Sum/Product Models don't do what they should (Trac #767)

[smk78]

The Sum|Multi model generator is not creating properly parameterised models:

'''Sum'''
Should do: scale_factor * {(scale_1 * model_1) + (scale_2 * model_2)} + background

Is doing : scale_factor * {(scale_1 * model_1 + background_1) + (scale_2 * model_2 + background_2)}

'''Product'''

Should do: scale_factor * model_1 * model_2 + background

Is doing : (scale_1 * model_1 + background_1) * (scale_2 * model_2 + background_2) + background

Migrated from http://trac.sasview.org/ticket/767

{
    "status": "closed",
    "changetime": "2017-09-15T15:38:14",
    "_ts": "2017-09-15 15:38:14.545678+00:00",
    "description": "The Sum|Multi model generator is not creating properly parameterised models:\n\n'''Sum'''\nShould do: scale_factor * {(scale_1 * model_1) + (scale_2 * model_2)} + background\n\nIs doing : scale_factor * {(scale_1 * model_1 + background_1) + (scale_2 * model_2 + background_2)}\n\n'''Product'''\n\nShould do: scale_factor * model_1 * model_2 + background\n\nIs doing : (scale_1 * model_1 + background_1) * (scale_2 * model_2 + background_2) + background\n",
    "reporter": "smk78",
    "cc": "",
    "resolution": "fixed",
    "workpackage": "SasView Bug Fixing",
    "time": "2016-10-10T21:30:14",
    "component": "SasView",
    "summary": "Sum/Product Models don't do what they should",
    "priority": "critical",
    "keywords": "",
    "milestone": "SasView 4.2.0",
    "owner": "lewis",
    "type": "defect"
}

[smk78]

Trac update at 2016/10/10 21:34:41: smk78 changed description from:

The Sum|Multi model generator is not creating properly parameterised models:

'''Sum'''
Should do: scale_factor * {(scale_1 * model_1) + (scale_2 * model_2)} + background

Is doing : scale_factor * {(scale_1 * model_1 + b/g_1) + (scale_2 * model_2 + b/g_2)}

'''Product'''

Should do: scale_factor * model_1 * model_2 + background

Is doing : (scale_1 * model_1 + b/g_1) * (scale_2 * model_2 + b/g_2) + background

to:

The Sum|Multi model generator is not creating properly parameterised models:

'''Sum'''
Should do: scale_factor * {(scale_1 * model_1) + (scale_2 * model_2)} + background

Is doing : scale_factor * {(scale_1 * model_1 + background_1) + (scale_2 * model_2 + background_2)}

'''Product'''

Should do: scale_factor * model_1 * model_2 + background

Is doing : (scale_1 * model_1 + background_1) * (scale_2 * model_2 + background_2) + background

[butlerpd]

Trac update at 2016/10/11 18:20:59:

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

[butlerpd]

Trac update at 2017/03/06 03:59:13: butler commented:

Is this really a blocker? Obviously the change would be much cleaner but not sure it is a blocker. By appropriate choices of values for scales and background the expected behavior can be achieved, just a bit .... ugly!

[smk78]

Trac update at 2017/03/06 10:17:55: smk78 commented:

Except no user actually knows that... :-)

[butlerpd]

Trac update at 2017/03/07 02:20:16: butler commented:

Welll... funny you should say that:-) Actually I would argue it will be quickly obvious to most "ordinary" users and that the people who will be most confused will be us, the developers. Indeed we have been talking so long about what we want it to be but cant keep track in the flurry of activities what has and has not been done that we (at least I do) get confused.

In fact, the very title of the ticket is incorrect. Sum and multiply do EXACTLY what they say. In fact the statement of the problem is a bit misleading. For example for the sum model it says:

Is doing : scale_factor * {(scale_1 * model_1 + background_1) + (scale_2 * model_2 + background_2)}

But really what it is doing is exactly what it says when you create the model namely:

Scale_factor * (Model_1 + Model_2).

All the parameters of model 1 and all the parameters of model 2 are present (and labeled as 1 or 2) and a separate non numbered scale factor is added. If the model in question has a scale and/or a background then it/they will show up, and if not (try for example hard sphere as one of the models) it/they won't. An ordinary user might asks us: what part of model_1 don't you guys understand? For me it would be the fact that when writing models I don't usually include scale and background anymore which are auto added (or not as the case may be), but an ordinary user will likely not be so encumbered by prejudice :-)

That said I think we should change the behavior as soon as we can as suggested in the ticket. I am just worried that we may not have the bandwidth to get it done soon enough for a release and questioning the priority of this particular ticket.

[butlerpd]

Trac update at 2017/03/07 13:42:13: butler commented:

ooops my bad. While last comment I think is correct, my previous one is not entirely so. Mainly for a sum model there is no way to decouple scale(time del rho etc) from background.

[ajj]

Trac update at 2017/03/16 20:23:38:

• ajj commented:

Checked current master. Changed name of ticket to reflect real nature of ticket.

This should be fixed as the behaviour is a pain - all the multiplication of backgrounds and the "!BackGround" parameter with inconsistent capitalisation is horrid.

Should probably be replaced by something nicer in sasmodels that can properly cope with more arbitrary model arithmetic, but behaviour could also be fixed in current implementation with suppression of background parameters etc.

Not a blocker for 4.1

• ajj changed milestone from "SasView 4.1.0" to "SasView 4.2.0"
• ajj changed priority from "blocker" to "major"
• ajj changed summary from "Sum/Product Models don't do what they claim" to "Sum/Product Models don't do what they should"