[draft] node tree

[exellian]

Currently the node tree is a top down structure where every node knows its child nodes but child nodes doesn't know their parent node. We have to discuss how we can work around this, if its an restriction, if we have to choose a different tree structure (linked tree)

[exellian]

The node tree is the core data structure behind a scene of a window/surface. It stores all necessary nodes that need to be rendered on screen. Therefore the node tree is also crucial for performance. In this issue I want to sketch my current thoughts about the node tree structure:

The enum Node rui::node::Node represents the current node tree structure. It defines some basic nodes which should cover primitive- and composition- nodes. With these basic nodes you should be able to build any ui you want. Therefore the enum will be extended to some point where everybody is satisfied. Currently the enum contains following base nodes:

• Rectangle (Simple rectangle that can have a background color)
• Border (Wraps another component and applies a border (same like css/html))
• Composition (Layers other nodes on top of each other)
• Image (trivial.)
• Text (Same features like in html/css)
• Component (Wraps a nodes and applies logic (State, behaviour, ...))

Nodes that should be implemented:

• Grid (should support swap operations + dynamic grid cell sizes + dynamic row/column count)
• Video
• Svg
• Simple 3D objects + lighted + shadowed

IMPORTANT: Every node is contained in a bounding rectangle and therefore we can optimize the rendering process as follows:

• Grid: Nodes in a grid a strictly separated. Therefore we only have to rerender the areas where nodes have changed.
• Composition: For two layers where one is on the top and one is at the bottom, if only nodes changed in the top layer then we only have to rerender the top layer, but the other way around we have to rerender both, because the top layer could depend on the bottom layer. I open up a new issue for a more detailed dependency graph. https://github.com/exellian/rui/issues/13
• Border: If the border gets more rounded we only have to rerender outer regions (regions that get cut away)
• Primitive Nodes (Video, Image, Text): Rerender the whole node

[exellian]

Currently wgpu doesn't has support for rendering only to certain areas of a render target (See Vulkan: renderArea property on the render pass struct) and it is also not efficient to create a own render pass for every node that needs to be updated. Therefore we probably have to overthink the design of the node tree and move to something more "gpu compatible":

We should not think of a tree but rather a simple list of layers in the ui. Then every group of layers which doesnt't has overlapping nodes that use background filter effects (like convolution filters) can be rendered in one render pass. So essentially if we have such a node tree than we first have to somehow flatten this tree in a list of primitives and then split it on such overlapping filter layers, that we can create the corresponding render passes. So we need somehow a hybrid data structure which is easy to build, easy to render and fast to build because we expect changes every frame.