A Framework for Analysis of Computational Imaging Systems
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1 A Framework for Analysis of Computational Imaging Systems Kaushik Mitra, Oliver Cossairt, Ashok Veeraghavan Rice University Northwestern University
2 Computational imaging CI systems that adds new functionality Light Field Capture Structured Lighting HDR Imaging Motion deblurring system CI systems that improves performance Extended depth of field Others: Multiplexed Light field Illumination Spectography
3 How does CI improve performance? Increased light throughput Short exposure Flutter Shutter Increased light throughput but blurry Sharp, but noisy Deblurred image Slide coutesy Amit Agarwal
4 How does CI improve performance? Well conditioned optical coding Flutter Shutter Large exposure Captured image Deblurred image Slide coutesy Amit Agarwal
5 Recovered image Captured image One key flaw: Signal prior has not been taken into account Short exposure Flutter Shutter Large exposure BM3D denoising BM3D deblurring BM3D deblurring SNR= 17 db SNR= 19 db SNR= 13.4 db
6 State-of-the-art systems use signal prior Denoising using BM3D Coded exposure video using dictionary learning Inpainting using GMM Dabov et al., 2011 Hitomi et al., 2011 Yu et al., 2011
7 Our goal: A comprehensive analysis Scene Image Computational camera Multiplexed image Multiplexed image Multiplexing matrix (Read+photon) Noise Signal prior P(x) Our analysis takes into account: Signal prior Multiplexing matrix Noise characteristics Figure courtesy Oliver Cossairt
8 Prior Work: Analysis of CI systems 1. Analysis under read noise withour prior 2. Analysis under affine noise withour prior Harwitt et al Ratner et al. 2007, Wuttig 2007, Hasinoff et al. 2008, Ihrke et al. 2010, Cossairt et al Relates performance to practical considerations such as illumination, sensor characteristics, etc. Cossairt et al. 2012
9 Our analysis framework: GMM as signal prior Advantages of GMM 1.Universal approximation property 3. State-of-the-art results Image processing Yu et al LF processing Sorenson et al., Analytically tractable A special case is Gaussian prior, whose MMSE can be computed analytically Mitra et al. 2012
10 Our analysis framework: Linear system Multiplexed image Multiplexing matrix Noise Motion blur Defocus blur Single pixel camera [Raskar 06] [Levin 08] [Cho 10] Light Field Capture [Hausler 72] [Nagahara 08] [Dowski, Cathey 96] [Levin et al. 07] [Zhou, Nayar 08] Reflectance [Wakin et al., 2006] High speed video [Lanman 08] [Veeraraghavan 07] [Liang 08] [Schechner 03] [Ratner 07] [Ratner 08] [Hitomi et al. 2011][Veera et al., 2011]
11 Slide courtesy Oliver Cossairt Our analysis framework: Affine noise model Noise Variance at i th Pixel: photon noise aperture, lighting, pixel size read noise electronics, ADC s, quantization J i : i th pixel intensity Signal dependent / independent noise Ignore Dark current, fixed pattern Noise PDF: Photon noise modeled as Gaussian (good approx. if #photons > 10) Photon noise spatially averaged
12 Complete specification of the framework Multiplexed measurement Multiplexing matrix Noise Learn patch-based GMM prior GMM Cluster 1: mean and PCA components GMM Cluster 2: mean and PCA components Cluster weight Cluster mean GMM patch prior Cluster covariance
13 MMSE as a performance metric Mean Squared Error (MSE) of an estimator is defined as: MMSE estimator: Defined as the estimator that achieves the minimum MSE Given by the posterior mean MMSE is the corresponding MSE error MMSE: a scalar that characterizes the performance of a system H
14 Computation of MMSE estimator The posterior PDF is also a GMM: old weight with new weights Probability of y coming from kth cluster and with new mean and covariance: The MMSE estimator (posterior mean):
15 Interpretation of MMSE Intra-cluster error, can be computed analytically Inter-cluster error needs MC simulations We have an analytical lower bound for the MMSE: Tight bound for fully-determined system H
16 Limitations of analysis Patch-based GMM prior GMM Cluster 1 GMM Cluster 2 Local multiplexing Shift invariant blur (motion and focus) Other assumptions: Linear systems Affine noise [Nagahara 08] [Dowsky 96] [Levin 08]
17 Practical implications of the analysis
18 Practical system performance depends on 1. Illumination condition 2. Scene reflectivity 3. Camera parameters F/#, Exposure time t, quantum efficiency q, pixel size p Average signal-level is given by: Average Signal (e - ) Illumination (lux) Reflectivity Aperture Exposure Time (s) Quantum Efficiency Pixel Size (m)
19 Average signal level for three form factors SLR camera Pixel size p SLR = 8 μm Machine vision camera (MVC) p MVC = 2.5μm Smartphone camera (SPC) p SPC = 1μm Typical values of average signal level J for different illumination levels Other parameters: q=.5, R =.5, F/11, t = 6ms, σ r =4
