From 0d6f329b1d9ecf3ba754a5affe80c669c56afc07 Mon Sep 17 00:00:00 2001
From: James R <justsomejames2@gmail.com>
Date: Mon, 2 Nov 2020 17:54:51 -0800
Subject: [PATCH] Do not sort sprite in front of plane if plane should render
 in front of sprite's plane

Say you have a higher plane in the foreground and a lower one behind. And then
insert a sprite above the plane in the background, the top of which is higher
than the height in the foreground. Should the sprite be drawn in front of the
foreground plane? I think not. Sprites drawing in front of a plane if only part
of them is above the plane is a rendering trick that allows sprites to extend
into the floor. This doesn't make sense if the plane they extend into would be
obscured anyway, or if they don't extend into the plane at all.
---
 src/r_things.c | 37 +++++++++++++++++++++++++++++++++++--
 src/r_things.h |  1 +
 2 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/src/r_things.c b/src/r_things.c
index 9702c7d3f..df1946ba6 100644
--- a/src/r_things.c
+++ b/src/r_things.c
@@ -1230,6 +1230,32 @@ fixed_t R_GetShadowZ(mobj_t *thing, pslope_t **shadowslope)
 	return groundz;
 }
 
+static void R_SetSpritePlaneHeights(vissprite_t *vis)
+{
+	ffloor_t *rover;
+
+	fixed_t top;
+	fixed_t bot;
+
+	vis->pt = vis->sector->floorheight;
+	vis->pb = vis->sector->ceilingheight;
+
+	for (rover = vis->sector->ffloors; rover; rover = rover->next)
+	{
+		if (rover->flags & FF_EXISTS)
+		{
+			top = P_GetFFloorTopZAt    (rover, vis->gx, vis->gy);
+			bot = P_GetFFloorBottomZAt (rover, vis->gx, vis->gy);
+
+			if (top <= vis->gzt && top > vis->pt)
+				vis->pt = top;
+
+			if (bot >= vis->gz && bot < vis->pb)
+				vis->pb = bot;
+		}
+	}
+}
+
 static void R_ProjectDropShadow(mobj_t *thing, vissprite_t *vis, fixed_t scale, fixed_t tx, fixed_t tz)
 {
 	vissprite_t *shadow;
@@ -1318,6 +1344,8 @@ static void R_ProjectDropShadow(mobj_t *thing, vissprite_t *vis, fixed_t scale,
 	shadow->xscale = FixedMul(xscale, shadowxscale); //SoM: 4/17/2000
 	shadow->scale = FixedMul(yscale, shadowyscale);
 	shadow->sector = vis->sector;
+	shadow->pt = vis->pt;
+	shadow->pb = vis->pb;
 	shadow->szt = (INT16)((centeryfrac - FixedMul(shadow->gzt - viewz, yscale))>>FRACBITS);
 	shadow->sz = (INT16)((centeryfrac - FixedMul(shadow->gz - viewz, yscale))>>FRACBITS);
 	shadow->cut = SC_ISSCALED|SC_SHADOW; //check this
@@ -1817,6 +1845,9 @@ static void R_ProjectSprite(mobj_t *thing)
 	vis->sector = thing->subsector->sector;
 	vis->szt = (INT16)((centeryfrac - FixedMul(vis->gzt - viewz, sortscale))>>FRACBITS);
 	vis->sz = (INT16)((centeryfrac - FixedMul(vis->gz - viewz, sortscale))>>FRACBITS);
+
+	R_SetSpritePlaneHeights(vis);
+
 	vis->cut = cut;
 	if (thing->subsector->sector->numlights)
 		vis->extra_colormap = *thing->subsector->sector->lightlist[light].extra_colormap;
@@ -2031,6 +2062,8 @@ static void R_ProjectPrecipitationSprite(precipmobj_t *thing)
 	vis->szt = (INT16)((centeryfrac - FixedMul(vis->gzt - viewz, yscale))>>FRACBITS);
 	vis->sz = (INT16)((centeryfrac - FixedMul(vis->gz - viewz, yscale))>>FRACBITS);
 
+	R_SetSpritePlaneHeights(vis);
+
 	iscale = FixedDiv(FRACUNIT, xscale);
 
 	vis->startfrac = 0;
@@ -2410,12 +2443,12 @@ static void R_CreateDrawNodes(maskcount_t* mask, drawnode_t* head, boolean temps
 				// bird: if any part of the sprite peeks in front the plane
 				if (planecameraz < viewz)
 				{
-					if (rover->gzt >= planeobjectz)
+					if (rover->pt >= planeobjectz && rover->gzt >= planeobjectz)
 						continue;
 				}
 				else if (planecameraz > viewz)
 				{
-					if (rover->gz <= planeobjectz)
+					if (rover->pb <= planeobjectz && rover->gz <= planeobjectz)
 						continue;
 				}
 
diff --git a/src/r_things.h b/src/r_things.h
index 400a63bcb..f95cd5c9e 100644
--- a/src/r_things.h
+++ b/src/r_things.h
@@ -171,6 +171,7 @@ typedef struct vissprite_s
 
 	// Precalculated top and bottom screen coords for the sprite.
 	sector_t *sector; // The sector containing the thing.
+	fixed_t pt, pb; // plane heights, also for sorting against 3D floors
 	INT16 sz, szt;
 
 	spritecut_e cut;