Re: [RFC] Gradient fill options

[Pierre <pie...@gm...>]

On 2/7/07, Ethan Merritt <merritt@u.washington.edu> wrote:
> On Wednesday 07 February 2007 11:44, Hans-Bernhard Br=F6ker wrote:
> > Pierre wrote:
> >
> > > I'm not sure to understand why it is biased. Each vertex color is
> > > known. The first vertex is the top left one, after they have been
> > > sorted.
> >
> > The bias lies in picking the top left vertex as "the first".
> > This can cause nasty effects like: the same triangle comes out subtly
> > different if the scene is rendered upside down or rotated by 90 degress=
,
> > even though the result should be exactly the same.
>
> That is true, but the failure mode is usually harmless. If you rotate the
> triangle in 3D its coloring may change very slightly, but if it is a
> single facet of a larger surface this will probably not be noticed since
> the adjacent triangles are similarly affected.

I will add a custom version to test these cases using numerical
results (output the edges values to a file and compare them). We can
then see if the error is acceptable. As you say, I do not expect any
noticeable differences.

> > the same issue can cause edges to be drawn differently if you switch th=
e
> > order of their vertices in the drawline() function call.
>
> This is the more important issue, as I tried to point out before.
> In rendering a tesselated surface, it is common that the same edge
> will be traversed in different directions in the course of rendering
> the two facets that it separates.  In order to assure smooth coloring
> across the break, it is crucial that the direction of traversal not
> affect the color assignment.

The "psychedelic example shows a case where the same edge is shared
between different triangles. I did not see any artifacts.

It was expected as the slopes and delta are not affected by the
position of the vertex, it is alway V(n) - V(n-1), where n is the
first or not should not change the constant slopes and delta. If yes,
we will see it with the numerical tests (hopefully :).

--Pierre