What does ImageData:getPixel return exactly? (new questions)

[Buddy4point0]

I'm making a pixel perfect collision system to use in all my future games.
I like to make engines and resources for myself to use when I start using new interpreters and languages.

So what I need to know is, what exactly does ImageData:getPixel return?
I need to know so I can check the alpha values of pixels in the image to see if something would collide.

I read on the wiki that it returns "r, g, b, a" but is this information in a table, as a string, what?
Thanks in advance, and sorry for all the questions.

[kikito]

I like to make engines and resources for myself to use when I start using new interpreters and languages.

I'll just leave this here. I totally agree with what it says. I have experienced it.

[nevon]

If it says r,g,b,a then my guess would be 4 integers.

Easiest way to find out is to just give it a try.

[Taehl]

I wouldn't do it that way if I were you: I'd keep a 2-dimensional sparse table which stored each 'pixel', then render it all with points or a spritebatch (depending on how big you're talking). I'm sure checking tables would be faster.

[Robin]

What the guys above me have said, and:

I read on the wiki that it returns "r, g, b, a" but is this information in a table, as a string, what?
Thanks in advance, and sorry for all the questions.

It is a list, which isn't a datatype or anything, but it means you can do:

Code: Select all

r, g, b, a = thingy:getPixel(x, y)

and

Code: Select all

print(thingy:getPixel(x, y)) -- prints the r, g, b, a values

[thelinx]

Or if you really want to store the values in an array:

Code: Select all

colortable = {thingy:getPixel(x,y)}

[Buddy4point0]

I like to make engines and resources for myself to use when I start using new interpreters and languages.

I'll just leave this here. I totally agree with what it says. I have experienced it.

I read it, and I don't agree with you or the creator of the article at all.
To be more specific, I have to make a pixel perfect collision system for what I'm working on right now. So I figure I might as well make it re-useable.

I wouldn't do it that way if I were you: I'd keep a 2-dimensional sparse table which stored each 'pixel', then render it all with points or a spritebatch (depending on how big you're talking). I'm sure checking tables would be faster.

Could you emphasize what you mean? I don't really understand what you're talking about.
I'm not sure what a spritebatch is, but this is what I was planning on doing:
Make a function that does this:

• Load the image.
• Convert it to data (so I an use the pixel functions).
• Create a table to store all the pixels in with a name according to the image name.
• Read all the pixels and save the data in the table.
• nil the image data.

Hopefully that would leave me with the image and a map that I could then use another function to read where it would collide.

It is a list, which isn't a datatype or anything, but it means you can do:

Code: Select all

r, g, b, a = thingy:getPixel(x, y)

and

Code: Select all

print(thingy:getPixel(x, y)) -- prints the r, g, b, a values

Ok, I looked on the Lua 5.1 Reference Manualand I don't see anything about lists.
What are they and how would I use the information in it? If you don't feel like answering though, it's fine because I got it figured out thanks to Thelinx.

Or if you really want to store the values in an array:

Code: Select all

colortable = {thingy:getPixel(x,y)}

And this would return it as if I had typed
colortable = {r=255, g=255, b=255}?

That's perfect if that's what it does! Thanks sooo much!

[TechnoCat]

Or if you really want to store the values in an array:

Code: Select all

colortable = {thingy:getPixel(x,y)}

And this would return it as if I had typed
colortable = {r=255, g=255, b=255}?

That's perfect if that's what it does! Thanks sooo much!

That would result in colortable = {255,255,255,255} if the pixel was completely opaque and white.
Notice the table isn't keyed.

[Buddy4point0]

So I've just written the function:

Code: Select all

function readImg(varName,fileName)
	local imageData = love.image.newImageData(fileName)

	local varName = {img = love.graphics.newImage(imageData),map = {}}

	local width = varName.img:getWidth()
	local height = varName.img:getHeight()
	local temp = {}

	for w = 1,width do
		varName.map[w] = {}

		for h = 1,height do
			temp = {imageData:getPixel(w,h)}

			if temp[4] > 0 then
				varName.map[w][h] = true
			else
				varName.map[w][h] = false
			end
		end
	end

	return varName
end

So when I call the function:

Code: Select all

readImg(player,"player.png")

I was hoping get:

Code: Select all

player = {}
player.img = (image)
player.map = {}
player.map[1][1] = true
player.map[1][2] = true
player.map[1][3] = true
ect...

But Love freezes and I get a runtime error. Any ideas why?

I uploaded what I have so far if that helps you guys help me.

Buddy4point0s Pixel Perfect Collision Engine Test (Broken-alpha).zip

(8.81 KiB) Downloaded 249 times

[Boolsheet]

Your function is trying to get a pixel outside of the image. This throws an exception and love quits.
ImageData in love goes from 0, 0 to width-1, height-1. To stay inside the image you could change

Code: Select all

imageData:getPixel(w,h)

to

Code: Select all

imageData:getPixel(w-1,h-1)

Some other things:
Creating tables in Lua has a certain time and memory cost. Creating thousands of them in a loop might hurt the performance considerably.
About lists; they're mentioned here: http://www.lua.org/manual/5.1/manual.html#2.5

Your function overwrites the first argument (varName) of the function, it will never be used. You can drop that argument and just keep fileName alone, it will work fine.
It is possible to pass a table to a function, if that is what you meant to do:

Code: Select all

function change_size(obj)
    obj.size = 500
end

local map = {size = 0}
change_size(map)
print(map.size) -- will print 500