Avoiding Implicit Scene Updates For Faster Blender Scripts

The Problem of Implicit Updates

Blender handles scene data and object dependencies in a way that can cause slowdowns in scripts. All data in a Blender scene is interconnected through a complex web of relationships. When one part of the scene data changes, dependent data needs to be updated. Blender handles these updates automatically behind the scenes through its dependency graph. However, for scripts that make many changes to the scene, these constant implicit updates can take up significant time.

For example, when a Python script adds new objects to the scene, Blender will automatically update dependencies – the render engine updates to include the new objects, dependency relationships get rebuilt, and the scene data itself is refreshed. With hundreds or thousands of objects being added, these background updates after each addition start accumulating and slowing the script down considerably.

Another example is animating object transformations through Python. Setting new locations, rotations or scales on animated objects triggers updates of the animation data and evaluation, which again slows things down unnecessarily for scripts that simply want to set keyframed values.

Table of Contents

Using Blender’s Dependency Graph

To manage dependencies between datablocks, Blender uses a “Dependency Graph” or “DG” that tracks relationships between datablocks like objects, meshes, materials, etc. This allows Blender to implicitly update dependent data when other data changes. The dependency graph hooks into all blender data to build a network of relationships.

For example, a material may depend on an image texture. When that image is changed, the material gets updated to reflect the new colors. A mesh object depends on the mesh data and also materials assigned to it, so changes to those will also trigger updates to the object.

The dependency graph enables Blender to ensure correctness and consistency in the scene data. But for scripts making many successive changes, constant updates waste time rebuilding dependencies, refreshing data and re-evaluating state.

Luckily, Blender allows ways to override the default dependency handling, so that scripts can make changes without incurring all the overhead of implicit updates after each step.

Disabling Implicit Updates

The disableImplicitContext() method is used to temporarily disable all implicit updates while a script makes changes. This prevents constant refreshing of data which saves significant time with large numbers of successive edits.

For example, when adding 500 cubes to the scene, the script can disable updates at the start, add all 500 cubes quickly without constant re-evaluation, and then re-enable updates after the additions are complete. This prevents slowdowns from having updates occur after each individual cube gets added.

Disabling context is most useful in cases where updates don’t need to happen until a complete set of edits is finished. Turning off auto updates basically batches up all dependency changes, avoiding rebuilding dependencies after each minor change.

Managing Dependencies Manually

Instead of wholesale disabling of updates, scripts can also gain more granular control over dependencies. The idea is to manually track specific dependencies as changes are made.

For example, when animating object transforms, a script can manually tag animations to only update those dependencies that are actually changing, instead of wasting updates on unrelated data.

This requires more complex logic tracking dependencies as they change, and manually refreshing only dependent data as needed. But for large complex scripts making many changes, the performance gains are well worth the extra logic.

Updating Only What is Needed

Besides disabling all updates, scripts can also explicitly refresh specific datablocks after making changes. The update() method will re-evaluate a datablock and its dependencies. The refresh() method refreshes a datablock itself without updating dependencies.

For example after setting animation keyframes, calling update() on the animated object will correctly update the animation without touching anything else. Or when assigning new materials, calling refresh() on those materials alone updates their internal state.

Used judiciously, these explicit refresh methods allow precision control over what gets updated. This becomes important for complex scenes and scripts making many diverse changes.

Recommendations and Best Practices

To summarize techniques for avoiding slowdowns from implicit updates in Blender scripts:

Use disableImplicitContext() and enableImplicitContext() to disable updates for bulk changes
Manually track dependencies and only refresh data blocks requiring updates
Where possible, batch up many changes between updates to avoid overhead
Use update() and refresh() to explicitly update only critical data

Some overall recommendations when writing scripts:

Analyze scripts to find where most time is lost to unnecessary updates
Focus optimization efforts on frequently executed code sections
Balance optimization vs readability and maintainability

Blender offers powerful ways to gain fine-grained control over scene updates. Understanding how to avoid unnecessary implicit updates can help scripts achieve dramatic speedups. But it requires analyzing script bottlenecks, tracking dependencies, and managing updates more manually. With some effort however, carefully optimized scripts can operate orders of magnitude faster.