Spritekit - Bottom of screen coordinates

Question

What are the coordinates for the bottom of the screen... or how can I create a "floor" at the bottom of the screen in spritekit?

Sorry, but I don't understand screen coordinates that well in spritekit.

Answer 1

You need to understand the Sprite Kit coordinate system as explained in Apple's Documentation here.

Here's how you create a floor at the bottom of the screen in SpriteKit:

SKNode *floor = [SKNode node];
node.physicsBody = [SKPhysicsBody bodyWithEdgeLoopFromRect:CGRectMake(CGRectGetMidX(self.frame),1.0 , CGRectGetWidth(self.frame), 1)];
[self addChild: floor];

Answer 2

You need some universal approach to get coordinates of corners on the screen.

Using code from that answer you can get CGRect with necessary information.

Example:

let screenRect = getVisibleScreen(
                 sceneRect: self.scene!.frame,
                 viewRect: self.view!.frame )

And then you can use it:

screenRect.minX
screenRect.maxX

screenRect.minY
screenRect.maxY

screenRect.width
screenRect.height

This is more then enough to calculate coordinates of "floor" or any other relative positions.

Answer 3

The location of the bottom of the screen will depend on what coordinate system you are using for your scene.

Out of the box, the bottom of the screen will be at y coordinate zero, but there are a few things that can happen that will affect that.

For instance, if you are using the scene editor in xCode, and your scene's anchorPoint property is something other than y=0, then the "origin" of your scene will not be at the bottom of the screen. In the recent xCode beta, they changed the default behavior to have the scene's origin at the center of the scene instead of the lower left corner, so that would explain why you might be seeing things in the center of the screen when you expect them to be at the bottom.

Also, the "bottom of the screen" will be relative to whatever parenting structure you have in your scene. For instance, if you place a background sprite in your scene, and want to attach a floor sprite to that which is at the bottom of the screen, you'll have to do some computing to figure out where to place it because you are going to inherit the translation and rotation of the floor's parent node (and any parents that node has).

To keep things simple, you can just place everything directly on the stage and manage their z-order manually. This will let you, basically, use the same coordinate system for everything. This is often fine; as long as you're not trying to do anything complex with your sprites, you don't need a complicated "tree" of nodes.

But even with this approach, the metrics of your scene are going to have to be handled dynamically. The width and height of your scene are going to depend on how you approach displaying your scene on different devices with different sizes. For instance, the top right of an iPhone 4 is going to be in a different place than the top right of an iPad Pro. A full discussion of how to deal with that is beyond the scope of your question, but generally, you'll probably want to use a "reference width" or a "reference height" for your scene, use .AspectFit or .AspectFill for the scaleMode, and set your scene's size accordingly. (I.e., inspect the view's frame to get the actual aspect ratio of your scene and set your scene size to match your reference metric on one axis and scale the other axis to match the device's aspect ratio.) This will let you use the same metrics for all devices (although one of your two axes will be fluid).