1. Tag-Cloud Drawing: Algorithm for Cloud Visualization
Owen Kaser, Daniel Lemire
Speaker: Yi-Ching Huang
2007/07/17

2. Introduction
Tag cloud can represent the frequencies of a hundred items
Tag cloud can be generated using
images
browser-specific technologies (ex: ActiveX)
plugins (ex: Flash)
complex HTML (using absolute positioning)
simple HTML with inline text or tables

3. Problem Definitions Problems Solutions Wasted space
Large clumps of white space
Solutions
Floorplanning / placement problem (EDA)
Hybrid of text justification (Knuth-Pass algo) and book-placement (Skiena)

4. Cloud Layout with Inline Text
1st algo: take an ordered list of tags and choose where to break line
Step 1: Greedy algorithm ,O(n)
Tags are added to the current line one by one, inserting a white space between them, until the line is full
Step 2: Dynamic programming ,O(n^2)
Knuth-Plass algorithm
Except that 2 things
The last line is not an exception
If a tag exceeds the maximal width, then it will be given a line of its own

5. Cloud Layout with Inline Text
2nd algo: reorder tags, attempting to decrease the badness
Finding an optimal ordering is NP-hard: when the required horizontal white space between tags is zero, we have the NP-hard Strip Packing Problem (SPP)
Dynamic programming to place the tags optimally and keep only the best solution
Next Fit Decreasing Height, Weight heuristic (NFDHW)

6. Strip Packing Problem First Fit Decreasing Height (FFDH)
Next Fit Decreasing Height (NFDH)

7. Cloud Layout with Arbitrary Placement
Min-cut placement
Placement approaches in EDA
Recursively decomposes a collection of tags by bipartitioning: splitting the tags into a “left” group and a “right” group

8. Slicing Floorplans & Slicing Tree

9. Nested Tables for Slicing Floorplans
Trick: each internal node in the slicing tree corresponds to a 2-element table in HTML

10. Tag Cloud with CSS

11. Result: Tag Cloud with Inline Text

12. Result: Tag Cloud with Arbitrary Placement

13. Conclusion Use simple HTML with CSS to layout tags
Two approaches for displaying tag cloud
inline text
arbitrary placement