[vtkusers] Slow text rendering with vtkCornerAnnotation and big windows

[David Lonie]

Hi Marc,

On Tue, Jul 28, 2015 at 10:19 AM, Marc Ruiz Altisent <
marc.ruiz+vtk at gmail.com> wrote:

> Hi, I'm the developer of a medical viewer that uses vtkCornerAnnotation to
> display image-related information in the corners of the window. I have
> found that updating the text for each frame is slow when the window is very
> big, like for example in a 5 MPixel medical display.
>
> Previously we used the vtkRenderingFreeTypeOpenGL module and annotation
> updates were instantaneous no matter the window size, but it doesn't
> support UTF-8, so we decided to not use it anymore.
>
> Now, with the default implementation in the vtkRenderingFreeType module,
> when the window is very big the updates take a considerable amount of time,
> to the point of making the application very laggy when scrolling through
> images. I have tried to limit the text size to a small number, but even
> then with a big window text rendering is slow and with a small window it's
> fast, so it's not directly related to text size. I don't know exactly what
> is causing the slowness, but according to profiling tests, the bottleneck
> is the vtkFreeTypeTools::RenderCharacter() method.
>
> Could the performance of text rendering be improved? Currently I could
> revert back to using the vtkRenderingFreeTypeOpenGL module, even with its
> limitations, but since it will disappear in VTK 6.3 this would just be a
> short-term workaround.
>

I suspect that vtkFreeTypeTools::RenderCharacter is not a bottleneck,
though it would be eating up a lot of time in a profiler as it's where most
of the heavy-lifting is done during text rendering.

After taking a quick skim through the code, the more likely problem is in
vtkCornerAnnotation::RenderOpaqueGeometry, which has the following loops:

      while (... /*annotation size is too small*/ &&  fontSize < 100)

        {

        fontSize++;

        for (int i = 0; i < NumTextPositions; i++)

          {

          this->TextMapper[i]->GetTextProperty()->SetFontSize(fontSize);

          this->TextMapper[i]->GetSize(viewport, tempi + i * 2);

          }

        /* Recompute dimensions of annotation */

        }


      /* Followed by the same thing, but decrease font size to fit */


So it basically renders the text repeatedly, attempting to fit it to a box.
Combining a high density display with the new DPI support in VTK and
repeatedly rendering to fit a box like this will likely eat up a lot of
time and thrash memory pretty hard.

It looks like it's currently not possible to 'seed' the fontsize via the
vtkCornerAnnotation API -- it just grabs the default font size from an
unmodified vtkTextProperty object (line 460) as a starting point.

Can you file a bug for this? At the very least the initial fontsize should
be able to be set, and at best this class should be refactored to use newer
text renderering APIs like vtkTextRenderer::GetBoundingBox, which won't
actually generate a texture but just compute the dimensions of the
resulting image.

Thanks,
Dave
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