linear horizontal sky scrolling

This is mostly this PR which I rejected for Crispy:
https://github.com/fabiangreffrath/crispy-doom/pull/629

Thank you very much @kitchen-ace for preparing this!

Source/m_menu.c

@@ -57,6 +57,7 @@
 #include "m_input.h"
 #include "r_draw.h" // [FG] R_SetFuzzColumnMode
 #include "r_sky.h" // [FG] R_InitSkyMap()
+#include "r_plane.h" // [FG] R_InitPlanes()
 
 // [crispy] remove DOS reference from the game quit confirmation dialogs
 #include "SDL_platform.h"
@@ -3598,8 +3599,8 @@ static const char *midi_player_strings[] = {
 #define G_X 250
 #define G_Y  44
 #define G_Y2 (G_Y+92+8) // [FG] remove sound and music card items
-#define G_Y3 (G_Y+44)
+#define G_Y3 (G_Y+44-8)
-#define G_Y4 (G_Y3+52)
+#define G_Y4 (G_Y3+52-16)
 #define GF_X 76
 
 setup_menu_t gen_settings1[] = { // General Settings screen1
@@ -3665,6 +3666,11 @@ enum {
   general_mouse2
 };
 
+enum {
+  general_sky1,
+  general_sky2
+};
+
 enum {
   general_corpse,
   general_realtic,
@@ -3700,25 +3706,30 @@ setup_menu_t gen_settings2[] = { // General Settings screen2
   {"Invert vertical axis", S_YESNO, m_null, G_X,
    G_Y + general_mouse2*8, {"mouse_y_invert"}},
 
-  {"Miscellaneous"  ,S_SKIP|S_TITLE, m_null, G_X, G_Y3 - 12},
+  {"Sky Drawing"  ,S_SKIP|S_TITLE, m_null, G_X, G_Y3 - 12},
+
+  {"Stretch Short Skies", S_YESNO, m_null, G_X,
+   G_Y3 + general_sky1*8, {"stretchsky"}, 0, R_InitSkyMap},
+
+  {"Linear Horizontal Scrolling", S_YESNO, m_null, G_X,
+   G_Y3 + general_sky2*8, {"linearsky"}, 0, R_InitPlanes},
+
+  {"Miscellaneous"  ,S_SKIP|S_TITLE, m_null, G_X, G_Y4 - 12},
 
   {"Maximum number of player corpses", S_NUM|S_PRGWARN, m_null, G_X,
-   G_Y3 + general_corpse*8, {"max_player_corpse"}},
+   G_Y4 + general_corpse*8, {"max_player_corpse"}},
 
   {"Game speed, percentage of normal", S_NUM|S_PRGWARN, m_null, G_X,
-   G_Y3 + general_realtic*8, {"realtic_clock_rate"}},
+   G_Y4 + general_realtic*8, {"realtic_clock_rate"}},
 
   {"Default skill level", S_CHOICE|S_LEVWARN, m_null, G_X,
-   G_Y3 + general_skill*8, {"default_skill"}, 0, NULL, default_skill_strings},
+   G_Y4 + general_skill*8, {"default_skill"}, 0, NULL, default_skill_strings},
 
   {"Default compatibility", S_CHOICE|S_LEVWARN, m_null, G_X,
-   G_Y3 + general_comp*8, {"default_complevel"}, 0, NULL, default_compatibility_strings},
+   G_Y4 + general_comp*8, {"default_complevel"}, 0, NULL, default_compatibility_strings},
 
   {"Show ENDOOM screen", S_CHOICE, m_null, G_X,
-   G_Y3 + general_endoom*8, {"show_endoom"}, 0, NULL, default_endoom_strings},
+   G_Y4 + general_endoom*8, {"show_endoom"}, 0, NULL, default_endoom_strings},
-
-  {"Stretch short Skies", S_YESNO, m_null, G_X,
-   G_Y3 + general_stretchsky*8, {"stretchsky"}, 0, R_InitSkyMap},
 
   {"<- PREV",S_SKIP|S_PREV, m_null, KB_PREV, KB_Y+20*8, {gen_settings1}},
 

Source/m_misc.c

@@ -181,6 +181,13 @@ default_t defaults[] = {
     "1 to stretch short skies"
   },
 
+  {
+    "linearsky",
+    (config_t *) &linearsky, NULL,
+    {0}, {0,1}, number, ss_gen, wad_no,
+    "1 for linear horizontal sky scrolling "
+  },
+
   { // killough 2/21/98
     "pitched_sounds",
     (config_t *) &pitched_sounds, NULL,

Source/r_main.c

@@ -78,6 +78,9 @@ int viewangletox[FINEANGLES/2];
 
 angle_t xtoviewangle[MAX_SCREENWIDTH+1];   // killough 2/8/98
 
+// [FG] linear horizontal sky scrolling
+angle_t linearskyangle[MAX_SCREENWIDTH+1];
+
 // killough 3/20/98: Support dynamic colormaps, e.g. deep water
 // killough 4/4/98: support dynamic number of them as well
 
@@ -294,6 +297,8 @@ static void R_InitTextureMapping (void)
       for (i=0; viewangletox[i] > x; i++)
         ;
       xtoviewangle[x] = (i<<ANGLETOFINESHIFT)-ANG90;
+      // [FG] linear horizontal sky scrolling
+      linearskyangle[x] = ((viewwidth/2-x)*((SCREENWIDTH<<6)/viewwidth))*(ANG90/(NONWIDEWIDTH<<6));
     }
     
   // Take out the fencepost cases from viewangletox.

Source/r_plane.c

@@ -96,12 +96,17 @@ static fixed_t xoffs,yoffs;    // killough 2/28/98: flat offsets
 
 fixed_t *yslope, yslopes[LOOKDIRS][MAX_SCREENHEIGHT], distscale[MAX_SCREENWIDTH];
 
+// [FG] linear horizontal sky scrolling
+boolean linearsky;
+static angle_t *xtoskyangle;
+
 //
 // R_InitPlanes
 // Only at game startup.
 //
 void R_InitPlanes (void)
 {
+  xtoskyangle = linearsky ? linearskyangle : xtoviewangle;
 }
 
 //
@@ -409,7 +414,7 @@ static void do_draw_plane(visplane_t *pl)
         for (x = pl->minx; (dc_x = x) <= pl->maxx; x++)
           if ((unsigned)(dc_yl = pl->top[x]) <= (dc_yh = pl->bottom[x])) // [FG] 32-bit integer math
             {
-              dc_source = R_GetColumn(texture, ((an + xtoviewangle[x])^flip) >>
+              dc_source = R_GetColumn(texture, ((an + xtoskyangle[x])^flip) >>
 				      ANGLETOSKYSHIFT);
               colfunc();
             }

Source/r_sky.h

@@ -41,6 +41,9 @@
 #define SKYSTRETCH_HEIGHT 228
 extern boolean stretchsky;
 
+// [FG] linear horizontal sky scrolling
+extern boolean linearsky;
+
 extern int skytexture;
 extern int skytexturemid;
 

Source/r_state.h

@@ -107,6 +107,9 @@ extern angle_t          xtoviewangle[MAX_SCREENWIDTH+1];  // killough 2/8/98
 extern fixed_t          rw_distance;
 extern angle_t          rw_normalangle;
 
+// [FG] linear horizontal sky scrolling
+extern angle_t          linearskyangle[MAX_SCREENWIDTH+1];
+
 // angle to line origin
 extern int              rw_angle1;