confusion regarding map output feature to the location of original image

[fishman2008]

We caculate Correspondence by this fomula:
x1 = x1_map * 2 + 1,
y1 = y1_map * 2 + 1,
x2 = x1_map * 2 + 1 + 12,
y2 = y2_map * 2 + 1 + 12.
First, how do you derive the equations here? Second, the axis of Red box in original image is (1, 1, 13, 13), which Correspond to (0, 0) in feature map. How about (0,0,12,12)? That is the first convolutional region, which should map to (0,0) in the output feature map? Could you clarify these? Thanks.

[BrightXiaoHan]

I think it's my fault. You can remove the "+1". But it's ok if calculate the same formulation in both training and evaluating process. And also the 1 pixel error can be fixed by bounding box regression process.