shape from texture nick vallidis march 20, 2000 comp 290 computer vision
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Shape From Texture
Nick Vallidis
March 20, 2000
COMP 290 Computer Vision
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Why Shape from Texture?
• Texture provides our visual systems with a huge amount of information
• Computers should gain lots of information from it too then, right?
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Sometimes texture is all you need
Source: Computer Analysis of Visual Texturesby Fumiaki Tomita and Saburo Tsuji
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So what is texture?
• One very restrictive definition: “Repeating patterns of local variations in image intensity which are too fine to be distinguished as separate objects”
• The patterns that repeat are sometimes referred to as texels– NOTE: not the same as a graphics texel as it is
made of more than one pixel!
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Tell me more about textures!
• There are basically two kinds:– Deterministic– Statistical
• It’s pretty much man-made (deterministic) vs. natural (statistical)
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Deterministic Texture Examples
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Statistical Texture Examples
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What’s the general approach?
• Texture segmentation– hard! This is still a big research area.
• Texture classification– There are many methods to do this.
• Shape from texture– We’ll just pretend we can do the first two...
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Many things to many people
• There isn’t “one” shape from texture algorithm.
• Textures are complex so there are many different aspects that can be taken advantage of.
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Comparison of a few approachesAuthor(year)
Surface cue Surfacetype
Originaltexture
Projectiontype
Bajcsy(1976)
Texture-gradient
Planar Unknown perspective
Ohta(1981)
Texture-gradient
Planar Unknown perspective
Nakatani(1980)
Converging-lines
Planar Parallel-lines
perspective
Kender(1979)
NTPM* Planar Known Ortho-graphic
Walker(1984)
Shape-distortion
Curved Unknown Ortho-graphic
*Normalized Texture Property Map
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Surface Orientation from Texture
• Statistical texture method
• Assumptions:– Texels are small line segments: “needles”– Needles distributed uniformly (in both angle
and position)– Only one, approximately-planar surface– Orthographic projection
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What we’re calculating
• The tilt, , and slant, , of the plane:
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Where do we get needles?
• Imagine straw covering a plane
• Use an edge detector and we’ve got needles! (this even gives us orientation!)
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Ok, so what do we do with them?
• The metric we’re working from is the needle’s angle with the X axis:
X axis
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Define some random quantities
• For every needle, define a vector: [cos(2), sin(2)]
• So we can tell the angle of the plane by the distribution of these vectors on the unit circle!
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Calculate some statistics
• Find the center of mass of the vectors:
N
iiN
C1
2cos1
N
iiN
S1
2sin1
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Calculate some statistics
• But C and S can be put in terms of and : (only holds for orthographic
projection)
cos1
cos12cos
C
cos1
cos12sin
S
(Sorry, no proof on this one…)
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We can solve for the orientation!
• By converting C and S to polar coordinates, we get a simple form to solve for and :
Q
Q
1
1arccos
)2(mod2
(where and )22 SCQ C
Sarctan
2
1
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Example!Original Texture/Needles
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Original vector distribution
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Rotated needles
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Rotated vector distribution
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Other texels
Source: ComputerAnalysis of VisualTextures
Source: Scale-Space Theory inComputer Vision by TonyLindeberg
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Other Texels II