Equirectangular projection/IMG2EQUI

Symbol	Meaning	Typical Range
`lon` $\theta _{c}$	Longitude of the view centre, will refer to the vertical $x$ -coordinate the centre pixel of the output image	$-180^{o}\leq \theta _{c}\leq +180^{o}$
`lat` $\varphi _{c}$	Latitude of the view centre, will refer to the vertical $y$ -coordinate of centre pixel of the output image	$-90^{o}\leq \varphi _{c}\leq +90^{o}$
$r\in \mathbb {R} ^{+}$	Radius of the virtual sphere (any positive number )	$r={\frac {width}{2\pi }}$
`HFOV` $\theta _{max}$	Horizontal Field‑Of‑View (HFOV) camera property of the output image (degrees)	$30^{o}\leq \theta _{max}\leq 120^{o}$
`VFOV` $\varphi _{max}$	Vertical Field‑Of‑View (VFOV) camera property of the output image (degrees)	$30^{o}\leq \varphi _{max}\leq 120^{o}$
`W, H` Width, Height	Width & height of the output image usually defined by the display sizes the output is produced for \| width:height = 9:6
`equirectImg`	Source equirectangular image (width `Ew`, height `Eh`) in general the ratio between `Ew:Eh=2:1`. The ratio is reasonable, because the range for $\theta$ has a span of $360^{o}$ degrees and the range of $\varphi$ is $180^{o}$ .	`Ew = 2096` `Eh = 1048`