lua -> how to modify these arguments to be const, or what should I do?

Question

all!

I came here because I have one problem bugging me for quite some time now. I am using love2d engine as a 'graphical' addition to lua scripting, but this problem is of lua type (I believe, at least).

I have a function:

createNew_keepOld = function (oldImgData, oldImgDraw)
    local newImgData = oldImgData --I am assigning old value to another variable
    local newImgDraw = oldImgDraw --I am doing the same thing as with data 

    for x = 0, newImgData:getWidth()-1 do
        for y = 0, newImgData:getHeight()-1 do
            local r, g, b, a = newImgData:getPixel(x, y)
            r = 2*r
            g = 2*g
            b = 0.5*b
            a = 2*a
            newImgData:setPixel(x, y, r, g, b, a)
        end
    end

    newImgDraw:replacePixels(newImgData)

    return newImgData, newImgDraw
end

When this code finishes, I get the change I need, but not WHERE I want it. I just want to produce two new variables which will store data and image objects. But, in process, original image gets changed.

Is there any way to declare:

name = function (const param, const param)

    return some_things
end

So that I get output I need without changing the original stuff? Or is there some problem with my code that I cannot see?

Thanks!

Answer 1

Actually the nature of this problem is both in Love2D and Lua. Anyway.

Quote from Lua Reference Manual 5.3:

Tables, functions, threads, and (full) userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.

Most types from Love2D are of userdata Lua type, so when passing them to your function you are actually passing reference to them, hence you modify the "old" versions in the very end. Those types usually have functions made for copying them.

ImageData does have one and Image does not, but you can do the following:

createNew_keepOld = function (oldImgData, oldImgDraw)
    local newImgData = oldImgData:clone()

    for x = 0, newImgData:getWidth()-1 do
        for y = 0, newImgData:getHeight()-1 do
            local r, g, b, a = newImgData:getPixel(x, y)
            r = 2*r
            g = 2*g
            b = 0.5*b
            a = 2*a
            newImgData:setPixel(x, y, r, g, b, a)
        end
    end

    local newImgDraw = love.graphics.newImage(newImgData, oldImgDraw:getFlags())

    return newImgData, newImgDraw
end

Note that I created entirely new Image based on copied ImageData and image flags from the old one.