[vtkusers] Scalars and diffuse lighting

[Andrew Ward]

I have just tried multiplying each channel of my original actor colour by
max(diffuseCoeff, ambientCoeff) and using this as the scalar colour in
places where the original actor colour should be preserved. This solves the
problem.

Thanks again David.

Andrew

On 13 April 2011 09:19, Andrew Ward <andrew.derek.ward at gmail.com> wrote:

> Thanks for your explanation David. This is a shame as I would expect it is
> a fairly common requirement for a user to toggle scalars on/off, which
> perhaps only affect part of an actor, therefore the other parts of the actor
> logically should stay looking exactly the same as before. I am assuming
> there is no way to only assign scalars to a portion of an actors cells.
>
> Regards,
>
>
> Andrew
>
>
>
> On 12 April 2011 18:59, David Gobbi <david.gobbi at gmail.com> wrote:
>
>> Okay, here is my best explanation of what is going on.  Someone
>> please correct me if there are mistakes.
>>
>> The use of lighting coefficients is limited when using scalars.
>> Generally, VTK uses the scalars as the diffuse color while ignoring
>> the diffuse coefficient (i.e. assuming it is 1.0).  The ambient and
>> specular colors and coefficients are used as specified in the
>> property, i.e. they are not taken from the scalars.
>>
>> This behavior can be slightly modified via the mapper
>> SetScalarMaterialMode method, by making it so that the ambient
>> color tracks the scalars, or so that both ambient and diffuse color
>> tracks the scalars.
>>
>> Long story short: diffuse coefficient is ignored when using scalars.
>> This does not appear to be a bug, I think this was done intentionally
>> to keep the mapper implementation from becoming too complex.
>> Kind of surprising that it isn't better documented, though.
>>
>>  - David
>>
>>
>> On Tue, Apr 12, 2011 at 10:49 AM, David Gobbi <david.gobbi at gmail.com>
>> wrote:
>> > Hi Andrew,
>> >
>> > I'm not 100% sure on this, but I think it has to do with the
>> > ScalarMaterialMode in the mapper.  Try doing this, for instance, to
>> > see if it helps:
>> >
>> > mapper->SetScalarMaterialModeToAmbientAndDiffuse();
>> >
>> > The PolyDataMapper seems to use glColor() for color scalars, but it
>> > uses glMaterial() for the property color, and I think that this is
>> > responsible for the different behavior.
>> >
>> >  - David
>> >
>> >
>> > On Tue, Apr 12, 2011 at 10:23 AM, andrew.derek.ward
>> > <andrew.derek.ward at gmail.com> wrote:
>> >> Hello,
>> >>
>> >> I am trying to determine how lighting works in colour scalar mode.
>> >>
>> >> I have an actor of colour (255,0,0) with ambient lighting set to 0.1
>> and
>> >> diffuse lighting set to 0.7
>> >>
>> >> Without scalars, this actor looks fine, but if I enable scalars giving
>> every
>> >> cell the same colour (255,0,0) the
>> >> diffuse lighting coefficient does not seem to have any affect.
>> >>
>> >> It is as though VTK locks the diffuse lighting coefficient to 1.0 when
>> >> scalars are enabled, giving me a brighter
>> >> looking actor than my original non-scalar coloured one.
>> >>
>> >> This appears to be similar to these unanswered questions:
>> >> http://www.vtk.org/pipermail/vtkusers/1999-July/050896.html
>> >> http://www.vtk.org/pipermail/vtkusers/2002-October/063444.html
>> >>
>> >> Any help/explanation greatly appreciated.
>> >>
>> >> Regards,
>> >>
>> >> Andrew
>> >
>>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.vtk.org/pipermail/vtkusers/attachments/20110413/db85c90d/attachment.htm>