HitFilm renders layers in a particular order. Understanding this render order makes it easier to construct your shots.
HitFilm renders from the bottom layer up. Therefore higher layers in the layer stack will obscure lower layers.
The contents of any embedded composite shot layers are rendered in their entirety before higher layers are rendered.
Mask and effects
Each layer can be heavily customized. Layer properties (eg Masks, Effects, Transform) are processed from top to bottom. Therefore masks are applied before effects.
As a practical example, if you add a circular mask to a layer, cutting out a hole, then add a glow effect, the glow will also be visible around the edges of the masked hole. This is because the hole is masked first, then the glow is applied.
When using a layer as a source for another effect it is important to understand that the source layer will be used before masks and effects are applied.
If you want the source to include applied effects and masks, the source layer should be made into an embedded composite shot. This will 'bake in' the effects and masks and the embedded composite shot can then be used as the source.
Using Grade layers will also flatten all layers below, baking in any effects and masks. Using the grade layer as a source will in fact use the entire flattened render of all the layers below it.
3D layer modes
3D layers can be set to three states: 2D, 3D plane and 3D unrolled.
When set to 2D, the content of the layer works in full 3D (for example, you can orbit a camera around a 3D object), but the rendered result of the layer is a 2D element. Therefore the 3D layer's interactions with other layers on the timeline is determined solely by its position in the layer stack (Z-depth is not a factor).
3D plane takes the 2D version and renders it as a 3D plane inside the 3D space. In the case of videos, images and planes this is like having a flat piece of card. With 3D objects and particle simulations it is like looking through a window onto a 3D scene. The 3D plane exists in 3D space, so its interactions are determined by its position in 3D space. In the case of 3D objects and particle simulations note that it is the flat 3D plane which exists in 3D space, not the plane's contents.
3D unrolled renders 3D content directly into the 3D space. Multiple 3D unrolled layers will accurately interact with each other in 3D space. This is the unified 3D space and enables powerful interactions - for example, you can position a green screened actor directly inside the cockpit of a 3D model helicopter, without needing any layering tricks.
When 3D and 2D layers are combined on the same timeline it can result in render batches. If one or more 2D layers are positioned between 3D layers, it will separate those 3D layers into separate render batches.
Therefore if you need 3D layers to interact with each other properly in 3D space, make sure they are not separated by any 2D layers.