What happens in a draw call?¶
The ngl_draw() call is split in 4 passes:
Visit the graph
Honor node resources (prefetch and release)
Update the nodes
Actual GL drawing
Visit¶
The first pass performed by ngli_node_honor_release_prefetch() is to visit
the scene.
By default, ngli_node_visit() will crawl every node parameter to find the
child nodes. It is meant to flag every node as active or inactive for a
given time. The final state of a node depends on the knowledge of the whole
graph because of diamond shaped tree. A typical case is an active branch and an
inactive branch sharing a common node; that leaf node must stay active even
though one its parent is inactive.
Note: if a branch is already inactive, the visitor is unlikely to follow it down, preventing an huge overhead on the tree with a large number of time range filtered branches.
Prefetch/Release¶
The second pass performed by ngli_node_honor_release_prefetch() will crawl
every node previously visited by the first pass and execute a prefetch or a
release according to how it’s been flagged. Previously unvisited nodes (and
their children) are ignored.
For medias, prefetching typically means starting to open the file in advance so it’s ready to playback immediately when its time arrive. Similarly, textures are allocated or released accordingly.
Update¶
The 3rd pass is executing a cascading update with ngli_node_update(). Each
node decides how it updates its state and trigger the update of its children
for a given time.
Draw¶
The 4th and final pass executed by ngli_node_draw() is similar to update in
the way it is cascading (explicit in every node). This pass does not require
time, its only purpose lies in drawing to the screen.