20 Common analysis and simulation framework Learn GMM prior of patch size GMM Cluster 1 GMM Cluster 2 Analytical computations: Without prior: Under GMM prior: Simulation computations: Perform per-patch reconstruction. Let y be the observed patch. Without prior: Under GMM prior: Performance measure: SNR gain w.r.t impulse imaging
21 Analysis of Extended DOF systems
22 Depth of Field and SNR Image Lens F Small apertures have large depth of field and low SNR Slide courtesy Oliver Cossairt
23 Focal Sweep: An example EDOF system Sensor Lens (depth) Point Spread Function (PSF) [Hausler 72, Nagahara et al. 08] Slide courtesy Oliver Cossairt
24 Focal Sweep: An example EDOF system Sensor Lens (depth) = t = 1 t = 2 t = 3 t = 4 t = 5 t = 6 t = 7 (400) (600) (900) (1200) (1500) (1700) (2000) [Hausler 72, Nagahara et al. 08] Integrated PSF Slide courtesy Oliver Cossairt
25 Depth Invariant PSF Focal Sweep PSF Traditional Camera PSF mm 750mm mm mm 2000mm mm mm 2000mm Extended depth of field with a single deconvolution Slide courtesy Oliver Cossairt
26 Recovery with GMM Recovery without prior Captured image Simulation performance Impulse (F/11) Low light condition (10 lux) Focal sweep (F/1) Pixel size = 8 um Exp time = 6 ms SNR = 4.8 db ~5.5 db multiplexing gain SNR = 10.2 db ~11.5 db gain due to prior SNR = 15.7 db SNR = 22.4 db Gain due to prior is much greater than gain due to multiplexing
27 Recovery with GMM Recovery without prior Captured image Simulation performance Impulse (F/11) High light condition (1000 lux) Focal sweep (F/1) Pixel size = 8 um Exp time = 6 ms SNR = 29.5 db ~1 db multiplexing gain SNR = 30.3 db ~1.5 db gain due to prior SNR = 30.8 db SNR = 31.9 db At high light condition, gain due to both prior and multiplexing is negligible.
28 SNR gain (in db) Analytic performance: SNR gain vs. illumination level (without prior case) FS multiplexing gain without prior Impulse Photon to read noise ratio (J/σ 2 ) I SLR I MV I SP Huge multiplexing gain at low light levels
29 SNR gain (in db) Analytic performance: SNR gain vs. illumination level (with prior case) Impulse GMM FS FS GMM multiplexing gain with prior Gain due to prior alone multiplexing gain without prior Impulse Photon to read noise ratio (J/σ 2 ) I SLR I MV I SP Under signal prior moderate multiplexing gain at low light levels
30 Other EDOF systems Depth invariant PSF systems Coded aperture systems [Dowski, Cathey 96] [Cossairt et al. 10] [Levin et al. 07] [Zhou et al. 08] [Veeraraghavan et al. 07] Depth invariant PSF Coded Camera PSF mm 750mm mm mm 2000mm mm mm 2000mm
31 SNR gain (in db) Analytic performance with Prior Impulse camera: F/11 Other cameras: F/1 SNR gain vs. light level J/σ 2 I SLR I MVC I SPC Good EDOF systems perform 9 db better than impulse imaging
32 Analysis of motion deblurring systems
33 Light throughput vs. motion blur Increasing exposure time Noise decreases but motion blur increases
34 Motion deblurring CI systems Coded exposure (Flutter shutter) [Raskar 06] Motion invariant photography [Levin 08] [Cho 10] Increased light throughput and inversion better conditioned Captured image has same motion blur for different motions Captured image Deblurred image Whole image deblurred using a single blur kernel
35 Recovered Captured Simulation Performance under signal prior t CI =33 t impulse Impulse imaging Flutter Shutter Motion invariant Low light condition (10 lux) SNR= -1.7 db Motion invariant 7 db better than impulse SNR= 13.5 db SNR= 16.8 db SNR= 20.9 db High light condition (1000 lux) Motion invariant 1.2 db better than impulse Recovered Captured SNR= 25.8 db SNR= 28.2 db SNR= 24.7 db SNR= 29.4 db
36 SNR gain (in db) Analytic Performance under signal prior t CI =33 t impulse SNR gain vs. light level J/σ 2 I SLR I MVC I SPC Motion invariant camera achieves a peak SNR gain of 7.5 db
37 Conclusion: Comprehensive analysis framework of CI Scene Image Computational camera Multiplexed image Multiplexed image Multiplexing matrix Affine noise Our analysis accounts for Signal prior (GMM) Optical coding (H) Noise (affine) Signal prior P(x) GMM Cluster 1 GMM Cluster 2
38 Conclusion: Practical implications 1. Illumination condition 2. Scene reflectivity 3. Camera parameters F/#, Exposure time t, quantum efficiency q, pixel size p We analyzed EDOF and motion deblurring systems for typical values of: Illumination conditions Scene characteristics Camera parameters
39 Conclusion: Our observations More gain due to prior than multiplexing Gain due to multiplexing modest when prior is taken into account CI systems provide significant advantage over impulse imaging under various illumination and camera parameters EDOF systems provides on average 7 db gain over impulse imaging Motion deblurring systems provides on average 4.5 db gain over impulse imaging
40 Future Work Analyze compressive systems: High speed video Light Field Capture Multi/Hyper-Spectral [Hitomi et al. 2011][Veera et al., 2011] [Lanman 08] [Veeraraghavan 07] [Liang 08] Single pixel camera [Sloane 79] [Hanley 99] [Baer 99] [Wetzstein et al., 12] [Wakin et al., 2006] Design optimal CI systems
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