How to get specific point and draw Rectangle on QImage?

[stampede]

Can you explain what you mean by "i would like to box the part i needed ask the pic shown," ?
If I'm correct, it looks like you want to draw a bounding box around the contours with the largest area, but I can only guess. Have you verified your sorting procedure ?

[YDYD]

Can you explain what you mean by "i would like to box the part i needed ask the pic shown," ?
If I'm correct, it looks like you want to draw a bounding box around the contours with the largest area, but I can only guess. Have you verified your sorting procedure ?

Yes，exactly [want to draw a bounding box around the contours with the largest area].

What do you mean by verified my sorting?
Do you mean the sorting of all the function?

[stampede]

What do you mean by verified my sorting?

It can't be more clear - have you checked that the list is sorted after the sorting code