PROCAMS 2005 San Diego Futures Laboratory MCS ANL CI UC Pixel Aligned Warping for Multiprojector Tiled Displays Mark Hereld and Rick Stevens Argonne National

PROCAMS 2005San Diego

Futures LaboratoryMCS ANL CI UC

Pixel Aligned Warpingfor

Multiprojector Tiled Displays

Mark Hereld and Rick StevensArgonne National Laboratory

Computation InstituteUniversity of Chicago



Introduction

Warping image content into projector frame buffers is a fact of life

Driven by geometrical constraints Never driven by clarity constraint

except in that geometrical constraints are sometimes driven by alignment with nearby or overlapping projected content

What if content crispness is more important?



pfcutoff 2

1

Pixel Exploit

Nyquist and the pixelLone illuminated pixel contains high frequency

components well beyond Nyquist cutoffAnti-aliasing techniques are the standard

antidote when sharp features don’t fall on pixel boundaries

Often exploited to represent thin, crisp featuresGraphics: single pixel width horizontal and

vertical lines, and pointsText: sharp and sometimes small fonts



Example



Warped

)),(),,((),( yxfyxfvu vu



Nearest Neighbor

Plate

Rift Line

)),(),,((),( yxfyxfROUNDvu vu



Warped w/Rifts



Detail



Possible Approaches

Modified nearest neighbor: soften rift boundary Stretch rift lines to avoid cutting through important stuff Perturb warp according to local measure of steepness Minimize an energy function that includes terms

reflecting affinity of features for pixel boundaries and gives weight to keeping proximal features together

Crystallize pixel values around nucleation centers related to locations of high-contrast features in the image

Segment the image into unwarpable islands of content

X coordinate of pixel

plateplate

rift

dX

of

warp



Possible Approaches







Possible Approaches




Segment the image into unwarpable islands of content)]),(),,(([),(

)],(),,([)),(1(],[

yxfyxfROUNDyxCONSTRAST

yxfyxfyxCONTRASTvu

vu

vu



Possible Approaches





x,y

u,v

f0u,f0v

Ci-1.Ci Ci

.Ci+1

k k k

feature blocking energy

target warp term

pixel mis-alignment

penalty

fu,fv



Possible Approaches







Possible Approaches







Islands in the Ocean

Lock contiguous blocks of high contrast pixels, islands, to one another and to nearest plate

Warp everything else, the ocean, in the usual fashion

Don’t worry about possible artifacts around the edges of the islands, the beaches



Algorithm Compute contrast of each pixel

w.r.t. 3x3 neighborhood Binarize Consolidate into islands

Morphological fill & open Pin island coordinates

to plate containing island centroid Warp ocean Transfer islands into warped ocean

cut-and-paste unwarped

9.0)(

)(

minmax

minmax II

II



Example



Contrast



Labeled



Labeled w/Rifts



PAW



Warped



Results

Interesting notion Simple algorithm

Islands in the Ocean

Very effectiveExcept when it’s

not• See “Weaknesses”

Large warpsRotationKeystone

Tiling introduces new problemsOverlap or

abutting

Large islandsCollisions

Strengths Weaknesses



Future

Robust rules for handling pathological cases gracefully

Fast implementation compatible with hardware accelerators

Handle blending areas in tiled displays Awareness of horizontal and vertical features

allow lines to stretch or shrink but not kink

Apply to image scaling Particularly high res laptop display driving XGA

projector



Summary

Pixel edges are commonly exploited to represent sub-Nyquist features

Warping can degrade this crispness

Many approaches to pixel aligned warping that could preserve much of this crispness at the expense of geometrical accuracy

Islands in the Ocean is effective

Documents

PROCAMS 2005 San Diego Futures Laboratory MCS ANL CI UC Pixel Aligned Warping for Multiprojector Tiled Displays Mark Hereld and Rick Stevens Argonne National