Re: [Vxl-users] vectorise image

[Ian S. <sc...@im...>]

On 16/05/2012 14:55, Friedmann Y. wrote:
> so how is it that the vectorised calcs are so much faster in matlab?

Taking the example of

a = some_large_matrix .^ 2

This is cheap to parse and dispatch.
The only interpreter-level operations are
1. Parse line.
2. look up variable "large_matrix" and store in internal register V1.
3. look up/create variable "a" once store in internal register V2.
4. Call internal function per_element_power(V1, 2, V2).

The internal function per_element_power will have been written in C (or 
C++, FORTRAN, or assembler) and will have been optimised by which ever 
compiler they used to compile MATLAB.

If you wrote the full loop version

for i=1:size(some_large_matrix,1)
   for j=1:size(some_large_matrix,2)
     a(i,j) = some_large_matrix(i,j) ^ 2;

then the interpreter would repeated be asking
1-9: Loop stuff
10. Parse loop internals.
11. lookup variable some_large_matrix and store in v1
12. lookup variable i and store in v2
13. lookup variable j and store in v3
14. call dereference_matrix(v1, v2, v3, v4)
15-18 - same again for a into v8
19. Call internal function full_power(v4, 2, v8)
20-25 Some more loop stuff - jump back to line 4-ish several thousand times.


I'm afraid these questions are getting a little far from VXL. I'd 
suggest reading a book, or taking a course on compilers and 
interpreters, if you want to know more.

Ian.