[vtkusers] Marking a pixel with a filled circle

[David Gobbi]

On Sun, Dec 6, 2009 at 9:36 AM, David Doria <daviddoria+vtk at gmail.com> wrote:
> I am trying to mark pixels in an image that have been found to be keypoints
> so I can visualize where the keypoints are. If I mark a pixel like this:
>
>   //set the pixel (10,10) to red
>   image->SetScalarComponentFromDouble(10, 10, 0, 0, 255.0);
>   image->SetScalarComponentFromDouble(10, 10, 0, 1, 0.0);
>   image->SetScalarComponentFromDouble(10, 10, 0, 2, 0.0);
>
> the red mark is often too small to even see (since it is only one pixel!).
> Are there any functions to draw a filled circle/square of a specified color
> in an image centered at a specified pixel?

The vtkImageCanvasSource2D has drawing functionality, and you can use
vtkImageBlend to alpha-blend the canvas output into an RGB or RGBA
version of your image.  But I wouldn't recommend it.  There is a
better way to do things!

If you want to annotate an image, don't do it by modifying the image.
Use a vtkActor instead.  For example, use a vtkSphereSource and set
the position of the vtkActor to the pixel location.  In an ideal
world, you could just use actor->SetPosition(image->GetPoint(i,j,k)),
but no such GetPoint() method exists because the interface to
vtkImageData sucks eggs.  Instead, you have to compute the x,y,z
coordinates of the i,j,k pixel yourself.

  David