about generate anchor

[Jackbrocp]

trafficstars

when i see generate_anchors process,I found that the axis of ancher for example:-100 -100 120 120. so why the axis of ancher is negetive integer???

[chrisway613]

Here is an example to illustrate this problem: Consider the 1st point (0, 0) in feature map, due to the feat stride is 16, the center point of 16x16 region in the input image is (8, 8). Therefore, we map (0, 0) to (8, 8) in the input image. Next, when scale is 8, the area of anchor will be (16x8) x (16x8), namely 128x128, if ratio is 1, then h=w=128. But the center point is (8, 8), so top left point is (8 - 128/2, 8 - 128/2) = (-56, -56), and bottom right point is (8 + 128/2, 8 + 128/2) = (72, 72). Finally, we get negative coordinates of some anchors.