Computer Vision

Homework 3: Scale-space blob detection

Basic project outline

1. Generate and apply a Laplacian of Gaussian filter at a variety of scales.

2. You will develop two separate implementations.

○ First, try increasing the size of the filter to detect blobs at different sizes.

○ Next, write code to instead scale the image down, and apply the filter.

3. Perform nonmaximum suppression in each scale space.

4. Display resulting circles.

Test images

The data folder contains four images to test your code, and the output folder contains sample

output images for your reference. Keep in mind, though, that your output may look different

depending on your threshold, range of scales, and other implementation details. In addition to

the images provided, also run your code on at least four images of your own choosing.

Running the code

Download the code and data and run evalCode.m. It runs a dummy implementation of the code

and draws the blob (a circle in the center of the image). On running the code the ouput should

be something like this (exact times will vary based on the machine):

evalCode

Elapsed time is 0.007286 seconds.

Elapsed time is 0.002676 seconds.

And two identical images:

Detailed instructions

Here are the key steps to implement the blob detector:

● Don’t forget to convert images to grayscale (rgb2gray command) and double

(im2double).

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● For creating the Laplacian filter, use the fspecial function (check the options). Pay careful

attention to setting the right filter mask size. Hint: Should the filter width be odd or even?

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● It is relatively inefficient to repeatedly filter the image with a kernel of increasing size.

Instead of increasing the kernel size by a factor of k, you should downsample the image

by a factor 1/k. In that case, you will have to upsample the result or do some

interpolation in order to find maxima in scale space. For full credit, you should turn in

both implementations: one that increases filter size detectBlobsScaleFilter.m, and

one that downsamples the image detectBlobsScaleImage.m. In your report, list the

running times for both versions of the algorithm and discuss differences (if any) in the

detector output. For timing, use tic and toc commands.

● Hint 1: think about whether you still need scale normalization when you downsample the

image instead of increasing the scale of the filter.

● Hint 2: For the efficient implementation, pay attention to the interpolation method you’re

using to upsample the filtered images (see the options of the imresize function). What

kind of interpolation works best?

● You have to choose the initial scale, the factor k by which the scale is multiplied each

time, and the number of levels in the scale space. I typically set the initial scale to 2, and

use 10 to 15 levels in the scale pyramid. The multiplication factor should depend on the

largest scale at which you want regions to be detected.

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● You may want to use a three-dimensional array to represent your scale space. It would

be declared as follows:

● scaleSpace = zeros(h,w,n); % [h,w] – dimensions of image, n – number of levels in scale

space

● Then scaleSpace(:,:,i) would give you the ith level of the scale space. Alternatively, if you

are storing different levels of the scale pyramid at different resolutions, you may want to

use a cell array, where each “slot” can accommodate a different data type or a matrix of

different dimensions. Here is how you would use it:

● scaleSpace = cell(n,1); %creates a cell array with n “slots”

● scaleSpace{i} = myMatrix; % store a matrix at level i

● To perform nonmaximum suppression in scale space, you should first do nonmaximum

suppression in each 2D slice separately. For this, you may find functions nlfilter, colfilt or

ordfilt2 useful. Play around with these functions, and try to find the one that works the

fastest. To extract the final nonzero values (corresponding to detected regions), you may

want to use the find function.

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● You also have to set a threshold above which the Laplacian response will count as a

detection. You should play around with different values and choose one you like best.

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● To display the detected regions as circles, you can use the drawBlobs.m. You should

display the highest scoring 1000 detected blobs for each image. If your code returns

fewer than 1000 blobs then you may have to lower your threshold. Hint: Don’t forget that

there is a multiplication factor that relates the scale at which a region is detected to the

radius of the circle that most closely “approximates” the region.

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For extra credit

● Implement the difference-of-Gaussian pyramid as mentioned in class and described in

David Lowe’s paper. Compare the results and the running time to the direct Laplacian

implementation.

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● Implement the affine adaptation step to turn circular blobs into ellipses as shown in the

lecture (just one iteration is sufficient). The selection of the correct window function is

essential here. You should use a Gaussian window that is a factor of 1.5 or 2 larger than

the characteristic scale of the blob. Note that the lecture slides show how to find the

relative shape of the second moment ellipse, but not the absolute scale (i.e., the axis

lengths are defined up to some arbitrary constant multiplier). A good choice for the

absolute scale is to set the sum of the major and minor axis half-lengths to the diameter

of the corresponding Laplacian circle. To display the resulting ellipses, you should

modify the circle-drawing function or look for a better function in the MATLAB

documentation or on the Internet.

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● The Laplacian has a strong response not only at blobs, but also along edges. However,

edge points are not “repeatable”. So, implement an additional thresholding step that

computes the Harris response at each detected Laplacian region and rejects the regions

that have only one dominant gradient orientation (i.e., regions along edges). If you have

implemented the affine adaptation step, these would be the regions whose characteristic

ellipses are close to being degenerate (i.e., one of the eigenvalues is close to zero).

Show both “before” and “after” detection results.

Helpful resources

● Blob detection on Wikipedia.

● D. Lowe, “Distinctive image features from scale-invariant keypoints,” International

Journal of Computer Vision, 60 (2), pp. 91-110, 2004. This paper contains details about

efficient implementation of a Difference-of-Gaussians scale space.

● T. Lindeberg, “Feature detection with automatic scale selection,” International Journal of

Computer Vision 30 (2), pp. 77-116, 1998. This is advanced reading for those of you

who are really interested in the gory mathematical details.

Grading checklist

As before, you must turn in both your report and your code. Your report will be graded based on

the following items:

1. The output of your circle detector on all the images (four provided and four of your own

choice), together with running times for both the “efficient” and the “inefficient”

implementation.

2. An explanation of any “interesting” implementation choices that you made.

3. An explanation of parameter values you have tried and which ones you found to be

optimal.

4. Discussion and results of any extensions or bonus features you have implemented.

Instructions for submitting the homework

On CTools submit the following files:

● detectBlobsScaleFilter.m

● detectBlobsScaleImage.m

● report.pdf

Also include additional code (e.g. for extra credit) and explain it in the report what each file

does.