Fractal model not useable with dQ data due to being undefined for negative q (Trac #835)

[smk78]

Reported by Rafal Kryszton and verified by smk78.

Affects Mac & Win.

The Fractal model in 4.1 builds and the 4.0.1 release is exhibiting unusual behaviour, not present in the 3.1.2 release, when dQ smearing is enabled.

An error is thrown: Plotting Error: Data has no positive values, and therefore cannot be log-scaled.

Depending on the root cause, this issue may affect other models.

To demonstrate:

Load the attached data. Send to Fitting. Select the Fractal model. Unselect all parameters.

With Instrumental Smearing = None, set fractal_dim to a number other than 2 and hit Enter. The model should recompute.

Now set Instrumental Smearing = Use dQ data and repeat. If fractal_dim is any value other than 2 the error is thrown and the model is not recomputed or disappears from view.

Reselecting Instrumental Smearing = None redisplays the model line.

Side Comment:

The .txt file is the users original data file. This reads into 4.1/4.0.1 BUT NOT into 3.1.2. The .csv version reads into all.

Migrated from http://trac.sasview.org/ticket/835

{
    "status": "closed",
    "changetime": "2017-03-21T15:12:03",
    "_ts": "2017-03-21 15:12:03.243777+00:00",
    "description": "Reported by Rafal Kryszton and verified by smk78.\n\nAffects Mac & Win.\n\nThe Fractal model in 4.1 builds and the 4.0.1 release is exhibiting unusual behaviour, not present in the 3.1.2 release, when dQ smearing is enabled.\n\nAn error is thrown: Plotting Error: Data has no positive values, and therefore cannot be log-scaled.\n\nDepending on the root cause, this issue may affect other models.\n\nTo demonstrate:\n\nLoad the attached data. Send to Fitting. Select the Fractal model. Unselect all parameters.\n\nWith Instrumental Smearing = None, set fractal_dim to a number other than 2 and hit Enter. The model should recompute.\n\nNow set Instrumental Smearing = Use dQ data and repeat. If fractal_dim is any value other than 2 the error is thrown and the model is not recomputed or disappears from view.\n\nReselecting Instrumental Smearing = None redisplays the model line.\n\nSide Comment:\n\nThe .txt file is the users original data file. This reads into 4.1/4.0.1 BUT NOT into 3.1.2. The .csv version reads into all.\n",
    "reporter": "smk78",
    "cc": "",
    "resolution": "fixed",
    "workpackage": "SasView Bug Fixing",
    "time": "2017-01-05T14:51:12",
    "component": "SasView",
    "summary": "Fractal model not useable with dQ data due to being undefined for negative q",
    "priority": "blocker",
    "keywords": "",
    "milestone": "SasView 4.1.0",
    "owner": "pkienzle",
    "type": "defect"
}

[smk78]

Trac update at 2017/01/05 14:52:10:

• smk78 changed attachment from "" to "rafal-sample-data.txt"
• smk78 commented:

Rafal data TXT

[smk78]

Trac update at 2017/01/05 14:52:28:

• smk78 changed attachment from "" to "rafal-sample-data.csv"
• smk78 commented:

Rafal Data CSV

[butlerpd]

Trac update at 2017/01/10 14:47:17:

• butler changed owner from "" to "richardh"
• butler changed status from "new" to "assigned"

[smk78]

Trac update at 2017/01/10 17:07:32: smk78 commented:

Update.

Following a suggestion on todays developers call, I have just repeated my tests above with the same dataset but with the interweaved data points removed.

It makes no difference.

So the error is coming from the dQ data.

There are no negative values in any of the data columns.

[smk78]

Trac update at 2017/01/10 17:09:40:

• smk78 changed attachment from "" to "rafal-sample-data-nointerweave.csv"
• smk78 commented:

Rafal Data CSV but with interwoven points removed (4 of them)

[sasview-bot]

Trac update at 2017/01/10 18:30:33: krzywon commented:

The further I look into this, the less I'm convinced this is related to #921. What I've noticed (in case this helps anyone):

(1) Any '''non-integer''' value for the fractal dimension when dQ data smearing is selected throws the "Plotting Error: Data has no positive values, and therefore cannot be log-scaled." error.
(2) Fitting the data with an initial integer value for the fractal dimension, the fractal dimension as a fittable parameter, and dQ data set results in a non-converging fit "Fitting did not converge!!!".
(3) Fitting the data as in (2), but with the fractal dimension as a fixed parameter results in a converging fit.
(4) Fitting the data as in (2), but with no dQ data set results in a non-integer value for the fractal dimension.

As far as I can tell, there is nothing forcing the fractal dimension to be an integer in sasmodels, but if this parameter is supposed to be an integer, this could be related to #912.

[ajj]

Trac update at 2017/02/07 14:46:08: ajj commented:

• The model doesn't work at all when any resolution is added and the fractal dimension is non integer.
• It is not dependent on dQ values from the data : putting in a custom pinhole on the model alone breaks it