[vtk-developers] Bug in scalar color mapping?

[David Gobbi]

Thanks, Ken.  That makes it very clear.

Now I know we need backwards compatibility, so maybe I should bite
my tongue, but it would be nice if the code would _combine_ the property's
color with the scalar color, e.g.

     if (this->ScalarMaterialMode == VTK_MATERIALMODE_AMBIENT)
       {
       FSSource = replace(FSSource,"//VTK::Color::Impl",
         "vec3 ambientColor = vertexColor.rgb*ambientColorUniform.rbg;\n"
         "vec3 diffuseColor = diffuseColorUniform.rgb;\n"
         "float opacity = vertexColor.a*opacityUniform;");
       }

etcetera.  If the mode is VTK_MATERIALMODE_AMBIENT the scalars would
combine with the property's ambient color, if VTK_MATERIALMODE_DIFFUSE
the scalars would combine with the property's diffuse color, and if the mode is
AMBIENT_AND_DIFFUSE the scalar color would combine with both.  As for
DEFAULT, my preference would be for it to be same as AMBIENT_AND_DIFFUSE.

 - David


On Thu, Sep 25, 2014 at 9:27 AM, Ken Martin <ken.martin at kitware.com> wrote:
> Depending on various settings, phase of the moon etc, scalar colors
> sometimes end up setting the ambient color and sometimes they set the
> diffuse color of a vertex and sometimes they set both IIRC.  Sometimes
> lighting is applied to the result and sometimes it isn't.
>
> The following snippet of code is what I am using for the OpenGL backend,
> which is very close to that VTK currently does. Textured scalars coloring
> is another beast. Hope that helps
>
>   if ( we have scalar colors)
>     {
>     if (this->ScalarMaterialMode == VTK_MATERIALMODE_AMBIENT ||
>           (this->ScalarMaterialMode == VTK_MATERIALMODE_DEFAULT &&
> actor->GetProperty()->GetAmbient() > actor->GetProperty()->GetDiffuse()))
>       {
>       FSSource = replace(FSSource,"//VTK::Color::Impl",
>                                   "vec3 ambientColor = vertexColor.rgb;\n"
>                                   "vec3 diffuseColor =
> diffuseColorUniform.rgb;\n"
>                                   "float opacity = vertexColor.a;");
>       }
>     else if (this->ScalarMaterialMode == VTK_MATERIALMODE_DIFFUSE ||
>           (this->ScalarMaterialMode == VTK_MATERIALMODE_DEFAULT &&
> actor->GetProperty()->GetAmbient() <= actor->GetProperty()->GetDiffuse()))
>       {
>       FSSource = replace(FSSource,"//VTK::Color::Impl",
>                                   "vec3 diffuseColor = vertexColor.rgb;\n"
>                                   "vec3 ambientColor =
> ambientColorUniform;\n"
>                                   "float opacity = vertexColor.a;");
>       }
>     Else // I believe this case is something like
> VTK_MATERIALMODE_AMBIENT_AND_DIFFUSE
>       {
>       FSSource = replace(FSSource,"//VTK::Color::Impl",
>                                   "vec3 diffuseColor = vertexColor.rgb;\n"
>                                   "vec3 ambientColor = vertexColor.rgb;\n"
>                                   "float opacity = vertexColor.a;");
>       }
>     }
>   Else // no scalars, just use material property colors
>     {
>     FSSource = replace(FSSource,"//VTK::Color::Impl",
>                                 "vec3 ambientColor =
> ambientColorUniform;\n"
>                                 "vec3 diffuseColor =
> diffuseColorUniform;\n"
>                                 "float opacity = opacityUniform;");
>     }
>
> Ken Martin PhD
> Chairman & CFO
> Kitware Inc.
> 28 Corporate Drive
> Clifton Park NY 12065
> ken.martin at kitware.com
> 518 881-4901 (w)
> 518 371-4573 (f)
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>
> -----Original Message-----
> From: vtk-developers [mailto:vtk-developers-bounces at vtk.org] On Behalf Of
> David Gobbi
> Sent: Thursday, September 4, 2014 7:33 PM
> To: VTK Developers
> Subject: [vtk-developers] Bug in scalar color mapping?
>
> I've run into very odd behavior when I use Ambient and Diffuse
> coefficients in combination with vtkDataSetMapper.  This is with VTK
> master.
>
> My property is set up like this, with AmbientColor="red" and
> DiffuseColor="green".
>
>   property->SetAmbient(0.51);
>   property->SetDiffuse(0.49);
>   property->SetAmbientColor(1.0, 0.0, 0.0);
>   property->SetDiffuseColor(0.0, 1.0, 0.0);
>
> My mapper, however, is set to MapScalars, so it should ignore the
> AmbientColor and the DiffuseColor, and use the scalars instead:
>
>   mapper->SetColorModeToMapScalars();
>   mapper->SetLookupTable(table);
>   mapper->UseLookupTableScalarRangeOn();
>
> The lookup table is a simple greyscale lookup table, and my polydata has
> scalars.  So I would expect my data to be rendered in greyscale.
> But it's not, it's rendered mint green!  See the left side of the attached
> image.
>
> I tried a different scalar mapping pathway:
>
>   mapper->InterpolateScalarsBeforeMappingOn();
>
> The result was totally different, now it's greyscale with ambient
> lighting, instead of the mix of ambient and diffuse lighting that I
> requested (see right of attached image).
>
>
> If I slightly modify the ambient and diffuse coefficients, the result is,
> once again, completely unexpected:
>
>   property->SetAmbient(0.49);  // was 0.51
>   property->SetDiffuse(0.51);  // was 0.49
>
> Now everything is red.  In the second attached image, the left shows
> InterpolateScalarsBeforeMappingOff() and the right shows On().
>
>
> Anyone know what is going on here?  Everything behaves just find if I set
> either the Ambient or the Diffuse coefficient to zero.  But when I use
> them together, VTK goes crazy.
>
>  - David