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

[Robert Osfield]

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.
>> _______________________________________________
>> osg-users mailing list
>> osg-users at lists.openscenegraph.org
>> http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
>
>
> _______________________________________________
> osg-users mailing list
> osg-users at lists.openscenegraph.org
> http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
>