Good day Fadhli!
Please be aware of the fact that orientation analyses are by no means a simple affair. To get an impression of important but by no means of all aspects, you may study this report about professional approaches to global/regional orientation salience. Make sure you understand why one should speak of orientation salience analysis and not orientation analysis.
Concerning your sample image, I should like to make some remarks:
- The image is index-colored which is suboptimum.
As far as I can see it doesn’t contain any color information, i.e. you should actually analyze the original gray-value image.
- A disc-shaped window is a good start but even better are window functions that show a soft slope, like those discussed here.
- The window isn’t applied in a way that the outside is zero.
As a consequence and if the whole image is analyzed, you get a large DC-component in the orientation-result.
The same holds true for the windowed part. Therefore, it is best to use DC-removal as discussed here.
- It appears as if the image is insufficiently bandlimited and it must even be be suspected that it is incorrectly sampled (digitized).
Please make sure that the bandlimit of the imaging optics accords with the Nyquist-frequency (half the sampling frequency) according to the Shannon-theorem.
- An aspect of orientation analysis that is not mentioned in the above cited report is the influence of low-frequency structures on the result. Most often they are caused by uneven illumination, etc. They can strongly influence the result although they are in fact irrelevant for many investigations. Carefully chosen highpass-filtering may help in such cases.
Concerning your image, such kind of influence isn’t very pronounced.
Here are three “Orientation Salience Functions” (OSF) computed from your sample image after conversion to 32bit gray-value:
- DC-removal only:
- Tukey-windowing (80% flat):
- Tukey-windowing (80% flat) plus highpass-filtering (limit @ 2% Nyquist):
Please note that the orientation angle starts from horizontal (0deg) and increases counter-clockwise.
You may also have a look at a more recent and related thread.