Re: [AD] polygon_cull_criterion()

[Jason Wilkins <fenix@xxxxxxxxxx>]

On Sat, 1 Apr 2000, Christian Schüler wrote:

> Allegro has a function called polygon_z_normal() in "math3d.c". This
> function alone is  completely ok, but the docs say you can use it for
> backface culling. Sorry, this is downright wrong. You could only use
> polygon_z_normal() for backface culling if one of these conditions are met:
> 
>    a) you are in homogenous coordinates
>    b) you have an orthogonal projection matrix

polygon_z_normal is for use after you have projected all your polygons
onto the plane z = 0.  I do not understand why you say it is 'downright
wrong'.  The docs may not be clear on when you can use this function, as
it certainly does not work if you give it unprojected polygons, but it
isn't 'downright wrong' because the function certainly can be used for
backface culling.

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