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Drivers freezing input value
Closed, ResolvedPublic


Blender Version
Broken: blender-2.80.0-git.03bd024c077d-windows64

Short description of error
I have a scene with a single light and a mesh with an emission material. I've set up a driver for the emission value, driven by the light energy.
The light energy is keyframed and controlled with an fcurve.
However, once the driver is connected to the light's energy, the light energy freezes and becomes will not update at any point in the fcurve. Changing the fcurve keyframe values or running the animation doesn't work.
If I delete the driver, the light energy animates properly.

I can't for the life of me work out the cause or exact steps to recreate this bug, it appears either the material or the light is permanently corrupted in some way.
Even copying the objects to a new blend, they remain corrupted.
I've tried removing and creating new animation data for the light, I've tried deleting the driver and creating a new one.
Absolutely nothing works. It's seemingly impossible without creating a new light and emission material.

I can create a new light and emission material in the same scene and setup and the exact same driver however, and this works.
I spent hours trying to suss this one out.
Horrible, random bug :(

See attached a simplified blend file demonstrating the problem. Notice how the light value doesn't change between frames 0-100 which is animated, but on deleting the emission material's driver, it works.
Also note how the timeline playhead only updates the energy value immediately on loading the blend. Try saving the file with the playhead in different positions and notice how the energy value is different on load. Also try moving the keyframes, this updates the energy value too.

Edit: Interestingly, if instead of adding Single Properties and referring to them in the Expression textbox, I can refer to the light directly with python (ie,["Point"].energy) in the Expression textbox and it works perfectly. However I'm warned by Blender that it is considerably slower doing it this way.

Event Timeline

I'm still experiencing this issue with the latest build.

Here's another example which I'll describe slightly differently:

In the NLA editor are 2 strips, the first sets the light to power 20, the other to power 0, labelled ON and OFF.
The emission strength of the lightbox shader is driven by the point light power.

Notice how moving the playhead over each nla strip does nothing to the light power value.

Turn on rendered view and try tweaking the ON strip. Observe the rendered view and the power property. Now exit tweak mode.
You should notice the light power change on entering tweak mode, hover over the property panel for it to redraw.
On Exiting you should notice the emission strength update.
The same behaviour is exhibited on the OFF strip.

Also note that rendering a still image works correctly, it's just in 3D view that this is an issue.

@Tom Wilson (hedgehog90), the report is still open and assigned to me. Not sure why it would be expected to be solved in the newer builds.
We've got more than a 1000 of reports for handful of developers. Please be patient. The issue will be solved.

My apologies. In your own time.