[cl-gd-devel] with-transformation off-by-one bug?

[Edi Weitz]

On Thu, 13 Sep 2007 08:29:56 -0500, "Michael J. Forster" <mike at sharedlogic.ca> wrote:

> I was thinking of lines on a graph as zero-based vectors.  Thus, a
> vector of length (or axis of width or height) n is indexed from 0 to
> n-1.  And, indeed, with the following...
>
> (let ((height 100)
>        (width 100)
>        (x-axis-width (list width 1))
>        (y-axis-height (list 1 height))
>        (x-axis-width-1 (list (1- width) 1))
>        (y-axis-height-1 (list 1 (1- height))))
>    (cl-gd:with-image* (width height)
>      (set-pixels x-axis-width)
>      (set-pixels y-axis-height)
>      (set-pixels x-axis-width-1)
>      (set-pixels y-axis-height-1)
>      ...))
>
> ... only the last two pixels are displayed on my 100x100 pixel
> image.  The axes are indexed from 0 to n-1.  If WIDTH and HEIGHT are
> 1, I only expect to see the pixel generated by...
>
> (set-pixels '(0 0))

I think where CL-GD and you disagree is that currently the outermost
"virtual" point (X2 or (+ X1 WIDTH)) is mapped to the first point
which is /not/ on the real image (and similar for the other axis),
i.e. the transformation arguments kind of describe a half-open
interval.  I can see how this is inconvenient and I wouldn't mind
changing that unless someone else provides important reasons not to do
it.  Could you please check if the attached patch does what you want?

As for your complaints about the x-axis transformation not being
consistent with the y-axis transformation, are you sure you're not
seeing rounding errors?  Or maybe this happened because of the "one
off" stuff plus the fact that the "virtual" y-axis usually points into
the opposite direction?

Cheers,
Edi.

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