how to detect the region of scales

I am detecting the position of a needle and scale markers in an analog gauge like the below. First, I transformed the gauge image into the rectangular image by polar coordinates. In the rectangular image, how can I detect the scale region marked by yellow color? Thank you.

(Analog gauge)

(Rectangular image)

(Target image)

Answer 1

[Image Analyst]

I don't see how you got the middle image. Somehow you transformed a circular hygrometer gauge with a scale of 0-100 into a rectangular thermometer gauge with range -30 to +50. I don't know how to do that. But anyway, once you've done that, you can just get the tick marks by looking at the scale, which should be in a fixed location. So

scaleImage = rectangleImage(row1:row2, col1:col2);

Once you've done one transform, you'll know what row1, row2, col1, and col2 are, of course.

[YoungTae Kwak]

Thank you for your answering. Our gauge images can vary for the input, so the position of tick markers can be different on the input gauge images. We just want to know the range of repeated tick markers. Good

[Image Analyst]

Then just find the center of the gauge and crop the image. Then scale it to a known size, like 1024x1024. Then do the spatial transform and the scale will be in the same place, guaranteed.