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Video Textures
This is a very cool paper.
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Why video texturing?
Static images are boring Video clips are finite in length We need something more interesting
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What is video texturing?
The process of extracting an infinitely varying sequence of continuous images from a finite length video clip
For the purposes of creating backward compatible videos, it is sufficient to generate a single video clip with continuity between the final and beginning frames
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The System
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The System
Objectives Find points in the video between which transitions
can be made Smooth the transition as much as possible Rendering
An infinite random play A finite video clip using a looping sequence to
simulate infinite play
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Representation
Probability Matrix Each entry indicates the probability of a certain
frame-to-frame transition being taken Good for dense set of possible transitions
Link Set Each entry indicates a link from one frame to
another and that transition Good for sparse set of possible transitions More typical
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Techniques
Splitting the video into distinct regions
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Techniques
Sprites
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Extraction
First Equalize brightness Stabilize video Enhance in any other way desired
Compute the L2 distance to determine
similarity between frames Store them in the matrix:
Dij = ||Li-Lj||2
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Extraction
Now transitions from frame i to frame j can be performed when Di+1,j is sufficiently small (i.e. similar)
Store the probability of these transitions in a probability matrix (or analogous structure):
Pij = exp(-Di+1,j/σ)
Then normalize each transition according to the rule:
ΣjPij = 1
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The Problem
This creates smooth frame-to-frame transitions, but…
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Preserving Dynamics
Smooth frame-to-frame transitions are not sufficient
Smooth motions must also be conveyed In other words
For a transition from i to j to be “good,” the frames i±m must be similar to the frames j±m
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Preserving Dynamics Simply take the similarity matrix D and check
for similarities between frames to form the filtered matrix D’
D’ij = Σk(wkDi+k,j+k) where -m≤k≤m-1 And m is the number of frames to check on either side (1 or 2 typically)
Now compute probabilities based on D’
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Preserving Dynamics
Graphically:
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Preserving Dynamics
Much better, but…
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Avoiding Dead Ends Now we have a dead end No problem…
Simply calculate the anticipated cost of transitioning from one frame to another D''ij = (D'ij)p + αΣkP''jkD''jk
High p favors the fewer good transitions Lower p favors the abundant poor transitions 0.99 <= α <= 0.999 makes a good alpha Alpha controls the affect that future transitions should have
P'' is now calculated with D''
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Avoiding Dead Ends
Solving the interdependencies has a simple iterative solution… However, it’s slow to converge
So simplify the calculation D''ij = (D'ij)p + α·mink(P''jkD''jk) This assumes sigma approaches 0 (i.e. Pij approaches 1) for good transitions… only the best transition is considered
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Avoiding Dead Ends See paper for details on iterative solution
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Sequencing Random Play
Random (infinite) play Begin sequence at any transition before the last
non-zero probability transition while (1)
Determine transition after frame i by finding frame j such that Pij is greatest
Generates video textures that never repeat exactly
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Sequencing Video Loops
Primitive loop Single transition from i to j
Must satisfy i ≥ j to guarantee loop Properties
Range = [j,i] Cost = D’ij
Length = i-j
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Sequencing Video Loops
Compound Loop Combination of one or more primitive loops (or
compound loops) Each loop must overlap with at least one other Properties
Range = Uk range(Tk)
Cost = Σk cost(Tk)
Length = Σk length(Tk)
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Sequencing Video Loops
Selecting Transition Sets Need to find the optimal loop for a given
sequence length This is just a simple DP algorithm
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Still Sequencing Video Loops
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Sequencing Video Loops
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Sequencing Video Loops
Scheduling Now we have a set of transitions to use All we know, based on the properties of
compound loops is that the set represents a continuous range of frames
Must schedule the set into a coherent sequence of transitions
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Sequencing Video Loops
Algorithm for Scheduling Primitive Loops 1. Schedule the transition at the end of the range
first 2. Remove transition from set – this may leave
one or more disjoint sets 3. Schedule the next transition from the first
disjoint set that begins after the beginning of the removed transition
4. Repeat 2-3 until no transitions remain
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Rendering
Between transitions Cross fade Morph or de-ghosting
Between film regions Feathering
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Questions?
Is “cyclified” a word?