I wrote:
The next stage is to detect bending and to measure the degree of bending.For this purpose, first, a region in the image that includes the top edge is identified. Then, a curve is fitted through the edge pixels in the region.
In the sentence above where should I put first? Did I use commas correctly?
should I say ...top edge is first identified?
I myself prefer to use first, or then in the beginning of sentence to show the order. Is it a good practice?
It's not incorrect, but it's also not stylistically ideal... As it currently reads, it feels like you're starting your sentence twice.
If you want to keep all of the clauses intact, you could keep "first" where it is, or you might also put it before or after "identified":
...a region in the image that includes the top edge is first identified.
OR
...a region in the image that includes the top edge is identified first.
As a matter of personal preference, though, I would take out the "For this purpose" and just leave:
The next stage is to detect bending and to measure the degree of bending. First, a region in the image that includes the top edge is identified.
