How to detect if a Sprite is on screen? (sprite container with deep hierarchy)

Mells

Hi friends,

(I write my questions in a FAQ style so that it's better indexed for people that might search for it later)

If I have a Sprite A that belongs to a SpriteSubgroup, that belongs to a SpriteGroup :
So we have SpriteGroup > SpriteSubGroup > SpriteA

SpriteGroup is moving and scaled (ex : *.5).
SpriteSubGroup is moving also, has its own scale (ex : *2), inside SpriteGroup, with its own speed.
SpriteA is moving also, has its own scale (ex : *.3) inside SpriteSubGroup, with its own speed.

How would you detect if SpriteA is visible on screen, whatever scale and local position it is?
I believe it has to deal with localToGloba() or globalToLocal() but I couldn't understand how to use it.

How do you deal with it?

thank you!

BJG

Re: localToGlobal, it looks like you can find the global position of the sprite with:

local x, y = SpriteA:localToGlobal(0, 0)

This seems to work even with extra nesting (ie localToGlobal seems to return the absolute screen position rather than the position of the parent) so I don't think you need worry about that. You could then test this against the screen limits while taking account of any scaling.

Mells

@BJG thank you, it helped

For those who might be looking for the answer later :


1. A working sample by @atilim :
Source

Hi,

There are two functions Sprite:localToGlobal and Sprite:globalToLocal to solve this problem. You can use these functions like:

1. Use Sprite:localToGlobal to find the global (screen) coordinates of the sprite before moving:

local x, y = sprite:localToGlobal(0, 0)

2. Move your sprite to another group.

anotherGroup:addChild(sprite)

3. Find the new local coordinates in this group by using Sprite:globalToLocal and set the position:

sprite:setPosition(anotherGroup:globalToLocal(x, y))

Hope this helps. Happy coding on your new game

2. A discussion on the topic :
How does globalToLocal and localToGlobal really work?