[osg-users] Changes in osgVolume from 3.0 to 3.4

[Alex Taylor]

Robert,

Thanks. This was helpful. I resolved my issue by just setting the alpha to
1.0 for all input values in my transfer function. This together with
specifying an IsosurfaceProperty gave me the effect I wanted.

Thanks again,

Alex
On Tue, Feb 16, 2016 at 10:16 AM Robert Osfield <robert.osfield at gmail.com>
wrote:

> Hi Alex,
>
> I'm a bit too busy to go rummaging through code to answer your
> questions so will do my best by providing some general questions.  I
> vaguely recall OSG-3.0 alpha, transparency and isosurface all
> overlapped a bit, so there was a bit of cross over.  In OSG-3.4
> transparency and isosurface value is now separate and can be applied
> together within the same shader.   There is also AlphaFunc which clips
> out fragments below a specific alpha value.  I hope this makes sense.
>
> Robert.
>
> On 16 February 2016 at 14:59, Alex Taylor <alextaylor at gmail.com> wrote:
> > Robert,
> >
> > Thanks again. I do have one quick follow up question related to something
> > you said.
> >
> > "Check the alpha values settings that you have set up for the
> VolumeTile."
> >
> > I'm trying to figure out exactly what you mean by the "alpha values
> > settings" on the VolumeTile. I define my VolumeTile as shown below. As
> you
> > can see, I'm not setting Alpha anywhere in the setup of my tile. The one
> > place where alpha comes in is when I set the TransferFunctionProperty on
> my
> > tile. This brings me to my question.
> >
> > It looks to me like the intention of Isosurface property is to specify
> the
> > isovalue for the isosurface rendering. What happens if you specify an
> > Isosurface property but your transfer function contains alpha values
> between
> > the ranges of (0,1)? Meaning, if you are defining an isosurface at
> isovalue
> > = X, do you also have to specify the alpha in your transfer function as a
> > unit step function U(I-X) with transition at X?
> >
> > I'm trying to understand the motivation for blending being on for the
> > Isosurface codepath in RayTracedTechnique and how the alpha in the
> transfer
> > function interacts with the specification of an IsosurfaceProperty. I had
> > originally thought/guessed that the alpha in your transfer function would
> > just be ignored if you specify an IsosurfaceProperty.
> >
> > - Alex
> >
> >             osg::ref_ptr<osg::Image> intensityImage =
> createTexture3D(data);
> >
> >             osg::ref_ptr<osg::Image> image_3d =
> > (volumeProperties.volumeTechnique == VolumeTechnique::FixedFunction) ?
> >
> >
> >
> osgVolume::applyTransferFunction(intensityImage.get(),volumeProperties.transferFunction.get())
> > :
> >
> >
> > intensityImage.release();
> >
> >
> >
> >             osg::ref_ptr<osgVolume::ImageLayer> layer = new
> > osgVolume::ImageLayer(image_3d);
> >
> >             tile->setLayer(layer.get());
> >
> > Where createTexture3D is defined as:
> >
> >     osg::Image* createTexture3D(const mxArray* src) {
> >
> >         if (src == NULL || mxGetNumberOfDimensions(src) != 3 ||
> > mxGetClassID(src) != mxUINT8_CLASS) {
> >
> >             return NULL;
> >
> >         }
> >
> >         size_t num_s = mxGetDimensions(src)[0];
> >
> >         size_t num_t = mxGetDimensions(src)[1];
> >
> >         size_t num_r = mxGetDimensions(src)[2];
> >
> >         // now allocate the 3d texture;
> >
> >         osg::ref_ptr<osg::Image> image_3d = new osg::Image;
> >
> >         image_3d->setImage(num_s, num_t, num_r,
> >
> >                 GL_LUMINANCE, GL_LUMINANCE, GL_UNSIGNED_BYTE,
> >
> >                 static_cast<uint8_T *>(mxGetData(src)),
> >
> >                 osg::Image::NO_DELETE);
> >
> >
> >
> >         return image_3d.release();
> >
> >     }
> >
> > Other than that, the only other specification Alpha comes from setting
> the
> > TransferFunctionProperty on the tile with my TransferFunction. This
> brings
> > me to my question.
> >
> > When
> >
> >
> >
> > On Fri, Feb 12, 2016 at 5:42 AM Robert Osfield <robert.osfield at gmail.com
> >
> > wrote:
> >>
> >> Hi Alex,
> >>
> >> On 11 February 2016 at 20:47, Alex Taylor <alextaylor at gmail.com> wrote:
> >> > Also, if it matters, I found that I can get the rendering I'd expect
> if
> >> > I
> >> > explicitly set the BlendFunc in the special case of Isosurface:
> >> >
> >> >   if (volumeProperties.useIsosurface){
> >> >             stateset->setAttribute(new osg::BlendFunc(GL_ONE,
> GL_ZERO),
> >> > osg::StateAttribute::ON);
> >> >    }
> >> >
> >> > I'm not sure why I need to do this in OSG 3.4 when I didn't in OSG
> 3.0,
> >> > probably another issue with the viewer or something on my end.
> >>
> >> From the details above and the picture it looks to me that OSG-3.4 is
> >> probably doing the right thing w.r.t the settings, such as alpha
> >> settings, you are using and OSG-3.0 implementation was incorrect and
> >> ignored these settings so you never saw this.
> >>
> >> Check the alpha values settings that you have set up for the VolumeTile.
> >>
> >> Robert.
> >> _______________________________________________
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> >> osg-users at lists.openscenegraph.org
> >>
> http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
> >
> >
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