converted to frequency

29
Q

What is the fourth stage of JPEG compression?

A

Frequency data is quantized

Human eye is very sensitive to small variations of brightness/colour but less to high frequency changes

30
Q

What is the fith stage of JPEG compression?

A

More repetition in the data quantized

Data is Huffman compressed

31
Q

What is PNG?

A

Image format that uses lossless compression which is replaced GIF
Handles true colour images
Better quality reproduction than GIF but no animation

32
Q

What is TIFF?

A

Image format that has lossless or no compression used to transfer image data without data loss

Intro to multimedia (5 decks)

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