1 / 19

Physical, Logical, Conceptual

Physical, Logical, Conceptual. DSA Lecture 2 2006-7. Abstraction Layers. WHAT. Conceptual What data is held An Image and its meta-data Entity-Relationship model (ERM) Logical How data is organised in storage Block and Directory structure Tables, keys Physical

asher
Télécharger la présentation

Physical, Logical, Conceptual

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Physical, Logical, Conceptual DSA Lecture 2 2006-7

  2. Abstraction Layers WHAT • Conceptual • What data is held • An Image and its meta-data • Entity-Relationship model (ERM) • Logical • How data is organised in storage • Block and Directory structure • Tables, keys • Physical • How data is stored in bits • JPEG as a stream of bytes • A Database as files and records stored in a DBMS-specific format Realisation (Refinement Reification) (Engineering, Model-Driven development Abstraction (Reverse Engineering) HOW

  3. On Flickr API: Image Data More..

  4. FireFox 1.5.0.7 IE 6.0.2800.1106

  5. Opera Browser properties

  6. JPEG and Exif • This image uncompressed would be • 1520 x 2032 x 3 bytes = 9 265 920 • 9 048.75 • File size is only 342kb • Compression ratio 0.0377952756 • JPEG achieves high compression • Lossy – cannot recover the full raw data • Level of compression is variable • JPEG • Joint Photographic Experts Group developed the standard for compression. • Defines how to create a byte stream for the image alone. • Wikipedia entry describes the algorithm. • JPEG is a BSI standard BS ISO/IEC 15444-6:2003 – we have license for these. • JPEG/JFIF • Defines how these bytes are packaged up unto a file for transmission. • This combines a JPEG compressed image (the ‘data’) with ‘meta-data’ (data about the image) in TIFF format (Tagged Image File Format) • Exif defines a set of tags and coded values to define this meta-data • Specification is 154 pages

  7. Meta data • “Data about Data” • The image is the Data • The make of the camera is metadata • The dangers • Thumbnails not updated with the main image • Date/time, location information

  8. Understanding the Physical Layer • Description of the implementation • Standard manual • Informal explanation • Hex viewer or editor • XVI32 (free) • 010 editor (30 day free, $49.95 US license) • Issues • How numbers are stored • How characters are stored • How strings are stored • How data is identified • How data is grouped • ..

  9. HEX dump of the file

  10. Hexadecimal • Hex – 6 in Latin – Hexagon • Deca – 10 in Latin – Decimal • Hexadecimal • Base 16 - 4 bits • Digits 0 – F (0 -15)

  11. Hexadecimal - Decimal • Nibble • 4 bits • 1 hex character • Byte • 8 bits • 2 nibbles • 2 hex characters • D8 • 13 x 16 + 8 = ? • 3C FA • 3 12 15 10 • 3 x 16^3 + 12 x 16 ^2 + 15 x 16 + 10 • ((3 x 16 +12) x 16 + 15) x 16 +10 = ?

  12. All JPEGs start with these bytes Marker : Start of Block Length of Block ‘Exif’ header Start of Data Intel Byte Order Start of IFD Tag No - 42 Offset to IFD (Image File Directory): 8 bytes

  13. Big-endian / Little-endian • Big-endian • Bytes in the order from most significant to least significant • 3C FA = 3C x 256 + FA = 15 610 • Motorola • coded MM in Exif • Little-endian • Bytes in the order from least significant to most significant • 2A 00 = 00 x 256 + 2A = 32 • Intel processors • coded II in Exif • Endianness • Affects addresses and dates • UK addresses are little-endian, Japanese big-endian • Email addresses little-endian, File paths big-endian • IP addresses ?

  14. No of directory entries (11) First entry Tag 010F ‘Make’ Data Type (string) String Length (24) CAMERA ‘Make’ Value (24 bytes) Relative Offset = 92 Absolute Position =Offset + start(0C) spaces Null (end of C string)

  15. Exercise • Each directory entry is 12 bytes long • Entry Type is coded – see table • Tags are coded • Decode 3 entries - distributed around the class

  16. Extract from the EXIF standard

  17. Logical structure

  18. Conceptual Model

  19. Workshop • Flickr Review • Review approach to research – see the blog for ideas • Get into groups • Physical layer work • The XVI32 editor can be downloaded from the autor’s site or run from the J: drive in J:/xvi • Find a Jpeg and identify some items of metadata • The lecture example is linked here • Find a MP3 file – it probably contains ID3 metadata – see the ID3 site for documentation – and identify some metadata. • Why don’t ID3 and Exif use the same logical structure using directories?

More Related