Mathew

Hello Everybody...

sorry for the late response... I was having tropuble in accesing the site..

I think i need the method which jdunham mentioned...

But could you both please explain it in a VI. It would be much clearer then..

thanksss

QUOTE (jdunham @ Mar 14 2009, 12:09 AM)

I think Justin's approach is pretty good, and might be the fastest to implement. If you are looking for more work and higher accuracy, you could try a two step approach, creating a curve fit along each polar axis.

For each radial line and each circle, use a curve fit to create a function that you can interpolate along any point (this is all available in the LabVIEW analysis libraries). You could fit the data to a polynomial or a spline, whichever seems more appropriate for your dataset. For the circles, don't forget to fit the same end point in both directions so that your fit doesn't have a terrible discontinuity where the angle rolls over.

Then you would take each point in the rectangular array, and express it in polar coordinates. Then evaluate those points in your nearest fits and combine them.

So for point xi, yi, compute [r(xi,yi), theta(xi,yi)], then find your nearest fits r(a), and r(a+1) and evaluate them at theta(xi,yi). Similarly, find your nearest fits theta(b) and theta(b+1), and evaluate them at r(xi,yi). Then take those four points and combine them either with a simple average or a weighted distance average like Justin showed.

Its a part of my prj wrk...any idea? I have to mapp the points in polar to rectangular.

Suppose we take the red point on the polar grid. When i convert it to Re & Im parts i get a number in rectangular grid, but this point is not actually present in the rectangular grid shown on right. This polar point was found by interpolating some points. How can i find a point in rectangulr grid then?

thanks

Hello

I am having an image. I converted this image to 2D array. Now i do the polar conversion in this array.Suppose on a polar grid i need values along 6 angles(60,120,180,240,300,360 degrees). I am having a 2D array in Rectangular grid. So i nterpolate values in 2D array and found the required values in polar grid. With these values i perfomed FFT and got a plot. Now i take the peak values from this FFT array and i need to go back in the reverse direction so that i may see as real picture what these peak values look like.

So as a first step i do the inverse FFT for these values. What would bethe next step.? How can i get those old points in 2D array from this interpolated values in polar grid?

Thanksss

See attachmenet to understand the rectangular polar grid system

Hello ..

Actually i tried with that.. but that is not the one which i need.I need to interpolate them in polar raster.

I have an NXN image. I made the 2D Fast Fourier Transform of this one. Now i need to binearly interpolate the Fourier complex spectrum in a polar raster. How is this possible.?

Thanks

Hello N

I have found 2D FFT for an image. Now i need to compute the Central Slices for a particular value of (r,theta). This is what i need and not direct complex to polar conversion.

thankss

QUOTE (Neville D @ Nov 12 2008, 06:19 PM)

If you have Cartesian coords, you can transform to polar using Complex to Polar function from the following pallet:

Programming>Numeric>Complex

(Its polymorphic).

N.

DO anyone know how to apply 2D Central slice theorem for 2D Fourier Transformed Image? FOr this forst we need to mapp the samples of FT in Cartesian coordinate system to Polar.

Can anyone help with this?

Thanks

Hello

I have an image of size M X N (pixel).

Fisrt of all i have read the image from the path location. Then I used Unflatten Pixmap VI to convert the image to array. Now i need to find the 2 D FFT. I have to do the FFT by fist computing the transforms of the rows in parallel and then the transforms of the columns of the result. For that i extracted the N Resolution value and gave it to ta for loop ( since there are N rows for the image.) Then I calculated the FFT for these rows. The resulting array is transposed to do the FFT for the columns. Is the VI i made correct? Please see the attchement for the image.

How can i get the 2D- FFT plot ?

Thankss

QUOTE (OlivierL @ Nov 3 2008, 04:38 PM)

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Hello Olivier

Thank you for the example. I tried to do the work replacing the input array by an pixel array from an array. But towards the end part of your VI , i didnt understand the use of (x+2y) ? What is the use of that one?

thnksss

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x+y 2 is just the default name LabVIEW gave to the result of the addition. There is nothing special there other than adding and dividing by two (since you are doing your operation on two copies of the same array, only in different orientation, row and column). This means that you need to do your interpolation on both "branches" of the code if you want it to be done in both dimensions and then merge those results together. I hopw this is a bit more clear now.

In my example, X and Transposed array are the individual results of your treatment in both dimensions, prior to being joined back together. Both treatment could potentially be done sequentially instead of in parallel and the results in one of the dimensions would be more accurate! *** Actually, as I realize it, this might be a better solution for interpolation, to go in one dimension first, then transpose and treat the second dimension. It will definitely save you time and trouble because your array sizes won't match in parallel. ***

Cheers,

Olivier

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Hello Olivier

Thanks. BUt i have one more problem now. I am now trying to do the 2D - FFT in another approach. Firstly I need to compute the FFT for rows (in parallel) and then the FFt transforms for the columns of the result. Then i have to map the samples of the final FFT from cartesian coordinate system to polar ones. How could this be implemented?

Thanks

QUOTE (OlivierL @ Oct 30 2008, 05:00 PM)

Hi Mathew,

Check the code below to have an idea of how to do it. It's not too complicated. Don't be surprised with the "weird" numbers at the end, they round up to the exact initial values.

Cheers,

Olivier

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Hello Olivier

Thank you for the example. I tried to do the work replacing the input array by an pixel array from an array. But towards the end part of your VI , i didnt understand the use of (x+2y) ? What is the use of that one?

thnksss

QUOTE (osvaldo @ Oct 30 2008, 02:45 PM)

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Hello Osvaldo

I cannot find anything on the link you gave.. i think its not working anymore...

Thankss

I am having an image of , say M X N Pixel size. At first I have to do a 2 D Fast Fourier Trnasformation of this image. Then an Interpolation and finally back an Inversve Fourier Transformation.

I have first tried to read the image from a folder path. Changed it to pixelmap and extracted the X & Y Resolution of the image. Now how can i do the rest. Is the X & Y resolution directly be given to the 2 D FFT function? COuld you please help me with this?

(I would prefer not to use any IMAQ functions.)

Thanks