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Analyzing Light and Shape in Van Eyck's Ghent Altarpiece Using Edge Detection Techniques

This research explores the intricate light sources and circular motifs in Jan van Eyck's "Ghent Altarpiece" (finished in 1432). Using advanced image processing techniques like the Hough Transform and Gabor filters, we focus on detecting circular shapes, particularly pearls. By converting the original image to black and white and significantly reducing its size, we streamline our analysis. The study reveals techniques for averaging pixel values within defined light wedges and evaluates the reliability of our light source findings, raising intriguing questions about Van Eyck's mastery of illumination.

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Analyzing Light and Shape in Van Eyck's Ghent Altarpiece Using Edge Detection Techniques

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Presentation Transcript

  1. Van Eyck: Follow the Light Krista Kelly Kevin Ma Advisors: Ingrid Daubechies & Rayan Saab

  2. The Ghent Altarpiece • Finished in 1432 • 24 framed panels • Religious painting • Light source significance

  3. Original size: [6668 4992 3] Pixels Rescaled and converted to black and white Resized: [1111 832] Reduced amount of data by a factor of 108 Which reduced calculation time significantly

  4. How to Analyze the Image? • There are other reflections but for consistency and purest results- focus on one shape • The pearl (dundundun…!) • Circuluar • Uniform • Existing codes available • Hough • Gabor

  5. Canny Filter • Matlab function: • e = edge(bwimage,`canny’, .5); • Shows up in both Hough and Gabor Canny .3 Canny .5

  6. Hough Transform • Edge detection technique • http://www.markschulze.net/java/hough/ • Returns a 3-D accumulator array • Voting on circle locations & radii

  7. Hough Transform - Raw

  8. Gabor Annulus • Circular wavelet filter • Inputs: • Kernel Size --- 80 • Wavelength (=100) --- f0 • Gaussian deviation (1-10 usually) --- • Radius of circle --- r

  9. Thresholding for Gabor Loop Control

  10. Gabor Continued… Gabor Annulus Without Thresholding Gabor Annulus With Thresholding

  11. Once the Gabor is used… Threshold again using “loop control” This is then put through a function called imdilate – a logical matrix that will threshold further so we can locate the pearls.

  12. Plot of Rough Pearl Locations

  13. Plot of Thresholded Pearl Locations

  14. “Pcards” There are no Aces, Kings, or Queens. • Given the locations of where pearls may be found • We create “Pcards”: small 40x40 images around each location and fill up what we call P, the deck of cards/images • The size of P is [40 40 #-of-cards] • Sample Pcard

  15. Use and abuse the Pcards • Apply the Hough Transform • Locates the relative center and radius on the Pcard • Extract the literal locations • Able to cut out the pearl • Now what about the light?

  16. Wedges – find the light • Used two variables, alpha and delta. • Cut out 20 degree wedges from -90 to 90 • Average the values by summing the pixel values and divide by the number of pixels. • Find the max • If n<3, disregard

  17. Raw Angles

  18. Thresholding • We discounted an angle using the following rule: • If the MaxValue<MeanValue*1.2 • Discount the angle

  19. Thresholded Angles

  20. Conclusions Our research suggests he was at least accurate enough to have a general direction • Is there more than one light source? • Was he really that good? • Error in our calculations is significant – • The answer is still up in the air Questions?

  21. References • Hough Transform Code: • http://www.mathworks.com/matlabcentral/fileexchange/26978-hough-transform-for-circles • Gabor Annulus Code & Paper: • http://red.cs.nott.ac.uk/~aqr//circle/ • A. Rhodes and L. Bai. Circle Detection Using a Gabor Annulus. Proceedings of the 22nd British Machine Vision Conference. Dundee, UK, 2011.

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