[vtk-developers] vtkTextActor (and vtkTextures?) with gradient backgrounds

[Karthik Krishnan]

On Tue, Oct 27, 2009 at 8:11 AM, Jeff Baumes <jeff.baumes at kitware.com> wrote:
> On Mon, Oct 26, 2009 at 10:19 PM, David Feng <dfeng at cs.unc.edu> wrote:
>>
>> It looks vtkTextActor doesn't play well with gradient backgrounds.  The
>> background disappears entirely when I add a vtkTextActor to my renderer,
>> which is embedded in a vtkRenderView.  I suspect it's because vtkTextActor
>> uses a vtkTexture, which enables GL_ALPHA_TEST (in the OpenGL version).  The
>> background is rendered as a 0.0 alpha colored quad (when gradient
>> backgrounds are enabled), and the alpha function is discarding all 0.0 alpha
>> fragments for the following reason:
>>
>>  // don't accept fragments if they have zero opacity. this will stop the
>>  // zbuffer from be blocked by totally transparent texture fragments.
>>  glAlphaFunc (GL_GREATER, static_cast<GLclampf>(0));
>>  glEnable (GL_ALPHA_TEST);
>>
>> Simply adding glDisable(GL_ALPHA_TEST) to the PostRender method didn't
>> seem to affect my output, so my diagnosis could be wrong.  If it is
>> something along these lines, I'm seems like all vtkTextures will clobber
>> gradient backgrounds.
>>
>> Regardless, it looks like vtkTextMapper doesn't cause these problems, so
>> I'll be switch to that for now.  Does anyone know of a simple way to have
>> the vtkTextMapper scale with viewport size?
>>
>
> The issue we have run into several times with the gradient background is
> that mappers/actors in VTK do not always reset their state after rendering.
> It is not textures in general since I've used textures with a gradient
> background before (e.g. in vtkGeoView).
> I believe the text mapper draws pixels directly on the screen, so they are
> always constant size. The text actor renders a texture to a quad in the
> scene, which is why it can scale.

That is right. We've noticed the issue and we tend to use
vtkTextMapper for this, and other reasons, in some of our
applications.

We do manage to control the scaling of the actor with the viewport
size. See the class vtkCornerAnnotation and its associated tests as an
example. This uses a vtkTextMapper and scales as the viewport size
changes. Essentially, it changes the font size on the text mapper in
its internal render method RenderOpaqueGeometry. So everytime the
viewport is scaled, the text mapper automatically assumes the right
size. You could do the same by wrapping your mapper/actor2D pair in
your own vtkActor2D.

Hope this helps.