Known Limitations¶

This page documents limitations of aep_parser that arise from the nature of parsing a binary file format rather than querying a running After Effects instance.

Property.value_at_time Accuracy¶

Spatial Properties (~0.015 Maximum Error)¶

Property.value_at_time() for spatial properties (position, 2D/3D) has a systematic ±0.015 deviation from After Effects' valueAtTime(). This is not a bug in the parser - it is caused by After Effects' internal spatial evaluation pipeline.

Evidence: even a perfectly straight, LINEAR-interpolated path from [0, 50, 0] to [100, 50, 0] (where the mathematically correct value is 20 × t) shows a sinusoidal deviation pattern in After Effects' own output, peaking at ±0.011. The same deviation appears regardless of whether the keyframe interpolation type is LINEAR or BEZIER.

After Effects appears to process all spatial properties through an arc-length reparameterisation pipeline (likely a polyline or spline approximation) that does not degrade gracefully to exact linear interpolation for straight paths.

Runtime-Only Attributes¶

Many ExtendScript attributes reflect the live state of After Effects and cannot be derived from the .aep file alone:

Attribute	Reason
`Application.memoryInUse`	Runtime memory state
`Application.isRenderEngine`	Launch mode flag
`Application.isWatchFolder`	Launch mode flag
`Application.fonts`	Installed fonts on the system
`Project.dirty`	Unsaved changes flag
`Viewer.maximized`	Window state not persisted in `.aep`

Expressions¶

Property.value When Expressions Are Enabled¶

When Property.expression_enabled is True, the value attribute contains the last static or keyframed value stored in the binary file - not the result of evaluating the expression. After Effects computes expression results at runtime using its expression engine; aep_parser has no expression evaluator.

prop = layer.transform.property(name="ADBE Position")
if prop.expression_enabled:
    # prop.value is the pre-expression value, not the expression result
    print(prop.expression)  # the expression string is available

Property.expression_error¶

Property.expression_error is always an empty string. After Effects computes expression errors at runtime when it evaluates the expression engine; this information is not stored in the binary .aep file.

Property Metadata¶

Property.default_value¶

Default values are set heuristically by the parser in defaults.py, not read from the binary format. They are used for Property.is_modified checks. Some default values may be inaccurate for non-standard property types.

Property.units_text¶

Property.units_text is not read from the binary format, it is based on a collection of samples. For some properties, the value may be an empty string even though After Effects displays a unit string in the UI.

Property.canSetExpression¶

Property.canSetExpression is not implemented. This attribute indicates whether an expression can be assigned to the property. Analysis of the binary format shows that this value is not stored in the .aep file. It is determined at runtime by After Effects based on context such as the layer type (camera, light, etc.), whether the layer is 3D, whether position dimensions are separated, and the light type.

Property.canVaryOverTime¶

Property.can_vary_over_time is parsed from the binary tdb4 chunk using a combined rule: the can_vary_over_time flag (byte 11 bit 1) OR the no_value flag (byte 57 bit 0), since NO_VALUE properties always report canVaryOverTime = true in ExtendScript. This achieves 100% accuracy across all 800 test files, with two match-name overrides for edge cases (ADBE Light Falloff Type, ADBE FreePin3 Outlines).

Output Module Templates¶

OutputModule.templates and RenderQueueItem.templates are always empty lists. After Effects populates them at runtime with template names from the application preferences, but they are not stored in the .aep file. The actual render settings are available through OutputModule.settings and RenderQueueItem.settings.

Proxy Sources¶

AVItem.proxySource and AVItem.useProxy are not parsed. Proxy footage information is stored in the binary format but not yet extracted.

Essential Properties¶

The Essential Graphics panel attributes are not parsed:

Property.alternateSource
Property.canSetAlternateSource
Property.essentialPropertySource
CompItem.motionGraphicsTemplateName
CompItem.motionGraphicsTemplateControllerCount

Missing Classes¶

The following ExtendScript classes do not exist in aep_parser:

Class	Reason
`System`	OS/machine info - not stored in `.aep`
`ImportOptions`	Import dialog settings - not stored in `.aep`
`FontsObject`	Runtime collection of installed fonts
`CharacterRange`	Text engine range object (AE 24.6+)
`ComposedLineRange`	Text engine range object (AE 24.6+)
`ParagraphRange`	Text engine range object (AE 24.6+)
`ItemCollection`	Use `project.items` (Python list) instead
`LayerCollection`	Use `comp.layers` (Python list) instead
`Settings`	Application settings - methods only, not stored in `.aep`
`Preferences`	Application preferences - methods only, not stored in `.aep`

File Paths¶

File paths in .aep files are stored as they were saved on the original system. They may be platform-specific (Windows backslashes vs. Unix forward slashes) and may not resolve on the current system. FileSource.file returns the path as stored without modification. FileSource.missing_footage_path provides the path that After Effects would display for missing footage.