Question about the getProjectionMatrix2 function

[Irving87]

Hello, can i ask some question about the getProjectionMatrix2 function in graphics_utils.py?

1. why the "left right top bottom" calculate in this way?

left = ((2 * cx - W) / W - 1.0) * W / 2.0
right = ((2 * cx - W) / W + 1.0) * W / 2.0
top = ((2 * cy - H) / H + 1.0) * H / 2.0
bottom = ((2 * cy - H) / H - 1.0) * H / 2.0

2. The P[0,2] and P[1,2] in the code are:

P[0, 2] = (right + left) / (right - left)
P[1, 2] = (top + bottom) / (top - bottom)

and z_sign = 1.0, so why don't they calculate like this:

P[0, 2] = -(right + left) / (right - left)
P[1, 2] = -(top + bottom) / (top - bottom)

or can you tell me how the camera coordiante looks like?

Any help would be much appreciated!