[PATCH 2/2] gdiplus: Revise closeness check for bezier curve flattening.

[Fabian Maurer]

More of a general question towards the wine devs:
According to gcc we have a lot of such float comparisons in the code. Isn't
that a potential general problem?

Regards,
Fabian Maurer

On Freitag, 4. Februar 2022 16:58:48 CET Robert Feuerbach wrote:
> The float equality and flatness calculation in flatten_bezier
> can fail due to the limited precision of the float math.
> The equality test was replaced with a simple check against
> the given flatness tolerance.
>
> Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=52492
> Signed-off-by: Robert Feuerbach <rjfeuerbach at gmail.com>
> ---
>  dlls/gdiplus/graphicspath.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/dlls/gdiplus/graphicspath.c b/dlls/gdiplus/graphicspath.c
> index ce2666eedab..c1bdf226e52 100644
> --- a/dlls/gdiplus/graphicspath.c
> +++ b/dlls/gdiplus/graphicspath.c
> @@ -109,7 +109,7 @@ static INT path_list_count(path_list_node_t *node)
>   *  - (x2, y2): first control point;
>   *  - (x3, y3): second control point;
>   *  - end     : pointer to end point node
> - *  - flatness: admissible error of linear approximation.
> + *  - flatness: admissible error of linear approximation in coordinate
> units. *
>   * Return value:
>   *  TRUE : success
> @@ -142,12 +142,13 @@ static BOOL flatten_bezier(path_list_node_t *start,
> REAL x2, REAL y2, REAL x3, R mp[2].X = (mp[1].X + mp[3].X) / 2.0;
>      mp[2].Y = (mp[1].Y + mp[3].Y) / 2.0;
>
> -    if ((x2 == mp[0].X && y2 == mp[0].Y && x3 == mp[1].X && y3 == mp[1].Y)
> || -        (x2 == mp[3].X && y2 == mp[3].Y && x3 == mp[4].X && y3 ==
> mp[4].Y)) -        return TRUE;
> -
>      pt = end->pt;
>      pt_st = start->pt;
> +    /* test for closely spaced points to avoid limited-precision errors in
> flatness check */ +    if((fabs(pt.X - mp[2].X) + fabs(pt.Y - mp[2].Y) +
> +        fabs(pt_st.X - mp[2].X) + fabs(pt_st.Y - mp[2].Y) ) <= flatness)
> +        return TRUE;
> +
>      /* check flatness as a half of distance between middle point and a
> linearized path */ if(fabs(((pt.Y - pt_st.Y)*mp[2].X + (pt_st.X -
> pt.X)*mp[2].Y + (pt_st.Y*pt.X - pt_st.X*pt.Y))) <=