Week 2 - Image formats Flashcards
State the 2 principle methods of representing graphical data?
- Bitmaps
* Vectors
Another name for bitmap images?
• Raster images (pixels in a grid)
What are bitmap images useful for?
- Type of sampled images that are produced by real-world applications.
- Good at representing complex variations in colours, shades and shapes
Describe GIF?
• Graphics Interchange Format to
enable graphical data to be transferred via their network
What’s the colour limit for GIF images?
• 256 colours
What type of compression are GIF images stored in?
• Lossless compression
What are GIF images useful for?
• Allows you to pick one colour from the colour map of the GIF to be transparent
WHat’s good about animated GIF’s?
- Several images in a GIF file & display them sequentially
- Simple animation
- Not good for long animations
Describe the GIF screen descriptor?
- The overall parameters for GIF images in the file.
* Defines the placement of the following image within the space defined in the screen descriptor.
What does M stand for in the GIF screen descriptor?
• 1 if Global colour map
follows Descriptor
What does CR stand for in the GIF screen descriptor?
• bits per pixel in the image
What does S stand for in the GIF screen descriptor?
• whether Global Colour Table
(GCT) is sorted
What does size stand for in the GIF screen descriptor?
• of GCT
What does background stand for in the GIF screen descriptor?
• - Index in the GCT
for the background colour
What does the aspect ratio stand for in the GIF screen descriptor?
• no. of pixels
Problems with GIF’s?
• Unisys and CompuServe held patents on the LZW
compression used in GIF.
• Unisys expected commercial information
service companies to license the technology from them.
Describe lossless compression?
- Stores data in more appropriate way.
- Doesn’t lose any important data.
- Original format can be restored.
Describe lossy compression?
- Removes parts of the data.
* Removes data which is unnoticeable by humans.
Why is it useful to use both types of compression on the same data?
- 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
Describe JPEG?
• Joint Photographic Experts Group.
• Set up by ISO (International
Organisation for Standardisation)
What is the dominant file format for high quality true colour images?
• JPEG (Joint Photographic Experts Group)
What is the compression ratio for JPEG?
• 30:1 with true colour images
Describe what happens in stage 1 of JPEG compression?
• Convert RGB to brightness & chrominance (Y, Cb, Cr)
Describe what happens in stage 2 of JPEG compression?
• Colour/ Chroma information is subsampled, normally by factor 2.
