Hack that fixes software drop shadow crashes by bypassing incorrectly set variables

src/r_draw.h

@@ -170,6 +170,7 @@ void R_DrawViewBorder(void);
 void R_DrawColumn_8(void);
 void R_DrawShadeColumn_8(void);
 void R_DrawTranslucentColumn_8(void);
+void R_DrawDropShadowColumn_8(void);
 void R_DrawTranslatedColumn_8(void);
 void R_DrawTranslatedTranslucentColumn_8(void);
 void R_Draw2sMultiPatchColumn_8(void);

src/r_draw8.c

@@ -416,6 +416,39 @@ void R_DrawTranslucentColumn_8(void)
 	}
 }
 
+// Hack: A cut-down copy of R_DrawTranslucentColumn_8 that does not read texture
+// data since something about calculating the texture reading address for drop shadows is broken.
+// dc_texturemid and dc_iscale get wrong values for drop shadows, however those are not strictly
+// needed for the current design of the shadows, so this function bypasses the issue
+// by not using those variables at all.
+void R_DrawDropShadowColumn_8(void)
+{
+	register INT32 count;
+	register UINT8 *dest;
+
+	count = dc_yh - dc_yl + 1;
+
+	if (count <= 0) // Zero length, column does not exceed a pixel.
+		return;
+
+	dest = &topleft[dc_yl*vid.width + dc_x];
+
+	{
+#define DSCOLOR 31 // palette index for the color of the shadow
+		register const UINT8 *transmap_offset = dc_transmap + (dc_colormap[DSCOLOR] << 8);
+#undef DSCOLOR
+		while ((count -= 2) >= 0)
+		{
+			*dest = *(transmap_offset + (*dest));
+			dest += vid.width;
+			*dest = *(transmap_offset + (*dest));
+			dest += vid.width;
+		}
+		if (count & 1)
+			*dest = *(transmap_offset + (*dest));
+	}
+}
+
 /**	\brief The R_DrawTranslatedTranslucentColumn_8 function
 	Spiffy function. Not only does it colormap a sprite, but does translucency as well.
 	Uber-kudos to Cyan Helkaraxe

src/r_things.c

@@ -837,6 +837,12 @@ static void R_DrawVisSprite(vissprite_t *vis)
 	else if (vis->mobj->sprite == SPR_PLAY) // Looks like a player, but doesn't have a color? Get rid of green sonic syndrome.
 		colfunc = colfuncs[COLDRAWFUNC_TRANS];
 
+	// Hack: Use a special column function for drop shadows that bypasses
+	// invalid memory access crashes caused by R_ProjectDropShadow putting wrong values
+	// in dc_texturemid and dc_iscale when the shadow is sloped.
+	if (vis->cut & SC_SHADOW)
+		colfunc = R_DrawDropShadowColumn_8;
+
 	if (vis->extra_colormap && !(vis->renderflags & RF_NOCOLORMAPS))
 	{
 		if (!dc_colormap)