Image Correction for Immersive Visualization Systems

1. Image Correction for Immersive Visualization Systems Alex Timchenko 02/03/2006

2. Project Background • Immersive Visualization System • TAMU TITF Grant 2002 - 2005 • NSF Grant - Ongoing • Principle Investigators • Dr. Frederic Parke • Dr. Donald House

3. Project Purpose • Development of a modular low-cost immersive environment • Visuals consists of a set of projectors back-projecting images on polygonal facets • Have to utilize readily available technology and materials • Powerful programming API to facilitate development of new applications for the system

4. 5-Facet Display Example

5. Need for Coherency • Temporal coherency • Each display must project a part of the world in-sync with the other projectors • Achieved through network protocols • Spatial coherency • Two adjacent facets must display adjacent information with as little of a seam as possible

6. The Problem • Image alignment on individual projectors We Want… We Get…

7. Problems with Projectors The projector is off-axis; lower distance shorter than upper distance Results in strong vertical perspective distortion

8. Problems with Projectors • Perspective distortion • Vertical distortion • Keystone • Horizontal distortion • Keystone correction not as common • More expensive • Keystoning reduces image quality • Difficult to manually align projector

9. Problems with Projectors • Barrel Distortion • Very difficult to manually correct • Makes perfect alignment virtually impossible

10. Problems with Projectors • Other Problems • Non-uniform Intensity • Non-uniform Color • Color difference between projectors

11. Solutions • Manual/Mechanical Solution • Very difficult to precisely align • No way to correct barrel distortion • Software based solution • Apply a real-time warp to the projected image, allowing the user to control the corners and midpoints of the polygon

12. Solutions • First approach: Quad-Grid • Each edge vertex is moveable • Moving the corners corrects perspective distortion • Moving the edge points corrects barrel distortion

13. First approach: Quad-Grid Good visual results Difficult to use Limited to quads Rendered with OpenGL evaluators Solutions

14. Solutions

15. Solutions • Next Approach: Web • Improved intuitive interface • Applicable to n-sided facets • Very simple structure

16. Solutions • Next Approach: Web • Difficult to find centroid • Texture “swims” across seams • Localized texture effect Creates slight shear and pinch Unaffected area pinch No side-effects

17. Solutions • Next Approach: N-Grid • Places a grid over an n-sided facet • Distributes texture difference over a larger area • Focuses distortion to corners • Structure is difficult to handle • OpenGL rendering issues

18. Solutions • Other approaches? • Spring-based meshes • Treat interior points as particles • Treat edges as springs • Spring Stability • Return to initial state

19. GPU Rendering • Instead of relying on OpenGL default texturing, control the warping through the GPU • Create a 2D displacement texture • Access the displacement texture to get an offset, then access the image with the UV coordinates and the offset

20. GPU Rendering • If we can find a displacement map for a mesh, we can use it • Requires fairly advanced GPU Hardware Texture Access Texture Access Texture Access Texture Access Common Functionality Parallel Texture Access Advanced Functionality Sequential Texture Access

21. GPU Rendering • Other advantages • Color correction • Easy to hue/color shift texel values • Brightness correction • Easy to adjust the brightness of texels • Intensity falloff correction by altering brightness based on some grayscale image

22. Conclusion • Image correction critical to immersive display systems • Commodity projectors introduce problems • Perspective and barrel distortion are difficult to correct manually • Software solutions based on grid structures and real-time image warping produces best results

23. Thank you for listening!