Python API support for creating CurveMapping, CurveProfile and ColorRamp properties. #105781

New Issue

Kévin Dietrich · 2023-03-15T05:14:18+01:00

Kévin Dietrich commented

2023-03-15 05:14:18 +01:00

Motivation

Add-on developers sometimes make use of curve mapping, color ramp, and curve profile properties. However, there is no way to create such properties in Python. To deal with this limitation, developers use hacks 1 which mostly revolve around creating a shader or texture node tree containing a node which has a curve mapping or color ramp property and use said nodes property for UI drawing and data evaluation. Besides limiting or even preventing development of procedural features based on curve or color functions, this can clutter the main database with many irrevelant node trees, and cause unrelated UI/data updates when the property is modified by the user.

Solution

The proposed solution is to allow creating curve mapping, color ramps, and curves profiles properties via the Python API, i.e. allowing my_curve: bpy.props.CurveMappingProperty(...)

Proposed implementation

Specific RNA property types for CurveMapping, CurveProfile, and ColorBand (ColorRamp in Python) are added (e.g. CurveMappingPropertyRNA, enum PROP_CURVE_MAPPING). The property types replace the current PROP_POINTER (e.g. a CurveMapping RNA property is now a PROP_CURVE_MAPPING instead of PROP_POINTER).

Since Python properties are stored using IDProperties, specific IDProperties types for curve mapping, curve profile, and color ramp are also added. IDProperties for those new types store a pointer to some CurveMapping, CurveProfile, or ColorBand object which is dynamically allocated; this mimics how such properties are stored in DNA.

We could consider other ways of storing the data in IDProperties, e.g. with a scalar arrays which describe the curves, instead of a pointer to some heap object, but this would mean adding conversion functions between the UI data and the IDProperty's data. Such conversion functions may not be too intrusive, but still a potential source of complexity and divergent behaviors (leading to bugs) between "native" mapping/profile/ramp and Python ones. Therefore, to keep things simple, I opt to simply store a pointer to some CurveMapping/CurvePorfile/ColorBand object (as could be seen in my original set of patches ([2], [3], & [4])).

API Future Proofing

By allowing creating such property types, some implementation details or limitations specific to the mapping/profile/ramp are exposed in the API. For example:

Curve mappings can only have a fixed number of curves (1, 3, or 4) which has to be set during creation. The behaviour is kept for the Python equivalent, albeit by using an enumeration on the type "scalar", "rgb", etc.
Color ramps only have a limited number of slots (32). However, this is not exposed in any API (C or otherwise), but could be the source of API breakage if exposed.
Curve profiles need a preset at creation time, again this could change in the future.

This list is non-exhaustive, as there might be other sources of API compatibility issues which I fail to consider.

Since add-on developpers are currently only using built-in mapping/profile/ramp widgets, no attempt was made to remove these limitations; the current work merely exposes the various C API for creating such widgets to Python as is. This all begs a few design consideration for future proofing the API.

Notes

Data storage in IDProperties and API future proofing seem to be the main points of contention.

Links

[2]: Python API: support creating Curve Mapping properties (https://archive.blender.org/developer/D15824)
[3]: Python API: support creating Curve Profile properties (https://archive.blender.org/developer/D15825)
[4]: Python API: support creating Color Ramp properties (https://archive.blender.org/developer/D15826)

**Motivation** Add-on developers sometimes make use of curve mapping, color ramp, and curve profile properties. However, there is no way to create such properties in Python. To deal with this limitation, developers use hacks [1] which mostly revolve around creating a shader or texture node tree containing a node which has a curve mapping or color ramp property and use said nodes property for UI drawing and data evaluation. Besides limiting or even preventing development of procedural features based on curve or color functions, this can clutter the main database with many irrevelant node trees, and cause unrelated UI/data updates when the property is modified by the user. **Solution** The proposed solution is to allow creating curve mapping, color ramps, and curves profiles properties via the Python API, i.e. allowing `my_curve: bpy.props.CurveMappingProperty(...)` **Proposed implementation** Specific RNA property types for CurveMapping, CurveProfile, and ColorBand (ColorRamp in Python) are added (e.g. `CurveMappingPropertyRNA`, enum `PROP_CURVE_MAPPING`). The property types replace the current `PROP_POINTER` (e.g. a CurveMapping RNA property is now a `PROP_CURVE_MAPPING` instead of `PROP_POINTER`). Since Python properties are stored using *IDProperties*, specific *IDProperties* types for curve mapping, curve profile, and color ramp are also added. *IDProperties* for those new types store a pointer to some CurveMapping, CurveProfile, or ColorBand object which is dynamically allocated; this mimics how such properties are stored in DNA. We could consider other ways of storing the data in *IDProperties*, e.g. with a scalar arrays which describe the curves, instead of a pointer to some heap object, but this would mean adding conversion functions between the UI data and the *IDProperty*'s data. Such conversion functions may not be too intrusive, but still a potential source of complexity and divergent behaviors (leading to bugs) between "native" mapping/profile/ramp and Python ones. Therefore, to keep things simple, I opt to simply store a pointer to some CurveMapping/CurvePorfile/ColorBand object (as could be seen in my original set of patches ([2], [3], & [4])). **API Future Proofing** By allowing creating such property types, some implementation details or limitations specific to the mapping/profile/ramp are exposed in the API. For example: 1. Curve mappings can only have a fixed number of curves (1, 3, or 4) which has to be set during creation. The behaviour is kept for the Python equivalent, albeit by using an enumeration on the type "scalar", "rgb", etc. 2. Color ramps only have a limited number of slots (32). However, this is not exposed in any API (C or otherwise), but could be the source of API breakage if exposed. 3. Curve profiles need a preset at creation time, again this could change in the future. This list is non-exhaustive, as there might be other sources of API compatibility issues which I fail to consider. Since add-on developpers are currently only using built-in mapping/profile/ramp widgets, no attempt was made to remove these limitations; the current work merely exposes the various C API for creating such widgets to Python as is. This all begs a few design consideration for future proofing the API. **Notes** Data storage in *IDProperties* and API future proofing seem to be the main points of contention. **Links** [1]: https://blender.stackexchange.com/questions/61618/add-a-custom-curve-mapping-property-for-an-add-on [2]: Python API: support creating Curve Mapping properties (https://archive.blender.org/developer/D15824) [3]: Python API: support creating Curve Profile properties (https://archive.blender.org/developer/D15825) [4]: Python API: support creating Color Ramp properties (https://archive.blender.org/developer/D15826)

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Python API support for creating CurveMapping, CurveProfile and ColorRamp properties. #105781