2D primitives ordering/layering

[emilk]

We don't currently define which order 2D shapes are painted. Once we support more primitives like filled rectangles, circles, text etc, it will be very important to be able to specify what is on top of what.

We also need good default, e.g. images should probably be furthest down whatever happens.
We should maybe default to painters-algorithm (whatever is logged first is painted furthest back), as that is what many users are used to from e.g. opencv.

Let's investigate nomenclature other libraries and tools use.

• Rust
• Python

[Wumpf]

related to

• Draw order & (order independent) transparency #702
• Treat camera projected primitives in 3D space more like 2D objects #1025

[emilk]

Let's avoid confusion by not calling this "Z". A depth-image contains Z-values, but can also have a draw-order/layer which is completely unrelated.

[emilk]

We should consider using defaults, like https://matplotlib.org/3.1.1/gallery/misc/zorder_demo.html does:

eg:

rgb image = 0,
depth image = 100
segmentation image = 200
rects/circles = 300
lines = 400
points = 500

[emilk]

Proposal: DrawOrder(f64) component for every 2D primitive.

Max one DrawOrder per entity path? i.e. a mono-component.

[emilk]

We should also have a default DrawOrder for all the 3D stuff projected into 2D, so that you can specify:

• Paint my image
• Project 3D stuff on top of the image
• Paint these rectangles on top of everything else

I don't think wee need to do this for the first version, but keep it in mind.

[emilk]

Implementation:

For 2D views

Simple way:

Paint 2D stuff first, using the DrawOrder as a Z.
Then clear the Z, and draw the 3D stuff on top.

Does not support showing 2D stuff on top of 3D stuff, but that's ok for now.

More complex way:

Paint 3D stuff to a texture.
Paint every 2D primitive as well as the above texture, respectiting its alpha.

The ID buffer complicates this.

For 2D planes in 3D views

Take the all the DrawOrders and sort on them, to map them into integers.

Take these integers as the ZOffset for each primitive. This will push them back the minimal amount to not produce visible distance between them, making them still appear flat.