Known Limitations¶

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

Property.value_at_time Accuracy on 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 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:

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; py_aep has no expression evaluator.

prop = layer.transform.property("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 synthesis.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).

Templates¶

OutputModule.templates and RenderQueueItem.templates are not available. 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.

Color Space Profiles Are Read-Only¶

Project.working_space and Project.display_color_space are read-only. The binary chunks that store these settings contain both the profile name and the full ICC profile data (base64-encoded binary, up to ~50 KB per profile). Updating only the profile name would leave stale ICC data, producing a file that After Effects may reject or silently revert to a default profile.

Generating the correct ICC data would require the Adobe ICC profile files installed on disk (under C:\Program Files (x86)\Common Files\Adobe\Color\ on Windows), which cannot be assumed in all environments. Bundling these profiles is not feasible due to licensing constraints.

Other color management settings (color_management_system, lut_interpolation_method, ocio_configuration_file, working_gamma, linearize_working_space, linear_blending, compensate_for_scene_referred_profiles) remain read/write as they do not depend on embedded ICC data.

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¶

Essential Property override values on precomp layers are parsed as regular properties under the "Essential Properties" group. However, the UUID linkage between overrides and their source controller definitions (LIST:OvG2) is not resolved. The following attributes are not parsed:

• Property.essentialPropertySource
• Property.alternateSource
• Property.canSetAlternateSource

Missing Classes¶

The following ExtendScript classes do not exist in py_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.