[vtkusers] Marking a pixel with a filled circle

[David Gobbi]

On Sun, Dec 6, 2009 at 5:25 PM, David Doria <daviddoria+vtk at gmail.com> wrote:
>
> vtkImageCanvasSource2D was exactly what I was looking/asking for, thanks!
> Although I agree that it is not a good plan to actually modify the pixels
> just to visualize something, this may be useful to write out images to
> remind myself of the results obtained at a later date.
>
> I made an example:
> http://www.vtk.org/Wiki/VTK_Examples_Drawing
> it attempts to read an input image and then draw on top of it. Since
> vtkImageCanvasSource2D is a source, this does not seem to be how it is
> supposed to operate. Instead I copied the Extent of the input image and draw
> on a blank image. I have two questions:
> 1) I set the draw color to what I believe is red ( (r, g, b, a) I was
> guessing). The resulting circle was white. Anyone know why?

Someone else will have to answer that... I don't really know much
about CanvasSource myself, other than that it exists.


> 2) How would I copy this image "onto" the input image so it was as if I drew
> on the input image?

As I said in my previous email, use vtkImageBlend to compose the
images.  The output will be the result of putting the two images
together.  They will have to be of the same data type.  If you are
planning to modify your image "in place", then don't.  The pipeline
gods will strike you down if you do.


> David - What you mentioned about the ImageData interface is interesting...
> it would definitely be nice to have good methods for determining:
> a) world space coordinates of an image actor pixel
> b) world space coordinates of an image actor pixel
> c) image space coordinates of a screen space point (ie. which pixel in the
> image does a clicked point in the window (on the image actor) correspond
> to?)

You must have meant to reverse (b).  It's a very good idea to add
these methods to vtkImageData, but it's something that has to be done
properly, i.e. whoever does it will have to hound the other developers
for input.  There are lots of developers who do image work with VTK,
but very few seem to be active on this list when you ask for
opinions... hmm.

About your item (c), though, I recently added a couple classes called
vtkSurfacePicker and vtkVolumePicker to VTK that do exactly that.  If
you click on a vtkImageActor, these pickers will give you the (i,j,k)
and (x,y,z) coordinates under the mouse.  They also give the CellId
and PointID so that you can easily check the data values, too, either
with interpolation (using cells) or without (using points).

   David