3D Scene Calibration for Infrared Image Analysis V. Martin, V. Gervaise, V. Moncada, M.H. Aumeunier, M. Firdaouss, J.M. Travere (CEA) S. Devaux (IPP), G. Arnoux (CCFE) and JET-EFDA contributors Workshop on Fusion Data Processing Validation and Analysis, ENEA Frascati, 26-28 March 2012

3D IR Scene Calibration Issue: a complex thermal scene JET #81313 KL7 (images in DL) Issue: a complex thermal scene Wide angle views with high geometrical effects: depth of field and curvature Many metallic materials (Be, W) with different and changing optical (reflectance) and thermal (emissivity) properties Objective: Match each pixel with the 3D scene model of in-vessel components for: getting the real geometry of the viewed objects reliable linking between viewed objects and their related properties Applications Image processing (e.g. event characterization) IR data calibration: Tsurf = f(material emissivity) Bulk Be W coated CFC Bulk Be Bulk Be Be coated linconel W coated CFC Bulk W

2D/3D Scene Model Mapping Methodology Calibration chain NUC Dead pixel Map Reference image 2D/3D scene models Knowledge base of the thermal scene Image Correction Image Stabilization 2D/3D Scene Model Mapping Image Processing Registered & Geo-calibrated Image Camera

Illustration of Motion in Images Camera vibrations lead to misalignments of ROIs (PFC RT protection) = false alarms or worth missed alarms Image stabilization is a mandatory step for heat flux deposit analysis based on Tsurf(t)-Tsurf(t-1) estimations

Image Stabilization Important factors for method selection Deformation type: planar (homothety), non-planar Target application: real-time processing, off-line analysis Data quality and variability: noise level, pixel intensity changes, image entropy Required precision level: pixel, sub-pixel Applications in tokamaks (non-exhaustive list) Motion amplitude Target application Precision required Difficulty JET KL7 wide-angle 5-10 pixels (camera vibrations) Hot spot detection PFC protection pixel low image entropy windowed up to 15 pixels (disruptions) Physics analysis (e.g. heat load during disruptions…) pixel intensity changes JET KL9 divertor tiles <1 pixel (sensor affected by magnetic fields) Physics analysis (power deposit influx) sub-pixel low resolution, slow motion, aliasing

Image Stabilization Classical Methodology Feature Detection Local descriptors: Harris corners, MSER, codebooks, Gabor wavelets (see Craciunescu talk), SIFT, SURF, FAST… Global descriptors: Tsallis entropy (see Murari talk), edge detectors… Fourier analysis: spectral magnitude & phase, pixel gradients, log-polar mapping… Feature Matching Spatial cross-correlation techniques: normalized cross-correlation, Hausdorff distance… Fourier domain: normalized cross-spectrum and its extensions Transform Model Estimation Shape preserving mapping (rotation, translation and scaling only) Elastic mapping: warping techniques… Image transformation 2D Interpolation: nearest neighboor, bilinear, bicubic… See Zitova’s survey, Image and Vision Computing, vol. 21(2003), pp. 977-1000

Proposed Algorithm Masked FFT-based image registration [1] Deterministic computing time Accelerating hardware compatible algorithm (e.g. FFT on GPU) → real time applications Local analysis with dynamic intensity-based pixel masking (e.g. mask the divertor bright region) with sub-pixel precision [2] Slow drift compensation and dynamic update of the reference image Robust to image intensity and structural changes Evaluation of the registration quality over time [1] D. Padfield, IEEE CVPR’10, pp. 2918-2925, 2010 [2] M. Guizar-Sicairos et al., Opt. Lett., vol. 33, no. 2, pp. 156-158, 2008

Principle of Fourier-based Correlation Let Iref a reference image, It an image at time t and DFT the Discrete 2D Fourier transform such as It ( x , y ) = Iref ( x-x0 , y-y0 ) Iref It max (NCC(Iref, It)) NCC(Iref, It) NCC is the Normalized Cross Correlation figure (image) and the position of the peak gives the coordinates of the translation ( x0 , y0 )

Sub-pixel Precision Up-sample k times the DFT of NCC (trigonometric interpolation): The peak coordinates ( x0 , y0 ) give F the translation with 1/k pixel of precision:

Reference Image Updating Goal: maintaining a good reliability of the motion estimator (NCC peak value) while image appearance changes during the pulse.

Reference Image Updating Solution: use the NCC peak value to trigger the update of Iref such as: update Iref update Iref update Iref update Iref NCC peak too low, no Iref update

Results JET #81313 (MARFE, disruption), KL7, 480x512 pixels, 50 Hz, 251 frames k=1/4 pixel

Results JET #80827 (disruption), KL7, 128x256 pixels, 540 Hz, 13425 frames k=1/2 pixel

Results JET #82278, KL9B (slow drift), 32x96 pixels, 6 kHz, 4828 frames 96 pixels 32 pixels

Computational Performance High frame rate performance using GPU 256x256, k=1/4 → 700 fps

From 2D to 3D Challenge Method transform pixel coordinates into machine coordinates: (x, y)  (r, θ, φ) Method Ray-tracing method from 3D/simplified CAD files

3D Scene Model for Image Processing S. Palazzo, A. Murari et al., RSI 81, 083505, 2010 Z Map (depth) 1 mm 2 2m 1 Blobs 1 & 2 must not be merged! 7m 2 1 2 V. Martin et al.

2D/3D Scene Model Mapping Integrated Framework An integrated software for IR data stabilization & analysis Set sub-pixel precision factor Set mask Load/save translations Used for PFC protection Used for temperature evaluation Used for event triggering Image Stabilization 2D/3D Scene Model Mapping Image Processing Image Correction Camera NUC Dead pixel Map Reference image 2D/3D scene models Knowledge base of the thermal scene Registered & Calibrated Image Plasma ImagiNg data Understanding Platform (PINUP)

Conclusion Summary Outlook Complex IR scenes require a new approach for reliable data analysis including image stabilization and 3D mapping. A robust and fast image stabilization algorithm with sub-pixel precision has been proposed. A first demonstration of 3D model for IR data analysis has been successfully carried out at JET on the wide-angle ITER-like viewing system (KL7). An integrated software (PINUP) implementing these features is available for users upon request. Outlook Test of the stabilization algorithm on visible imaging data (JET KL8) with rotation compensation Full integration of 3D scene models into PINUP Improvement of image processing algorithms (e.g. hot spot detection) with 3D information