Summary by Alexander Jung 6 months ago
* Traditionally neural nets use max pooling with 2x2 grids (2MP).
* 2MP reduces the image dimensions by a factor of 2.
* An alternative would be to use pooling schemes that reduce by factors other than two, e.g. `1 < factor < 2`.
* Pooling by a factor of `sqrt(2)` would allow twice as many pooling layers as 2MP, resulting in "softer" image size reduction throughout the network.
* Fractional Max Pooling (FMP) is such a method to perform max pooling by factors other than 2.
### How
* In 2MP you move a 2x2 grid always by 2 pixels.
* Imagine that these step sizes follow a sequence, i.e. for 2MP: `2222222...`
* If you mix in just a single `1` you get a pooling factor of `<2`.
* By chosing the right amount of `1s` vs. `2s` you can pool by any factor between 1 and 2.
* The sequences of `1s` and `2s` can be generated in fully *random* order or in *pseudorandom* order, where pseudorandom basically means "predictable sub patterns" (e.g. 211211211211211...).
* FMP can happen *disjoint* or *overlapping*. Disjoint means 2x2 grids, overlapping means 3x3.
### Results
* FMP seems to perform generally better than 2MP.
* Better results on various tests, including CIFAR-10 and CIFAR-100 (often quite significant improvement).
* Best configuration seems to be *random* sequences with *overlapping* regions.
* Results are especially better if each test is repeated multiple times per image (as the random sequence generation creates randomness, similar to dropout). First 5-10 repetitions seem to be most valuable, but even 100+ give some improvement.
* An FMP-factor of `sqrt(2)` was usually used.
![Examples](https://raw.githubusercontent.com/aleju/papers/master/neural-nets/images/Fractional_Max_Pooling__examples.jpg?raw=true "Examples")
*Random FMP with a factor of sqrt(2) applied five times to the same input image (results upscaled back to original size).*
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### Rough chapter-wise notes
* (1) Convolutional neural networks
* Advantages of 2x2 max pooling (2MP): fast; a bit invariant to translations and distortions; quick reduction of image sizes
* Disadvantages: "disjoint nature of pooling regions" can limit generalization (i.e. that they don't overlap?); reduction of image sizes can be too quick
* Alternatives to 2MP: 3x3 pooling with stride 2, stochastic 2x2 pooling
* All suggested alternatives to 2MP also reduce sizes by a factor of 2
* Author wants to have reduction by sqrt(2) as that would enable to use twice as many pooling layers
* Fractional Max Pooling = Pooling that reduces image sizes by a factor of `1 < alpha < 2`
* FMP introduces randomness into pooling (by the choice of pooling regions)
* Settings of FMP:
* Pooling Factor `alpha` in range [1, 2] (1 = no change in image sizes, 2 = image sizes get halfed)
* Choice of Pooling-Regions: Random or pseudorandom. Random is stronger (?). Random+Dropout can result in underfitting.
* Disjoint or overlapping pooling regions. Results for overlapping are better.
* (2) Fractional max-pooling
* For traditional 2MP, every grid's top left coordinate is at `(2i-1, 2j-1)` and it's bottom right coordinate at `(2i, 2j)` (i=col, j=row).
* It will reduce the original size N to 1/2N, i.e. `2N_in = N_out`.
* Paper analyzes `1 < alpha < 2`, but `alpha > 2` is also possible.
* Grid top left positions can be described by sequences of integers, e.g. (only column): 1, 3, 5, ...
* Disjoint 2x2 pooling might be 1, 3, 5, ... while overlapping would have the same sequence with a larger 3x3 grid.
* The increment of the sequences can be random or pseudorandom for alphas < 2.
* For 2x2 FMP you can represent any alpha with a "good" sequence of increments that all have values `1` or `2`, e.g. 2111121122111121...
* In the case of random FMP, the optimal fraction of 1s and 2s is calculated. Then a random permutation of a sequence of 1s and 2s is generated.
* In the case of pseudorandom FMP, the 1s and 2s follow a pattern that leads to the correct alpha, e.g. 112112121121211212...
* Random FMP creates varying distortions of the input image. Pseudorandom FMP is a faithful downscaling.
* (3) Implementation
* In their tests they use a convnet starting with 10 convolutions, then 20, then 30, ...
* They add FMP with an alpha of sqrt(2) after every conv layer.
* They calculate the desired output size, then go backwards through their network to the input. They multiply the size of the image by sqrt(2) with every FMP layer and add a flat 1 for every conv layer. The result is the required image size. They pad the images to that size.
* They use dropout, with increasing strength from 0% to 50% towards the output.
* They use LeakyReLUs.
* Every time they apply an FMP layer, they generate a new sequence of 1s and 2s. That indirectly makes the network an ensemble of similar networks.
* The output of the network can be averaged over several forward passes (for the same image). The result then becomes more accurate (especially up to >=6 forward passes).
* (4) Results
* Tested on MNIST and CIFAR-100
* Architectures (somehow different from (3)?):
* MNIST: 36x36 img -> 6 times (32 conv (3x3?) -> FMP alpha=sqrt(2)) -> ? -> ? -> output
* CIFAR-100: 94x94 img -> 12 times (64 conv (3x3?) -> FMP alpha=2^(1/3)) -> ? -> ? -> output
* Overlapping pooling regions seemed to perform better than disjoint regions.
* Random FMP seemed to perform better than pseudorandom FMP.
* Other tests:
* "The Online Handwritten Assamese Characters Dataset": FMP performed better than 2MP (though their network architecture seemed to have significantly more parameters
* "CASIA-OLHWDB1.1 database": FMP performed better than 2MP (again, seemed to have more parameters)
* CIFAR-10: FMP performed better than current best network (especially with many tests per image)
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