Re: [Vxl-users] Access an image in blocks

[Ian S. <ian...@st...>]

Alois Komenda wrote:
> Hello
> 
> I've got some questions:
> 
> 1) I have to calculate a value for each pixel of an picture using the values of its environment (at least a 3x3-environment, more likely 5x5). Is there a way to access the required pixels in a block mode to speed up the calculation?


you can pre-calculate the offsets to a regular neighbourhood of pixels.
In vil_image_view im1
...........
.....ABC...
.....DEF...
.....GHI...
...........

where the pixel at A is at memory address ptr_a,
then the memory addresses of positions of the other labelled pixels are
ptr_b = ptr_a + (  im1.istep());
ptr_c = ptr_a + (2*im1.istep());
ptr_d = ptr_a + (                  im1.jstep());
ptr_e = ptr_a + (  im1.istep() +   im1.jstep());
ptr_f = ptr_a + (2*im1.istep() +   im1.jstep());
ptr_g = ptr_a + (                2*im1.jstep());
ptr_h = ptr_a + (  im1.istep() + 2*im1.jstep());
ptr_i = ptr_a + (2*im1.istep() + 2*im1.jstep());

The terms in the brackets can be precalculated for any given image (or 
indeed for any image with identical memory layout.)


> 
> 2) Is there a function to build a quad-tree of an image available?

A simple search at http://www.isbe.man.ac.uk/search_vxl.html fails to 
find any.

> 
> 3) Are there methods for classification like minimum distance in vxl?

You may have some specific meaning of classification in mind other than 
"statistical classification", since I am not aware of any standard 
technique called "minimum distance".

However, you can find a range of statistical classifiers in the 
contrib/mul/clsfy library, described in 
http://paine.wiau.man.ac.uk/pub/doc_vxl/contrib/mul/clsfy/html/hierarchy.html

Ian.