[ptx] Subpixel accurate template matching algorithm ???

[Terje Mathisen]

ptx-bounces at email-lists.org wrote:

> Hi everybody,
> 
> I'm just looking for algorithm which could solve this issue :
> Having a template image, it should return every occurrence of this pattern
> in a given image with a good subpixel accuracy.
> (I suppose we could set every necessary informations for the accuracy of the
> algorithm for dealing with exposure difference, noise, etc). The aim of the
> algorithm is not for control point but to retrieve positions on a picture
> with crosses as patterns : I need the
> exact locations of all crosses.
> Do you have ideas about the best approach ?

This sounds like the problem I attempted when I wanted to write a lens 
distortion specifier:

I printed a big (A0) sheet containing a rectangular grid, then tried to 
automatically locate the coordinates of all the grid junctions.

I couldn't get this to run solidly enough, so instead I did it mostly 
manually. :-(

The next step was to write a perl program which took the list of 
coordinates, and generated a PTOptimizer project with horizontal or 
vertical lines defined between each pair of relevant points.

Terje

PS. The best method I did find was to use a small template cross, 
convert the input image into B&W, and then do a convolution between the 
template and target image. Even without any rotation of the template, 
this did tend to find useful maxima close to the proper crossing points.

An algorithm that knew how the target image was supposed to look could 
find each horizontal line, scan along it, an d determine where the 
vertical crossings were.

-- 
- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"
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