1 / 15

Announcements

Announcements. Final exam Tuesday, March 13, 2:30-4:20, JHN 175 Closed book, closed notes Sample exams handed out today Prof’s office hours extra hour this week: Wednesday 4-5 next week: Monday 12:30-2pm. Photometric Stereo. Readings

cshawver
Download Presentation

Announcements

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Announcements • Final exam • Tuesday, March 13, 2:30-4:20, JHN 175 • Closed book, closed notes • Sample exams handed out today • Prof’s office hours • extra hour this week: Wednesday 4-5 • next week: Monday 12:30-2pm

  2. Photometric Stereo • Readings • Optional: Woodham’s original photometric stereo paper • http://www.cs.ubc.ca/~woodham/papers/Woodham80c.pdf Merle Norman Cosmetics, Los Angeles

  3. image intensity of P Diffuse reflection • Simplifying assumptions • I = Re: camera response function f is the identity function: • can always achieve this in practice by solving for f and applying f -1 to each pixel in the image • Ri = 1: light source intensity is 1 • can achieve this by dividing each pixel in the image by Ri

  4. Shape from shading • Suppose • You can directly measure angle between normal and light source • Not quite enough information to compute surface shape • But can be if you add some additional info, for example • assume a few of the normals are known (e.g., along silhouette) • constraints on neighboring normals—“integrability” • smoothness • Hard to get it to work well in practice • plus, how many real objects have constant albedo?

  5. L1 L3 L2 Photometric stereo N V Can write this as a matrix equation:

  6. Solving the equations

  7. Least squares solution: What’s the size of LLT? Solve for N, kd as before More than three lights • Get better results by using more lights

  8. Computing light source directions • Trick: place a chrome sphere in the scene • the location of the highlight tells you where the light source is

  9. Recall the rule for specular reflection For a perfect mirror, light is reflected about N • We see a highlight when V = R • then L is given as follows:

  10. Computing the light source direction • Can compute θ (and hence N) from this figure • Now just reflect V about N to obtain L Chrome sphere that has a highlight at position h in the image N h H rN θ c C sphere in 3D image plane

  11. V2 V1 N Depth from normals • Get a similar equation for V2 • Each normal gives us two linear constraints on z • compute z values by solving a matrix equation orthographic projection

  12. Results… Input (1 of 12) Normals Normals Shadedrendering Texturedrendering

  13. Results… from Athos Georghiades http://cvc.yale.edu/people/Athos.html

  14. Limitations • Big problems • doesn’t work for shiny things, semi-translucent things • shadows, inter-reflections • Smaller problems • camera and lights have to be distant • calibration requirements • measure light source directions, intensities • camera response function

  15. Trick for handling shadows • Weight each equation by the pixel brightness: Gives weighted least-squares matrix equation: Solve for N, kd as before

More Related