Driver evaluates to wrong value. #40747

New Issue

Gustav Nilsson · 2014-06-21T20:01:04+02:00

Gustav Nilsson commented

2014-06-21 20:01:04 +02:00

System Information
Windows 8.1 64-bit, Nvidia GTX 580 3GB

Blender Version
Broken: 2.70 - 19e627c, 2.71RC2 - a19c1be

Short description of error

The driver expression "var == 5" evaluates to False when the variable value is 5.

Exact steps for others to reproduce the error

BugReport.blend

Using the file Iincluded

When playing the animation, the cube starts appearing and diappering on certain frames,
when it should be visible on every frame.

I have a driver activating and deactivating a mask modifier based on the position of an empty.

In the driver, the expression "var == 5" is sometimes evaluated to False,
even though the variable value is always 5.000.

The frames it fails on seems to differ between the two Blender versions I tried.

In Blender 2.70, on frame 76, the variable value is exactly 5,
which I can confirm by writing just "var" as the expression, and getting an even 5 as a result.

But the expression "var == 5" returns a 0.

Switching from Local Space to Transform Space,
makes the expression "var == 5" alaways evaluate to True, stopping the cube from disappearing.

The value of the variable is displayed as 5.000 when both using
Local Space and Transform space, but only Transform Space works.

To recreate the issue from scratch.

Add three objects.

Parent Object 1 and Object 2 to Object 3.

Move "Object 2" 5 units along Z.

Add a driver to Object 1 with Transform Channel variable that uses the Z location of Object 2 in Local Space.

Set the expression to var == 5 (the same height as Object 2 has).

Moving Object 3 makes the expression "var == 5" evaluate to False on certain frames.

Changing from Local Space to Transform Space stops the problem.

**System Information** Windows 8.1 64-bit, Nvidia GTX 580 3GB **Blender Version** Broken: 2.70 - 19e627c, 2.71RC2 - a19c1be **Short description of error** The driver expression "var == 5" evaluates to False when the variable value is 5. **Exact steps for others to reproduce the error** [BugReport.blend](https://archive.blender.org/developer/F95194/BugReport.blend) *Using the file Iincluded* When playing the animation, the cube starts appearing and diappering on certain frames, when it should be visible on every frame. I have a driver activating and deactivating a mask modifier based on the position of an empty. In the driver, the expression "var == 5" is sometimes evaluated to False, even though the variable value is always 5.000. The frames it fails on seems to differ between the two Blender versions I tried. In Blender 2.70, on frame 76, the variable value is exactly 5, which I can confirm by writing just "var" as the expression, and getting an even 5 as a result. But the expression "var == 5" returns a 0. Switching from Local Space to Transform Space, makes the expression "var == 5" alaways evaluate to True, stopping the cube from disappearing. The value of the variable is displayed as 5.000 when both using Local Space and Transform space, but only Transform Space works. *To recreate the issue from scratch.* Add three objects. Parent Object 1 and Object 2 to Object 3. Move "Object 2" 5 units along Z. Add a driver to Object 1 with Transform Channel variable that uses the Z location of Object 2 in Local Space. Set the expression to var == 5 (the same height as Object 2 has). Moving Object 3 makes the expression "var == 5" evaluate to False on certain frames. Changing from Local Space to Transform Space stops the problem.

Gustav Nilsson commented

2014-06-21 20:01:04 +02:00

Changed status to: 'Open'

Gustav Nilsson commented

2014-06-21 20:01:04 +02:00

Added subscriber: @GustavN

Bastien Montagne commented

2014-06-21 21:21:29 +02:00

Changed status from 'Open' to: 'Archived'

Bastien Montagne closed this issue

2014-06-21 21:21:29 +02:00

Bastien Montagne self-assigned this 2014-06-21 21:21:29 +02:00

Bastien Montagne commented

2014-06-21 21:21:29 +02:00

You are facing the infamous float precision issue (though it would be same thing with doubles, actually). Just have a look at the console, you'll that often, value is actually e.g. 5.000000476837158, not 5.0.

This is a universal issue with floating numbers - exact equality is often not really usable, you should rather check difference between the two numbers is below a given threshold, e.g. you can use this expression:

abs(var - 5.0) < 1e-4

Anyway, thanks for the report, but this is not a bug.

You are facing the infamous float precision issue (though it would be same thing with doubles, actually). Just have a look at the console, you'll that often, value is actually e.g. `5.000000476837158`, not `5.0`. This is a universal issue with floating numbers - exact equality is often not really usable, you should rather check difference between the two numbers is below a given threshold, e.g. you can use this expression: ``` abs(var - 5.0) < 1e-4 ``` Anyway, thanks for the report, but this is not a bug.

Gustav Nilsson commented

2014-06-21 21:32:10 +02:00

Thanks for clarifying the issue (and providing a solution)! I'm sorry for taking up your time.

May I just ask why this doesn't happen when using Transform Space? I thought the only difference between Local Space and Transform Space was the inclusion of constraints. If this is a rounding error, shouldn't it happen for both spaces and not just Local Space?

Thanks for clarifying the issue (and providing a solution)! I'm sorry for taking up your time. May I just ask why this doesn't happen when using Transform Space? I thought the only difference between Local Space and Transform Space was the inclusion of constraints. If this is a rounding error, shouldn't it happen for both spaces and not just Local Space?

Bastien Montagne commented

2014-06-21 22:02:28 +02:00

Well, transform space contains the “raw” data, without any matrix manipulation in-between, hence its values have suffered no deviation at all. Local space has gone at least across one matrix creation/reading, so…

Gustav Nilsson commented

2014-06-21 22:12:47 +02:00

Awesome! Now I understand why it behaved like it did.

Thanks again! :D

Awesome! Now I understand why it behaved like it did. Thanks again! :D

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Driver evaluates to wrong value. #40747