Re: A "slanted edge" analysis program

Bart van der Wolf wrote:

"Lorenzo J. Lucchini" <ljlbox@xxxxxxxxxx> wrote in message news:P8_Ze.49524$nT3.24502@xxxxxxxxxxxxxxxxxxxxxxxx

Lorenzo J. Lucchini wrote:
> [snip]

Since http://ljl.150m.com appears to be down,

I was wondering about that, so it wasn't my setup afterall ;-)

You know, it's free hosting... I'm starting to worry, though, it's been down for quite a while now.

the file can also be found at
  http://ljl.741.com/slantededge-alpha1.tar.gz

Hope this one stays up! Anyway, I guess I'll make a release on SourceForge after I've fixed the really bad bugs.

Although the source maybe helpful, I'm running on a Windows XP platform. I don't mind looking into the project, but I have too little time to compile it myself (I know compilation itself doesn't take long, but I'd have to install a compiler first). A pre-compiled Win32 version might help.

I know. The main problem is that I'm using the FFTW library for taking Fourier transforms, and while there seems to be a Windows version available (several, actually), well... the site is down. The web doesn't like me apparently.

I'll see what I can do, perhaps I can just write a slow, simple DFT myself (I have no idea how difficult it is, I'll have to read a bit) as a compile alternative to FFTW.


Anyway, do you have a SourceForge account? With one, I can just add you to the project, and then you'll be able to access SF's shell servers. This way you could easily run - and even easily compile - the program remotely, without installing anything special on your computer.


Now some of the most important questions I have:

- What is Imatest's "10%" and "90%"? Initially, I took these as the minimum and maximum pixel values that can constitute the "edge". But it appears clear that the showed ESF also contains lower and higher values; besides, it always seem to go from -6 pixels to +8 pixels from the edge center. Is there a reason for this?
So, now I suppose "10%" and "90%" are simply used to compute (guess what) the 10%-90% rise.
Which in turns call for: should I normalize the image before doing anything else? I currently normalize so that 0.5% of pixels clip to black and 0.5% clip to white.

- Is it ok to take a (single) Fourier transform of the Hanning-windowed LSF? Without windowing, I get weird results, but with windowing, I'm afraid I'm affecting the data. My MTF's always look "smoother" than Imatest's and SFTWin's ones, and too high in the lower frequencies.

- How many samples should my ESF/LSF have? I understand that it only depends on how high you want your frequencies to be -- i.e., if I want to show the MTF up to 4xNyquist, I should have 4x more samples than there are real pixels. Is this correct?

- How do I reduce frequencies spacing in the DFT? If I just transform the LSF (or a Hanning'ed LSF), I get ridiculously low frequencies resolution. What I'm doing now to overcome this is... add a lot of zeroes at the end of the original LSF. But, somehow, I think this is kinda stupid.

- The method I'm currently using for getting a smooth, uniformely-spacing sampled ESF from the point I have is naive and very slow. The sources I've read suggest using "LOESS curve fitting" for this. I've had some trouble finding good references about this, and it seems very complicated anyway. The question is: is something simpler good enough?


by LjL
ljlbox@xxxxxxxxxx
.

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