Strata: Layered Coding for Scalable Visual Communication

Strata: Layered Coding for Scalable Visual Communication

Wenjun Hu Jingshu Mao Zihui Huang Yiqing Xue Junfeng She Kaigui Bian Guobin (Jacky) Shen

You may have seen these…

Smartphone cameras as “receivers”

… everywhere

Existing codes: All or nothing

Camera view

Minimum resolution needed

Multi-resolution information display

Diverse camera hardware

Lumia 1020 ~38MP, 30fps

Nexus 5 8MP, 30fps

iPhone 4 5MP, 30fps

iPhone 5s 8MP, 120fps

Capture distance vs resolution

Our goal

Multi-resolution encoding and decoding Analogous to HM and SVC for scalable channel/source coding Decoding performance scales with receiver capability and channel condition

In the rest of the talk…

Undersampled channel Strata design Strata performance Focus on spatial, analogous for temporal See paper for temporal mixing, code design, and performance results

Undersampled channel

Spatial undersampling

Original image

Captured at 28m , enlarged

Spatial undersampling

In theory, linear mixing of pixel colors In practice, messier… (Auto-)Focus, exposure, contrast Noise at block edges …

Color mixing experiment

Patterns of alternating squares

Color mixing results

Color mixing results

For the same distance, close to linear color mixing

Color mixing results

Darker when further away White is better preserved

Color mixing results

Less grayscale difference if minimal contrast

Strata Design

Toy example

2 layers Black overall Also small blocks Small white blocks are interference

A simple SNR (like) model

Signal = # of small blocks of the intended color Noise = # of small blocks of other colors Both noise and interference This actually reflects color mixing weights for monochrome codes

A simple SNR (like) model

If SNR > 1, the overall block carries at least 1 bit of information Its color can be determined Key: Control noise/interference “Reserve” the color of some blocks

Basic design

2-layer structure, with reserved block

Reserved block

Base layer block

Enhancement layer block

Ensures black majority Contiguous to mitigate issues in color mixing

Harnessing additional bits

Bits from reserved block shape

2 bits from 4 different positions

Example 2-layer code

Base layer block

Enhancement layer block

Recursively adding layers

Divide each enhancement block further Follow the same rule otherwise

Reserved block

(In)Dependency between layers: structural, but not the information encoded

Base layer (2nd layer) block

Enhancement layer (3rd layer) block

Harnessing more bits

Adding smaller (third-layer) blocks in the reserved block

Harnessing more bits

Doubles the third-layer capacity Can further add fourth-layer blocks

Choice of parameters

Reserved block size Efficiency vs accuracy tradeoff Need ? when not knowing input statistics “Branching factor” Control of granularity 16 offers a good tradeoff Details in the paper

Decoding

Decoding

Divide into blocks Start with Layer 1 Determine per-block colors 11 blacks, 5 whites Majority gives overall block color (black)

Decoding

Continue with finer blocks Stop if can’t divide further

Strata performance

Example Strata image

20cm x 20cm on screen, no error correction

Decoder implementation

Online Android app & Offline version

Multi-level decoded info

Strata test display

Performance metric

Goal recap: The amount of information decoded scales with capture resolution/rate Metric: decodable bit/layer count

Performance (spatial)

Performance (spatial)

Strata vs Single-layer code

Strata vs Single-layer code

Strata vs Single-layer code

Strata balances capacity and supported distance

Other comparison results

Strata vs frequency domain encoding Strata vs multi-level grayscales Strata vs group of codes Details in the paper

Related work

Temporal barcode design (inter-frame) Unsynchronized 4D Barcodes Spatial barcode design (per-frame layout) PixNet, COBRA Visual tags Hierarchical coding on LED arrays Other work on visible light communications E.g., Visual MIMO

Conclusion

Diverse screen/surface-camera channels Hardware diversity Capture conditions vary Strata: Layered coding for scalability The amount of information decoded scales with diverse channel conditions

Thank you

Questions?