I think I have seen the black horizontal lines merely move from the top to the bottom of the sprite with this approach. Will have to check again when I'm at home.

Indeed, with this fix I get a black line at the bottom of the sprite:

woofsav1.zip

Indeed, with this fix I get a black line at the bottom of the sprite:

I don't have that at any resolution:

I've had this happen before when I changed it in the wrong place. Are you sure you don't have any remnants of the old changes? Also what is your resolution?

Are you sure you don't have any remnants of the old changes?

Nope, this is the only change:

$ git diff
diff --git a/src/r_things.c b/src/r_things.c
index c998b20..6a64ff4 100644
--- a/src/r_things.c
+++ b/src/r_things.c
@@ -441,7 +441,7 @@ void R_DrawVisSprite(vissprite_t *vis, int x1, int x2)
       else
         colfunc = R_DrawColumn;         // killough 3/14/98, 4/11/98
 
-  dc_iscale = abs(vis->xiscale);
+  dc_iscale = abs(vis->xiscale) + 1;
   dc_texturemid = vis->texturemid;
   frac = vis->startfrac;
   spryscale = vis->scale;

Also what is your resolution?

(gdb) p video
$1 = {
  width = 1278,
  height = 600,
  pitch = 1920,
  unscaledw = 426,
  deltaw = 53,
  xscale = 196608,
  yscale = 196608,
  xstep = 21846,
  ystep = 21846,
  fov = 1267733622
}

Interestingly, the line vanishes if I switch off widescreen rendering:

(gdb) p video
$2 = {
  width = 960,
  height = 600,
  pitch = 1920,
  unscaledw = 320,
  deltaw = 0,
  xscale = 196608,
  yscale = 196608,
  xstep = 21846,
  ystep = 21846,
  fov = 1073741824
}

No wait, still there:

This is probably what Killough meant with this comment:

If I'm not mistaking, "sparkles" was meant to be a transparent pixels at the junction of two-sided midtextures with floors/ceilings. These ones:

Interesting that #1400 affect this issue (it seems to be improved). I believe the problem is in centery/ projection and related calculations. Eternity uses float there: https://github.com/team-eternity/eternity/blob/451a67e29cf32658a5bb9d731eb7e21eb5769ff7/source/r_draw.cpp#L133 (view.ycenter is float).

Interesting that #1400 affect this issue (it seems to be improved).

Doesn't really change much for me.

Interesting that #1400 affect this issue (it seems to be improved).

Doesn't really change much for me.

Right, for some reason it's harder for me to catch this. For example, your save doesn't show any black lines, only after some turning.

This approach doesn't work. I tested this WAD in Eternity and it too has black lines on this sprite. I think the problem is rare, so we can postpone it.