# convTri

## PURPOSE

Extremely fast 2D image convolution with a triangle filter.

## SYNOPSIS

function J = convTri( I, r, s, nomex )

## DESCRIPTION

Extremely fast 2D image convolution with a triangle filter.

Convolves an image by a 2D triangle filter (the 1D triangle filter f is
[1:r r+1 r:-1:1]/(r+1)^2, the 2D version is simply conv2(f,f')). The
convolution can be performed in constant time per-pixel, independent of
the radius r. In fact the implementation is nearly optimal, with the
convolution taking only slightly more time than creating a copy of the
input array. Boundary effects are handled as if the image were padded
symmetrically prior to performing the convolution. An optional integer
downsampling parameter "s" can be specified, in which case the output is
downsampled by s (the implementation is efficient with downsampling
occurring simultaneously with smoothing, saving additional time).

The output is exactly equivalent to the following Matlab operations:
f = [1:r r+1 r:-1:1]/(r+1)^2;
J = convn(convn(J,f,'valid'),f','valid');
if(s>1), t=floor(s/2)+1; J=J(t:s:end-s+t,t:s:end-s+t,:); end
The computation, however, is an order of magnitude faster than the above.

When used as a smoothing filter, the standard deviation (sigma) of a tri
filter with radius r can be computed using [sigma=sqrt(r*(r+2)/6)]. For
the first few values of r this translates to: r=1: sigma=1/sqrt(2), r=2:
sigma=sqrt(4/3), r=3: sqrt(5/2), r=4: sigma=2. Given sigma, the
equivalent value of r can be computed via [r=sqrt(6*sigma*sigma+1)-1].

For even finer grained control for very small amounts of smoothing, any
value of r between 0 and 1 can be used (normally if r>=1 then r must be
an integer). In this case a filter of the form fp=[1 p 1]/(2+p) is used,
with p being determined automatically from r. The filter fp has a
standard deviation of [sigma=sqrt(2/(p+2))]. Hence p can be determined
from r by setting [sqrt(r*(r+2)/6)=sqrt(2/(p+2))], which gives
[p=12/r/(r+2)-2]. Note that r=1 gives p=2, so fp=[1 2 1]/4 which is the
same as the normal r=1 triangle filter. As r goes to 0, p goes to
infinity, and fp becomes the delta function [0 1 0]. The computation for
r<=1 is particularly fast.

The related function convBox performs convolution with a box filter,
which is slightly faster but has worse properties if used for smoothing.

This code requires SSE2 to compile and run (most modern Intel and AMD
processors support SSE2). Please see: http://en.wikipedia.org/wiki/SSE2.

USAGE
J = convTri( I, r, [s], [nomex] )

INPUTS
I      - [hxwxk] input k channel single image
r      - integer filter radius (or any value between 0 and 1)
filter standard deviation is: sigma=sqrt(r*(r+2)/6)
s      - [1] integer downsampling amount after convolving
nomex  - [0] if true perform computation in matlab (for testing/timing)

OUTPUTS
J      - [hxwxk] smoothed image

EXAMPLE - matlab versus mex
r = 5; s = 2; % set parameters as desired
tic, J1=convTri(I,r,s); toc % mex version (fast)
tic, J2=convTri(I,r,s,1); toc % matlab version (slow)
figure(1); im(J1); figure(2); im(abs(J2-J1));

EXAMPLE - triangle versus gaussian smoothing