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.
Layer dimension modes
Layers can be set to three states: 2D, 3D plane and 3D unrolled. This can be changed using the icon next to the layer on the timeline.
When set to 2D, standard 2D layers are rendered flat to the screen, with no depth. This is ideal for layer compositing, such as working with greenscreen video or adding simple titles. 3D content layers are rendered in 3D (for example, you can orbit a camera around a muzzle flash), but the rendered result of the layer is still a flat 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 layer and renders it as a 3D plane inside the 3D space. In the case of videos, images, embedded composite shots and planes this is like having a flat piece of card. The 3D plane exists in 3D space, so its interactions are determined by its position in 3D space.
3D unrolled renders 3D content directly into the 3D space. Multiple 3D unrolled layers will accurately interact with each other in 3D space. Embedded composite shots containing 3D layers will be rendered in full 3D, while still only occupying a single layer on the timeline.
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.