| Feature detection |
|---|
| Edge detection |
| Corner detection |
| Blob detection |
| Ridge detection |
| Hough transform |
| Structure tensor |
| Affine invariant feature detection |
| Feature description |
| Scale space |
Pyramid, orpyramid representation, is a type ofmulti-scalesignalrepresentation developed by thecomputer vision,image processing andsignal processing communities, in which a signal or an image is subject to repeatedsmoothing andsubsampling. Pyramid representation is a predecessor toscale-space representation andmultiresolution analysis.
There are two main types of pyramids: lowpass and bandpass.
A lowpass pyramid is made by smoothing the image with an appropriate smoothing filter and then subsampling the smoothed image, usually by a factor of 2 along each coordinate direction. The resulting image is then subjected to the same procedure, and the cycle is repeated multiple times. Each cycle of this process results in a smaller image with increased smoothing, but with decreased spatial sampling density (that is, decreased image resolution). If illustrated graphically, the entire multi-scale representation will look like a pyramid, with the original image on the bottom and each cycle's resulting smaller image stacked one atop the other.
A bandpass pyramid is made by forming the difference between images at adjacent levels in the pyramid and performing image interpolation between adjacent levels of resolution, to enable computation of pixelwise differences.[1]
A variety of different smoothingkernels have been proposed for generating pyramids.[2][3][4][5][6][7] Among the suggestions that have been given, thebinomial kernels arising from thebinomial coefficients stand out as a particularly useful and theoretically well-founded class.[3][8][9][10][11][12] Thus, given a two-dimensional image, we may apply the (normalized) binomial filter (1/4, 1/2, 1/4) typically twice or more along each spatial dimension and then subsample the image by a factor of two. This operation may then proceed as many times as desired, leading to a compact and efficient multi-scale representation. If motivated by specific requirements, intermediate scale levels may also be generated where the subsampling stage is sometimes left out, leading to anoversampled orhybrid pyramid.[11] With the increasing computational efficiency ofCPUs available today, it is in some situations also feasible to use wider supportedGaussian filters as smoothing kernels in the pyramid generation steps.
In a Gaussian pyramid, subsequent images are weighted down using a Gaussian average (Gaussian blur) and scaled down. Each pixel containing a local average corresponds to a neighborhood pixel on a lower level of the pyramid. This technique is used especially intexture synthesis.
A Laplacian pyramid is very similar to a Gaussian pyramid but saves the difference image of the blurred versions between each levels. Only the smallest level is not a difference image to enable reconstruction of the high resolution image using the difference images on higher levels. This technique can be used inimage compression.[13]
A steerable pyramid, developed bySimoncelli and others, is an implementation of a multi-scale, multi-orientationband-pass filter bank used for applications includingimage compression,texture synthesis, andobject recognition. It can be thought of as an orientation selective version of a Laplacian pyramid, in which a bank ofsteerable filters are used at each level of the pyramid instead of a single Laplacian orGaussian filter.[14][15][16]
In the early days of computer vision, pyramids were used as the main type of multi-scale representation for computing multi-scale imagefeatures from real-world image data. More recent techniques includescale-space representation, which has been popular among some researchers due to its theoretical foundation, the ability to decouple the subsampling stage from the multi-scale representation, the more powerful tools for theoretical analysis as well as the ability to compute a representation atany desired scale, thus avoiding the algorithmic problems of relating image representations at different resolution. Nevertheless, pyramids are still frequently used for expressing computationally efficient approximations toscale-space representation.[11][17][18]
Levels of a Laplacian pyramid can be added to or removed from the original image to amplify or reduce detail at different scales. However, detail manipulation of this form is known to produce halo artifacts in many cases, leading to the development of alternatives such as thebilateral filter.
Someimage compression file formats use theAdam7 algorithm or some otherinterlacing technique.These can be seen as a kind of image pyramid.Because those file format store the "large-scale" features first, and fine-grain details later in the file,a particular viewer displaying a small "thumbnail" or on a small screen can quickly download just enough of the image to display it in the available pixels—so one file can support many viewer resolutions, rather than having to store or generate a different file for each resolution.