Simple Rectangle Program - Don't Understand A Body of Code

Questions
1

The code below is a program that creates rectangles that obey gravity from touches on the screen. I don’t have any troubleshooting questions for this, but I want to know what a certain block of code means. The entire program is below followed by the section I don’t understand.

The Entire Code:


-- Main Class

function setup()
    print("Welcome to BOX STACKER!!!")
    
    boxes = {}
    
    local ground = Box:createBox(WIDTH/2, 10, WIDTH, 20, 0)
    ground.type = STATIC
    table.insert(boxes, ground)

end

function touched(touch)
    if touch.state == BEGAN then
            table.insert(boxes, Box:createBox(touch.x, touch.y, math.random(20, 80), math.random(20, 80), 0, vec2(touch.x, touch.y)))
    end
end

function collide(contact)
    if contact.state == BEGAN then
        sound(DATA, "ZgNAHAAfQEBATExAAAAAAFBxST5jD7M+TABAf0BAQEBAQEBA")
    end
end

function draw()
    background(40, 40, 50)
    
    for k, box in pairs(boxes) do
        Box:drawBox(box)
    end

end

-- Box Class

Box = class()

function Box:createBox(x, y, w, h, r)
    local box = physics.body(POLYGON, vec2(-w/2, h/2),
    vec2(-w/2, -h/2), vec2(w/2, -h/2), vec2(w/2, h/2))
    
    box.x = x
    box.y = y
    box.angle = r
    box.interpolate = true
    box.restitution = 0
    
    return box
end

function Box:drawBox(box)
    pushStyle()
    pushMatrix()
    
    stroke(148, 224, 135, 255)
    translate(box.x, box.y)
    rotate(box.angle)
        
    if box.shapeType == POLYGON then
        strokeWidth(3.0)
        local points = box.points
        for j = 1,#points do
            a = points[j]
            b = points[(j % #points)+1]
            line(a.x, a.y, b.x, b.y)
        end
    end
    
    popMatrix()
    popStyle()
end

The Section I Don’t Understand:


function Box:drawBox(box)
    pushStyle()
    pushMatrix()
    
    stroke(148, 224, 135, 255)
    translate(box.x, box.y)
    rotate(box.angle)
        
    if box.shapeType == POLYGON then
        strokeWidth(3.0)
        local points = box.points
        for j = 1,#points do
            a = points[j]
            b = points[(j % #points)+1]
            line(a.x, a.y, b.x, b.y)
        end
    end
    
    popMatrix()
    popStyle()
end

Can anyone explain what this section of code means line by line? I want to be able to understand it so I can utilize it in other programs. I’d appreciate the effort. Thank you.

2

Hi, pushStyle() and popStyle() pretty much just make sure the colour of stroke or fill do not carry over to other parts of the program. Push and pop matrix do the same for transformations. stroke() sets the colour of the stroke, translate() translates (so the new origin is the centre of the box) and rotate rotates so that the x-axis points in the direction of the box’s angle. Then it checks if the shape is a polygon and if it is it sets the stroke width to something else. Next it makes a local table which has the points of the box on it. The for loop goes through each of the points of the box and draws a line from that point to the next point ((j % #points)+1 will be j+1 for j<#points and 1 if j=#points)
Note that this draws any polygon, not just a rectangle.

3

Thank you very much!

4

@TheBroskateers Just a suggestion. Anytime you don’t understand what code does, try looking in the build in reference and read what the command does. That way you get a full explanation of each command. Also, you might want to just skim thru the reference just to see what’s there. You’ll find some interesting thing there for later use.

5

Thank you too. I do read the reference a lot, but sometimes I just don’t understand the technical jargon, and I need someone to explain it in more laymen terms.

6

@TheBroskateers - I had a similar problem, so I wrote some ebooks to hopefully help people like yourself, here

https://www.dropbox.com/sh/mr2yzp07vffskxt/AACqVnmzpAKOkNDWENPmN4psa