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Fourier Descriptors For Shape Recognition. Applied to Tree Leaf Identification By Tyler Karrels. Motivation. Shape recognition can be useful any time we want to detect a distinct shape. Seek one description that can characterize all shapes to a desired accuracy.

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fourier descriptors for shape recognition

Fourier Descriptors For Shape Recognition

Applied to Tree Leaf Identification

By Tyler Karrels

motivation
Motivation
  • Shape recognition can be useful any time we want to detect a distinct shape.
  • Seek one description that can characterize all shapes to a desired accuracy.
  • The Fourier coefficients have this property. We can approximate with the first M coef.
  • The goal is automatic tree identification.
preparation
Preparation
  • Needed a database of leaves.
  • Collected leaves from 6 species.
  • Took over 200 pictures of various orientations.
theory
Scale Invariant

- A big square and a small square should both be classified as squares.

Shift Invariant

- Where the square exists in an image should not affect performance.

Rotation Invariant

- Any rotated version of the square should be recognized.

And it should work!

Theory

Imagine a picture with a square in it.

A useful shape representation should be…

The Centroid Contour Distance Curve (CCDC)

can satisfy these.

method
Method
  • Reduce the resolution by a factor of 6

- original is 3 Mega pixels, now 0.5 Mega pixels

- increases speed of future calculations

2. Convert original image to grayscale image.

(Black Cherry Tree Leaf)

slide7

threshold

3. Calculate an appropriate grayscale threshold.

4. Threshold to produce a binary image.

slide8

5. Find and label 6 longest boundaries in the image. At this point, the user must select the leaf’s boundary.

Once the database is

completed, eccentricity can

help detect the leaf correctly.

slide9

After the leaf is identified it is extracted from the larger image with some additional zero padding.

6. Fill in any interior holes in the leaf.

7. Remove stem by erosion with a square of size 10x10 pixels.

slide10

Next create a window to overlay on leaf image to extract the stem.

8. Dilate a few times by 15 pixel square to enlarge the image of the leaf with no stem

  • The goal is to create an area larger than the leafs area so we can subtract the images.
  • Notice that we cannot reconstruct the sharp points of the leaf.

Original Leaf

slide11

The set difference is taken and the resulting image is again thresholded to convert back to binary image (removes -1 entries).

9. Select the area with longest boundary. This should be the leafs stem.

  • The point on the stem closest to the leaf body will be the starting point of the boundary sequence.

Extracted Stem (enlarged)

slide12

Assuming a stem is present this method should accurately reproduce the same starting point on different leaves giving us a rotation invariant representation.

Starting Point

Centroid

10. Calculate centroid.

slide13

Centroid

11. Calculate the Centroid Contour Distance Curve (CCDC) in the clockwise direction, beginning at the derived starting point.

what s next
Take the Discrete Fourier Transform using the FFT.

Normalize the transform to [0,1] for a scale invariant representation.

Approximate with a reasonable amount of accuracy.

Store results in a database for use later in comparing and classify a leaf.

Distance function between database entry and unidentified leaf will be calculated with Time-Warp Distance and/or CCDC, angle code histogram, and eccentricity.

What’s Next?