Frequently Asked Questions

• Q: The program asks me for a password .
• A: Yeah I know. Sorry, but that bit needs to be in there. To obtain one, please contact Dr. Nikos Papamarkos .

• Q: I started the program, I want to detect some circles! What must I do first?
• A: Load an image from the File menu, or from the big "Load" key with the floppy.

• Q: I did that! What must I do next?
• A: You must proceed to edge detection of your image. Choose a mask from the "Edge Detection" menu and hit the "Detect Edges" button.

• Q: Alright, I got a weird white and black thing! What i do now?
• A: These are the edges of your image. You can keep it "as is" for the next step, or (recommendded) convert it to a two-level binary image by use of the Otsu method, or by setting the threshold yourself.

• Q: My image is simple enough, can't I skip these first parts?
• A: If your image is a simple sketch with clear shapes, or is already an edge detection image, you may wish to use the "Consider as Binary" or "Consider as Edges" buttons, which will take you directly to the respective steps without altering your image. If your image is reasonably "binary", make sure that it's WHITE ON BLACK before applying CHT, GHT or EHT. Use the Negative key for that.

• Q: I ruined my original or some of the intermediate images by applying filters etc. Can I restore them?
• A: The edge detection images come from the original one, so you can repeat the edge detection if you aren't satisfied. Similarly, the binary images can come from the original or from the edge detection image. You can reload the original image with the green recycle button :-)

• Q: I detected some circles, changed the tolerance, and it didn't detect them all the circles again!
• A: Yeah, I know. This is because the detection procedure is destructive for the GHT/CHT/EHT spectrum. If you wish to change the tolerance after a detection, you should go back to the GHT tab and repeat the calculations. Keeping a "spare" spectrum would be even more cruel on the memory, sorry :-(

• Q: Why can't the program detect ALL circles in the image??
• A: OK here goes. Reasons for this may include:
  1. Is the diameter you're looking for within the Minimum Diameter and Maximum Diameter settings?
  2. Is your tolerance value (%) low enough? 70% should be optimal, values over 80 are overkill with CHT and GHT. EHT on the other hand performs well with values > 85%. For images with a lot of noise, you may have to be "forgiving" and lower this setting.
  3. If using EHT, are the Third Point Max.Distance and EHT Radius Tolerance parameters correctly set?
  4. Is your inital image too noisy or too poor ? Consider enhancing it before detection.
  5. Have tried to "feed" CHT or GHT with a too crisp image? Use an edge detection image or turn on fuzziness settings from the "Tricks" menu. CHT and GHT love fuzzy images. ;-)
  6. On the converse, have you tried to "feed" a too fuzzy edge detection image to EHT? EHT prefers crisp images :-)

• Q: I get too many false detections.
• A: With CHT/GHT, it might not be a good idea to search for diameters below 10. Try avoiding small diameters, use EHT instead or increase the tolerance to e.g. 80 or 85%. Also, avoid using images which gave big "globs" or "concentrations" of white pixels. Delete those manually, or use edge detection to remove them.

• Q: Why can't the program just detect ALL circles in an image without all this mumbo-jumbo?
• A: Because "all circles" means:
1. A lot of diameters to check for, up to absurd numbers e.g. 500 or 600.
2. An AWFUL lot of memory and processing power.

Also, many of the methods used in this program have to work with precise costraints and predefined tolerances or suffer from more subtle limitations, and cannot be made so generic without significant effort and resources. They can surely be improved, but they will never be absolutely limit-free, settings-free and error-free.

• Q: OK...what's the closest one can get, then?
• A: Select EHT, X-Step and Y-Step to be 1:2 or 2:1, Enable "Fine EHT" and leave R-Step at 1, 2 or 3. This can take a loooong time though. :-(

• Q: OMG, The EHT misses some pretty obvious circles in an image, and they are WITHIN range !1!!
• A: This is probably due to "vote stealing", e.g., smaller diameters get priority when searching for the second and third point in an image, and lead in turn to repeated or lost votes. To contrast that you can either:
  • Restrict the difference between minimum and maximum diameter. Reading this will help you understand what's good and what's not.
  • Increase the "Third Point Max Distance" (but only up to a certain point).
  • Enable the "Fine EHT" option, and set R-Step high enough so that it skips the smaller radii fast. (e.g. with a Min.Diameter of 10 and a Max.Diameter of 90, you can set R-Step to 10, so that it skips the smallest radii (10-20) and focuses on the biggest ones, in successive passes. This works, but costs extra computational time.

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