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fast.ai lesson08

Follow Dec 16, 2019 · 4 mins read
fast.ai lesson08
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This is the part of Journey which Jeremy recommended us to do. One of the concepts I have to know.

1) Kaiming Initializtion in Pytorch was in trouble.1 2) Jeremy started to dig in, in lesson09, but I didn’t know why the size of tensor is 2 and even understand this spreadsheet data.3


Homework

Read Visualizing and Understanding Convolutional Networks paper


What is a convolution?

A convolutional neural network is that your red, green, and blue pixels go into the simple computation, and something comes out of that, and then the result of that goes into a second layer, and the result of that goes into the third layer and so forth.

Visualization

one kernel
  • Refer this site for visualizing CNN filtering
Matthew D Zeiler & Rob Fergus Paper

Lecture01

{align-items: center;}

  • nine examples of the actual coefficients from the first layer.

Convolution can be represented as matmul

CNNs from different viewpoints

{align-items: center;}

  • [A B C D E F G H I J] is 3 by 3 image data flatten to vector.

  • As a result, convolution is a just matrix just two things happens
    • Some of entries are set to zeros at all the times
    • same color always have the same weight. That called weight time / wegith sharing
  • So, we can implement a convolution with matrix multiplication. But, we don’t do that because it’s slow!

Padding

  • What most of libraries do is just put zeros asdie of matrix

  • fast.ai uses reflection paddings (what is this? Jeremy said he uttered it)

Kernel has rank 3

  • As standard picture input would be 4 5, it would be actually 3d, not 2d.
  • If we make kernel as a 3x3 size, we pass over same kernel all the different Red, Green, Blue Pixels.
    • This could make problem, because, if we want to detect frog, which is green, we would want more activations on the green(I made a test cell in my colab 6)

How can we find a side-edge, a gradient and area of constant weight?

Not top-edge!

  • One kernel can find only the top-edge, so we should stack the kernels 7
  • So, we pass it through bunch of kernels to the input images, and that process gives us height x width x corresponding number of kernels.

  • Usually that number of chanel is 16
  • And if we want to get the more channels and features, we should repeat that process
    • This process gives rise to memory out of control, we do the stride

####

conv-example.xlsx

  • 2 convolutional filters
  • At a second layer, filter is 3x3x2 tensor, because to add up together the first layer’s channel.

Reference

  1. Problem was math.sqrt(5) was not kaiming initialization formula, Implementation in Pytorch 

  2. size of tensor, lecture09 

  3. conv-example.xlsx 

  4. Why do computer use red, green and blue instead of primary colors 

  5. Grayscale is a group of shades without any visible color. … Each of these dots has its own brightness level as well and, therefore, can be converted to grayscale. A grayscale image is one with all color information removed. 

  6. Testing RGB and grayscale 

  7. stack kernel and make new rank of tensor at output, Lesson06-2019