Creating Seamless Display Walls with a Single PC Grant Wallace and Han Chen

Motivation Inexpensive, High Resolution, Collaborative Display Single PC Avoids clustering issues  Run a desktop environment across display  Run favorite applications without modification Best for small tiled displays Automatic alignment Improves display quality Reduces maintenance expense

Example: PPPL Control Room PPPL can benefit from a small tiled display (2-3 projectors) Should allow scientists to easily run common applications Should have automatic alignment Projectors difficult to access - ceiling mounted Keystone unavoidable do to projector placement

The Easy Part Single PC Tiled Display without alignment Buy PC, quad graphics card and projectors Plug everything together Tell OS to span desktop across displays

Some Already Solved Challenges Aligning an application We have a system to produce an alignment transformation for each projector Scalable Alignment of Large-Format Multi- Projector Displays Using Camera Homography Trees IEEE Visualization 2002 We can apply these transformations in real-time to an application

LCD lens Stages: 1. Capture 2. Optimization 3. Warping PCProjectorScreen Projector Alignment

The Remaining Challenge: Aligning the Desktop Apply an alignment transformation to a desktop environment Requires system hacking Must intercept or redirect the OS rendering calls Possible Hardware Alternatives Vendor based hardware solutions - expensive NVKeystone  Only warps 1 output  No programming interface (API)

Application Level Solution Divide the output displays into 2 groups Group 1 shows the usual desktop Group 2 runs our application What our application does Reads back pixels from Group 1 frame buffer Applies alignment transformation to Group 1 pixels Redisplays pixels in full screen mode on Group 2 Displays Trade off Lose half of our display outputs Gain automatic alignment

Diagram of Desktop Warping Windows Desktop Rendered VGA 4 VGA 1 VGA 3 VGA 2 Application Shows Desktop Grab Pixels from desktop Warp Pixels to Application

The Resulting Display Projectors initially roughly aligned and keystoned Geometry corrected for seamless appearance

Application Specifics Uses DirectX to read desktop FB Texture mapping warps pixels to App FB Requires Desktop FB and App FB have shared memory on a per-pair basis Performs warping at 30Hz Consumes ~25% CPU on 800 MHz PC Reduces a Quad card to a Dual Card

The Result… Inexpensive Collaborative Walls Questions and comments