Mike Williams <mik### [at] nospamplease> wrote in 
news:3BA### [at] econymdemoncouk:

> Thorsten says "Not properly confirmed.  Just being able to reproduce the
> exact behavior is not enough.  At least a tiny bit of experimentation
> should be done to be able to say it is a bug and not a precision
> problem."

I'm wondering just what kind of precision he has in mind if blatant 
lighting errors as a result of using rotate x*90 (that's with zero decimal 
places) can be dismissed as a precision problem.  This kind of reasoning is 
getting close to defining -1^(1/2)=3 as correct.

Working with the example scene, it's plainly obvious that the lighting 
calculations break down on the beveled portions of the prism when the prism 
is transformed to face the camera.  This problem depends on the 
orthographic camera and on the prism being transformed to face the camera 
head-on by using either rotate x*90 or a matrix.

The prism is lit correctly if it is rotated by 90.01 or 89.99 degrees (if 
using rotate), if  a shear component is introduced into the second row of 
the matrix (if using a matrix transform), or if the perspective camera is 
used.

Simplified scene, with matrix transform:

light_source {<10, 10, 50> rgb 1}
camera {orthographic location z*4 look_at 0 right x*3 up y*3}

prism
{                                
    conic_sweep
    linear_spline
    1, .5, 5, <-1, -1>, < 1, -1>, <1, 1>, <-1,  1>, <-1,-1>    

    matrix
    <
        1,0,0,
        0,0,-1,
        0,1,0,
        0,0,0
    >

//    rotate  x*-90

    pigment {rgb 2}
}

background {rgb 0.25}

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