Region Processing Flashcards
Describe Convolution
- t(x,y) template
- at each image LOCATION
- SUM of products
- output VALUE
How to normalize result of convolution template?
-normalize output directly (255*9/9)
or
-convolve with [1/9 1/9 1/9 …]
Why is the distributivity of convolution useful?
efficiency gain
compute composite filter + convole > convolve,+ convole
If convolving with a nxn template requires n^2 multiplications and additions per output pixel, how many does convolving with two nx1?
2n
What are 3 applications of convolution?
- Smoothing (noise reduction)
- Sharpening (edge enhancement)
- Template matching (finding objects)
Simplest way to smooth?
average neighbour values
If you have a neighbourhood of m pixels to smooth what values would you put in the convolutional filter?
1/m
Way to avoid smoothing all pixels?
Conditional smoothing
Replace pixel with smoothed version only if their difference is less than δ
How does median smoothing work?
Pick median of set
What does Gaussian smoothing help prevent?
ringing artefact (echoes of boundary)
In summary which smoothing techniques (convolution) exist?
- average neighbourhood values
- conditional
- median
- gaussian
What is the definition of an edge?
An edge, is an extended,
significant,
local change in image intensity
Edge detection methods?
- Roberts Cross
- Prewitt/Sobel
- Canny/Deriche
- Laplacian of Gaussian
- Template matchingg
What’s a different more common name for edge enhancement?
Sharpening
What is the roberts/cross process of edge enhancement?
- enhance the edges of an image
- enhance the ones orthogonal to it
- combine these
- RESULT: edge magnitude image, tangent of orientation image
What is the problem with roberts cross edge detector?
Noise sensitivity -> corruption of edge strength
What edge enhancement operators are less sensitive to noise than Roberts Cross?
Prewitt/Sobe
What is the issue with both Roberts/Cross and Prewitt/Sobe?
Scale σ
What happens as the scale parameter is increased?
- all objects consired at first
- smaller objects discarded
Process of edge enhancement with scale?
- smooth image (Gaussian template +σ)
- differentiation or double differentiation
What are the requirements for the Canny/Deriche Edge Detector?
- accurate localisation
- single response to an edge
Two definitions of “local”
- close PIXEL distance
- OBJECT
Which operator uses the Second Derivative?
Marr-hildreth
What does the Second Derivative improve?
Localizaiton of edge pixel
When looking at double derivative graph, where is the edge location?
Where the graph crosses the axis (“zero-crossing”)
What template does Marr-Hildreth use?
Laplacian of Gaussian
What is template matching?
technique for measuring similarities-> FIND things
How do we measure dissimilarity of template g and and image f?
sum(f-g)^2
