"Kima" <nomail@nomail> wrote:

> To make the equation work, I must omit 0.5, and I don't know where it comes
> from.
>
> Simple example:
>
> Consider the angle of view of 90, then, direction_length = right_length. Since
> tang(45) = 1, 0.5 is superfulous.
>
> I can think of 0.5 necessary if only using full_length
> (left_lenth+right_lenght). Or amd I making another stupid mistake?

I feel your pain.   I've been through this before, and I found the answer, but
it was too long ago for me to remember where the description / diagram /
explanation lies.

_Somewhere_ in the documentation, and probably in a forum thread or two lies a
discussion of how the camera angle is DEFINED.

IIRC, it isn't what you think it might be, but half of that.

My advice to you - for a lot of things - will be to pretend like POV-Ray and the
documentation don't exist, and look up how "standard computer graphics" stuff
works.   In this case, the view frustum and projection matrices.
The diagrams and explanations and equations will be a lot more extensive,
varied, and often clearer.   A lot of it will discussing OpenGL and other
software, but that's irrelevant, since it's all using the same fundamentals.


https://www.scratchapixel.com/lessons/3d-basic-rendering/perspective-and-orthographic-projection-matrix/projection-matr
ices-what-you-need-to-know-first?url=3d-basic-rendering/perspective-and-orthographic-projection-matrix/projection-matri
ces-what-you-need-to-know-first

Take a look at that first diagram, where FOV=90.   It only intuitively
illustrates what I'm trying to recall, but I hope it will establish that it's
not a figment of my imagination or memory.

Perhaps one of the developers can chime in with details or a link.

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