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Intro to Multimedia > Week 2 - Image formats > Flashcards

Week 2 - Image formats Flashcards

1
Q

State the 2 principle methods of representing graphical data?

A
  • Bitmaps

* Vectors

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2
Q

Another name for bitmap images?

A

• Raster images (pixels in a grid)

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3
Q

What are bitmap images useful for?

A
  • Type of sampled images that are produced by real-world applications.
  • Good at representing complex variations in colours, shades and shapes
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4
Q

Describe GIF?

A

• Graphics Interchange Format to

enable graphical data to be transferred via their network

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5
Q

What’s the colour limit for GIF images?

A

• 256 colours

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6
Q

What type of compression are GIF images stored in?

A

• Lossless compression

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7
Q

What are GIF images useful for?

A

• Allows you to pick one colour from the colour map of the GIF to be transparent

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8
Q

WHat’s good about animated GIF’s?

A
  • Several images in a GIF file & display them sequentially
  • Simple animation
  • Not good for long animations
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9
Q

Describe the GIF screen descriptor?

A
  • The overall parameters for GIF images in the file.

* Defines the placement of the following image within the space defined in the screen descriptor.

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10
Q

What does M stand for in the GIF screen descriptor?

A

• 1 if Global colour map

follows Descriptor

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11
Q

What does CR stand for in the GIF screen descriptor?

A

• bits per pixel in the image

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12
Q

What does S stand for in the GIF screen descriptor?

A

• whether Global Colour Table

(GCT) is sorted

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13
Q

What does size stand for in the GIF screen descriptor?

A

• of GCT

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14
Q

What does background stand for in the GIF screen descriptor?

A

• - Index in the GCT

for the background colour

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15
Q

What does the aspect ratio stand for in the GIF screen descriptor?

A

• no. of pixels

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16
Q

Problems with GIF’s?

A

• Unisys and CompuServe held patents on the LZW
compression used in GIF.
• Unisys expected commercial information
service companies to license the technology from them.

17
Q

Describe lossless compression?

A
  • Stores data in more appropriate way.
  • Doesn’t lose any important data.
  • Original format can be restored.
18
Q

Describe lossy compression?

A
  • Removes parts of the data.

* Removes data which is unnoticeable by humans.

19
Q

Why is it useful to use both types of compression on the same data?

A
  • Lossy compression is good at increasing the repetition in a data set
  • Lossless compression is good at reducing the size of a data set with lots of repetition
20
Q

Describe JPEG?

A

• Joint Photographic Experts Group.
• Set up by ISO (International
Organisation for Standardisation)

21
Q

What is the dominant file format for high quality true colour images?

A

• JPEG (Joint Photographic Experts Group)

22
Q

What is the compression ratio for JPEG?

A

• 30:1 with true colour images

23
Q

Describe what happens in stage 1 of JPEG compression?

A

• Convert RGB to brightness & chrominance (Y, Cb, Cr)

24
Q

Describe what happens in stage 2 of JPEG compression?

A

• Colour/ Chroma information is subsampled, normally by factor 2.

25
Q

Describe what happens in stage 3 of JPEG compression?

A
  • Image split into 8x8 blocks.

* Spatial data converted into frequency data.

26
Q

Describe what happens in stage 4 of JPEG compression?

A
  • Frequency data is quantized.

* Human vision is sensitive to small variations in brightness/colour than high-frequency brightness changes.

27
Q

Describe what happens in stage 5 of JPEG compression?

A

• Quantized data is then Huffman compressed.

28
Q

What is the one luma component (Y)?

A

• Brightness

29
Q

What are the two chroma components (Cb, Cr)?

A

• Colours

30
Q

The human eye is less sensitive to changes in ______ than __________?

A
  • Colour

* Brightness

31
Q

State what PNG file format is?

A
  • Portable Network Graphics is a new bit-mapped graphics.

* Lossless compression

32
Q

State what PNG file format is?

A
  • Tagged image file format
  • Handle any kind of image efficiently
  • Lossless compression

Intro to Multimedia (11 decks)

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