fix SKYDEFS scroll speed

[fabiangreffrath]

Fixes #2107

[fabiangreffrath]

I tried this approach for interpolation but couldn't spot a difference between capped and uncapped rendering, probably because we are already at sub-pixel-per-second precision. What do you say @rfomin ?

diff --git a/src/r_plane.c b/src/r_plane.c
index 914a8e71..068bd894 100644
--- a/src/r_plane.c
+++ b/src/r_plane.c
@@ -39,6 +39,7 @@
 #include "doomstat.h"
 #include "doomtype.h"
 #include "i_system.h"
+#include "i_video.h"
 #include "m_fixed.h"
 #include "r_bmaps.h" // [crispy] R_BrightmapForTexName()
 #include "r_data.h"
@@ -415,7 +416,17 @@ static void DrawSkyTex(visplane_t *pl, skytex_t *skytex)
 
     dc_texturemid += skytex->curry;
 
-    angle_t an = viewangle + (skytex->currx << (ANGLETOSKYSHIFT - FRACBITS));
+    fixed_t deltax;
+    if (uncapped)
+    {
+        deltax = LerpFixed(skytex->prevx, skytex->currx);
+    }
+    else
+    {
+        deltax = skytex->currx;
+    }
+
+    angle_t an = viewangle + (deltax << (ANGLETOSKYSHIFT - FRACBITS));
 
     for (int x = pl->minx; x <= pl->maxx; x++)
     {
diff --git a/src/r_sky.c b/src/r_sky.c
index fc5a7148..908bbc04 100644
--- a/src/r_sky.c
+++ b/src/r_sky.c
@@ -176,12 +176,16 @@ void R_UpdateSky(void)
     }
 
     skytex_t *background = &sky->skytex;
+    background->prevx = background->currx;
+    background->prevy = background->curry;
     background->currx += background->scrollx;
     background->curry += background->scrolly;
 
     if (sky->type == SkyType_WithForeground)
     {
         skytex_t *foreground = &sky->foreground;
+        foreground->prevx = foreground->currx;
+        foreground->prevy = foreground->curry;
         foreground->currx += foreground->scrollx;
         foreground->curry += foreground->scrolly;
     }
diff --git a/src/r_skydefs.h b/src/r_skydefs.h
index 0f20b3cf..c4d54810 100644
--- a/src/r_skydefs.h
+++ b/src/r_skydefs.h
@@ -37,8 +37,10 @@ typedef struct
     double mid;
     fixed_t scrollx;
     fixed_t currx;
+    fixed_t prevx;
     fixed_t scrolly;
     fixed_t curry;
+    fixed_t prevy;
     fixed_t scalex;
     fixed_t scaley;
 } skytex_t;

[rfomin]

I tried this approach for interpolation but couldn't spot a difference between capped and uncapped rendering, probably because we are already at sub-pixel-per-second precision. What do you say @rfomin ?

Interpolation works, try to reduce the game speed to notice it. I'm using this test WAD: sky.zip

Looks good!

[rfomin]

How about scrolly interpolation? Also we need to add check for pause/menu:

diff --git a/src/r_plane.c b/src/r_plane.c
index 068bd894..bc919251 100644
--- a/src/r_plane.c
+++ b/src/r_plane.c
@@ -414,18 +414,20 @@ static void DrawSkyTex(visplane_t *pl, skytex_t *skytex)
     dc_texheight = textureheight[texture] >> FRACBITS;
     dc_iscale = FixedMul(skyiscale, skytex->scaley);
 
-    dc_texturemid += skytex->curry;
-
-    fixed_t deltax;
-    if (uncapped)
+    fixed_t deltax, deltay;
+    if (uncapped && leveltime > oldleveltime)
     {
         deltax = LerpFixed(skytex->prevx, skytex->currx);
+        deltay = LerpFixed(skytex->prevy, skytex->curry);
     }
     else
     {
         deltax = skytex->currx;
+        deltay = skytex->curry;
     }
 
+    dc_texturemid += deltay;
+
     angle_t an = viewangle + (deltax << (ANGLETOSKYSHIFT - FRACBITS));
 
     for (int x = pl->minx; x <= pl->maxx; x++)