Note on Course: Introduction to Computational Thinking

Computational science can be summed up by a simplified workflow:

• data ==> input ==> process ==> model ==> visualize ==> output

Topics Include:

• Image Analysis
• Particle Dynamics and Ray Tracing
• Epidemic Propagation
• Climate Modeling

Module 1: Images

• https://computationalthinking.mit.edu/Spring21/images/

1. Install Julia and Pluto

• Step 1: Download from https://julialang.org/downloads/
• Step 2: Open Julia and Install Pluto
  • To install Pluto, we want to run a package manager command. To switch from Julia mode to Pkg mode, type ] (closing square bracket)
• Step 3: import Pluto
• Step 4: Pluto.run()

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julia> ]
(@v1.7) pkg> add Pluto
julia> import Pluto
julia> Pluto.run()

2. Images as Data and Arrays

2.1. Pixel

If we open an image on our computer or the web and zoom in enough, we will see that it consists of many tiny squares, or pixels (“picture elements”).

• Each pixel is a block of one single colour
  • black = RGB(0, 0, 0)
  • red = RGB(1, 0, 0)
  • green = RGB(0, 1, 0)
  • blue = RGB(0, 0, 1)
• and the pixels are arranged in a two-dimensional square grid.

Note that an image is already an approximation of the real world

• it is a two-dimensional, discrete representation of a 3D reality.

Interactivity using Sliders
The Slider type is defined in the PlutoUI.jl package.

2.2. Download and Load 2D Images

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begin
    url = "https://i.imgur.com/VGPeJ6s.jpg"
    # download to a local file. The filename is returned
    download(url, "dog.jpg")
    using Images
    dog = load("dog.jpg")
    # Image/Pixel is a RGB Type
    typeof(dog) # Matrix{RGB{N0f8}} (alias for Array{RGB{Normed{UInt8, 8}}, 2})

    size(dog) # (height, width) = (3675, 2988)

    typeof(dog[100,400]) # RGB{N0f8}
    # Crop the head
    (h,w) = size(dog)
	head = dog[(h ÷ 2):h, (w ÷ 10): (9w ÷ 10)]
    # ÷ is typed as \div + press <TAB>
end

2.3. Images Flip and Concatentaion as Matrix

2.4. Locate and Modify Pixels

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using

• This creates a new variable called number_reds, whose value is the value shown by the slider. When we move the slider, the value of the variable gets updated. Pluto is a reactive notebook, other cells which use the value of this variable will automatically be updated too!