Revision Sesh Flashcards
“Salt-and-pepper” noise is image noise that appears as white and black pixels in the image. Which image filter can be used to reduce/eliminate salt-and-pepper noise? Describe how the filter works.
the median filter is a non-linear filter that replaces the value of a pixel by the median of the value of a pixel by the median of the values in mask of the filter.
Median = “middle value” - i.e. that value m so that half of the values in the set are less than or equal to m, and the other greater than or equal to m.
What is the median filter useful for?
The median filter is useful for removing/reducing certain Types of random noise (impulse noise, random occurences of black and white pixels, salt-and-pepper noise).
Will lead to less blurrinng compared to averaging.
Describe spatial filtering.
Kernel is moved across the image. Centre of mask is aligned with current central pixels.
What is convolution in terms of spatial filtering?
Mask multiplied with pixel value and summed to give new value for central pixel.
What is the purpose of smoothing filters?
They “smoothe” the image and used for blurring and for noise reduction.
Typically used in pre-processing for:
- removal of small details from an image (e.g. prior to object extraction)
- bridging of small gaps in lines or curves
- reducing the effect of image noise
Describe the averaging filter (mean filter).
Output is simply the average of the pixel contained in the neighbourhood of the filter mask.
What are gaussian filters purpose?
Useful for smoothing images (and better than mean filtering).
