Calculate pixel angle from the center having hFov and vFov

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Once that I calibrated my camera and I know horizontal and vertical fields of view (through calibrationMatrixValues), is it possible to know the X and Y angles (from the center) of a particular pixel?

Suppose I have a camera with fields of view of 100° (h) and 80° (v) and a resolution of 500x400. The angle/pixel ratio is (0.2°, 0.2°). So the central pixel will be (0°, 0°), its left neighbor (-0.2°, 0), the topmost central pixel (0°, +80°) and so on.

Is actually this relation constant through all the image or there is a formula to perform this calculation? Is the obtained information reliable?

This is going to be the first step for triangulating objects in a multi camera environment.