Render Layers

PixiJS Layer API Guide

The PixiJS Layer API provides a powerful way to control the rendering order of objects independently of their logical parent-child relationships in the scene graph. With RenderLayers, you can decouple how objects are transformed (via their logical parent) from how they are visually drawn on the screen.

Using RenderLayers ensures these elements are visually prioritized while maintaining logical parent-child relationships. Examples include:

• A character with a health bar: Ensure the health bar always appears on top of the world, even if the character moves behind an object.

• UI elements like score counters or notifications: Keep them visible regardless of the game world’s complexity.

• Highlighting Elements in Tutorials: Imagine a tutorial where you need to push back most game elements while highlighting a specific object. RenderLayers can split these visually. The highlighted object can be placed in a foreground layer to be rendered above a push back layer.

This guide explains the key concepts, provides practical examples, and highlights common gotchas to help you use the Layer API effectively.

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Key Concepts

1. Independent Rendering Order:

   • RenderLayers allow control of the draw order independently of the logical hierarchy, ensuring objects are rendered in the desired order.

2. Logical Parenting Stays Intact:

   • Objects maintain transformations (e.g., position, scale, rotation) from their logical parent, even when attached to RenderLayers.

3. Explicit Object Management:

   • Objects must be manually reassigned to a layer after being removed from the scene graph or layer, ensuring deliberate control over rendering.

4. Dynamic Sorting:

   • Within layers, objects can be dynamically reordered using zIndex and sortChildren for fine-grained control of rendering order.

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Basic API Usage

First lets create two items that we want to render, red guy and blue guy.

const redGuy = new PIXI.Sprite('red guy');
redGuy.tint = 0xff0000;

const blueGuy = new PIXI.Sprite('blue guy');
blueGuy.tint = 0x0000ff;

stage.addChild(redGuy, blueGuy);

Now we know that red guy will be rendered first, then blue guy. Now in this simple example you could get away with just sorting the zIndex of the red guy and blue guy to help reorder.

But this is a guide about render layers, so lets create one of those.

Use renderLayer.attach to assign an object to a layer. This overrides the object’s default render order defined by its logical parent.

// a layer..
const layer = new RenderLayer();
stage.addChild(layer);
layer.attach(redGuy);

So now our scene graph order is:

|- stage
    |-- redGuy
    |-- blueGuy
    |-- layer

And our render order is:

|- stage
    |-- blueGuy
    |-- layer
        |-- redGuy

This happens because the layer is now the last child in the stage. Since the red guy is attached to the layer, it will be rendered at the layer's position in the scene graph. However, it still logically remains in the same place in the scene hierarchy.

3. Removing Objects from a Layer

Now let's remove the red guy from the layer. To stop an object from being rendered in a layer, use removeFromLayer. Once removed from the layer, its still going to be in the scene graph, and will be rendered in its scene graph order.

layer.detach(redGuy); //  Stop rendering the rect via the layer

Removing an object from its logical parent (removeChild) automatically removes it from the layer.

stage.removeChild(redGuy); // if the red guy was removed from the stage, it will also be removed from the layer

However, if you remove the red guy from the stage and then add it back to the stage, it will not be added to the layer again.

// add red guy to his original position
stage.addChildAt(redGuy, 0);

You will need to reattach it to the layer yourself.

layer.attach(redGuy); // re attach it to the layer again!

This may seem like a pain, but it's actually a good thing. It means that you have full control over the render order of the object, and you can change it at any time. It also means you can't accidentally add an object to a container and have it automatically re-attach to a layer that may or may not still be around - it would be quite confusing and lead to some very hard to debug bugs!

5. Layer Position in Scene Graph

The layer’s position in the scene graph determines its render priority relative to other layers and objects.

// reparent the layer to render first in the stage
stage.addChildAt(layer, 0);

Complete Example

Here’s a real-world example that shows how to use RenderLayers to set ap player ui on top of the world.

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Gotchas and Things to Watch Out For

1. Manual Reassignment:

   • When an object is re-added to a logical parent, it does not automatically reassociate with its previous layer. Always reassign the object to the layer explicitly.

2. Nested Children:

   • If you remove a parent container, all its children are automatically removed from layers. Be cautious with complex hierarchies.

3. Sorting Within Layers:

   • Objects in a layer can be sorted dynamically using their zIndex property. This is useful for fine-grained control of render order.

   rect.zIndex = 10; // Higher values render later
   layer.sortableChildren = true; // Enable sorting
   layer.sortRenderLayerChildren(); // Apply the sorting

4. Layer Overlap:

   • If multiple layers overlap, their order in the scene graph determines the render priority. Ensure the layering logic aligns with your desired visual output.

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Best Practices

1. Group Strategically: Minimize the number of layers to optimize performance.
2. Use for Visual Clarity: Reserve layers for objects that need explicit control over render order.
3. Test Dynamic Changes: Verify that adding, removing, or reassigning objects to layers behaves as expected in your specific scene setup.

By understanding and leveraging RenderLayers effectively, you can achieve precise control over your scene's visual presentation while maintaining a clean and logical hierarchy.