Create custom graph and access values from Python

Question

I need to allow the user to create their own transfer function y = f(x)

(I will then use this transfer function to warp the scene)

Currently I'm using a mathematical formula:

y = exp( 1/(x^2 + 1) )

But it doesn't allow a (non-mathematical) artist to shape the curve, and it is awkward even for a mathematician to turn an arbitrary shape into a mathematical formula.

Ideally I would like the user to be able to draw the curve freehand, or maybe in the same manner as constructing curves in Blender.

In fact, can I get away with using Blender's existing curve tool? Say I create a new scene, I suppose I can then create a curve that is constrained to the xy axis. But then how could I get y = f(x) from a Python script?

And also, is there some 2D curve tool, that might allow the curve to be designed from within some panel?

Is there any solution path I'm missing?

Answer 1

After a bit of investigation it seems you can't use Graph Editor for "standalone" FCurves. However, you can link them to any object and name them as you like. I used the default Cube in example below, but it might be as well an empty mesh.

Below script uses Cube as object that "holds" your curves within its animation_data.action. It creates them if they do not exist, and if they exists current one is picked.

import bpy

placeholder = bpy.context.scene.objects["Cube"]
if not placeholder.animation_data:
    placeholder.animation_data_create()
if not placeholder.animation_data.action:
    placeholder.animation_data.action = bpy.data.actions.get("curves") or bpy.data.actions.new("curves")
action = placeholder.animation_data.action

print(placeholder)
name = 'my_curve'

fcurves = action.fcurves
for f in fcurves:
    if f.data_path == name:
        break
else:
    f = action.fcurves.new( name )
    f.select = True
    f.keyframe_points.insert( 0, 0 )
    f.keyframe_points.insert( 100, 10 )

Then you have get to edit curve you can add points by duplicating existing ones using shift-d.

After shaping it you can use FCurve.evaluate() (API docs - bpy.types.FCurve). For example like this:

for i in range(100):
    print(i, f.evaluate(i))