Fix Incorrect Polygon Swapping Behavior and Implement Correct Rules for Shifting Right Edges Left (#1816)

* fix polygons being swapped incorrectly

"borrowed" this from noods
needs verification that the >= and <= signs aren't actually supposed to be > and <

* proper rules for moving vertical right slopes left

* nvm most of that was actually pointless

that's on me for not checking

src/GPU3D.cpp

@@ -1067,7 +1067,7 @@ void SubmitPolygon()
 
     dot = ((s64)v1->Position[0] * normalX) + ((s64)v1->Position[1] * normalY) + ((s64)v1->Position[3] * normalZ);
 
-    bool facingview = (dot < 0);
+    bool facingview = (dot <= 0);
 
     if (facingview)
     {

src/GPU3D_Soft.cpp

@@ -742,6 +742,11 @@ void SoftRenderer::RenderShadowMaskScanline(RendererPolygon* rp, s32 y)
     s32 zl = rp->SlopeL.Interp.InterpolateZ(polygon->FinalZ[rp->CurVL], polygon->FinalZ[rp->NextVL], polygon->WBuffer);
     s32 zr = rp->SlopeR.Interp.InterpolateZ(polygon->FinalZ[rp->CurVR], polygon->FinalZ[rp->NextVR], polygon->WBuffer);
 
+    // right vertical edges are pushed 1px to the left as long as either:
+    // the left edge slope is not 0, or the span is not 0 pixels wide, and it is not at the leftmost pixel of the screen
+    if (rp->SlopeR.Increment==0 && (rp->SlopeL.Increment!=0 || xstart != xend) && (xend != 0))
+        xend--;
+
     // if the left and right edges are swapped, render backwards.
     if (xstart > xend)
     {
@@ -955,6 +960,11 @@ void SoftRenderer::RenderPolygonScanline(RendererPolygon* rp, s32 y)
 
     s32 zl = rp->SlopeL.Interp.InterpolateZ(polygon->FinalZ[rp->CurVL], polygon->FinalZ[rp->NextVL], polygon->WBuffer);
     s32 zr = rp->SlopeR.Interp.InterpolateZ(polygon->FinalZ[rp->CurVR], polygon->FinalZ[rp->NextVR], polygon->WBuffer);
+    
+    // right vertical edges are pushed 1px to the left as long as either:
+    // the left edge slope is not 0, or the span is not 0 pixels wide, and it is not at the leftmost pixel of the screen
+    if (rp->SlopeR.Increment==0 && (rp->SlopeL.Increment!=0 || xstart != xend) && (xend != 0))
+        xend--;
 
     // if the left and right edges are swapped, render backwards.
     // on hardware, swapped edges seem to break edge length calculation,

src/GPU3D_Soft.h

@@ -233,16 +233,8 @@ private:
 
         s32 SetupDummy(s32 x0)
         {
-            if (side)
+            dx = 0;
-            {
+            
-                dx = -0x40000;
-                x0--;
-            }
-            else
-            {
-                dx = 0;
-            }
-
             this->x0 = x0;
             this->xmin = x0;
             this->xmax = x0;
@@ -278,7 +270,6 @@ private:
             else
             {
                 this->xmin = x0;
-                if (side) this->xmin--;
                 this->xmax = this->xmin;
                 this->Negative = false;
             }
@@ -309,7 +300,7 @@ private:
 
                 if (XMajor)              dx = Negative ? (0x20000 + 0x40000) : (Increment - 0x20000);
                 else if (Increment != 0) dx = Negative ? 0x40000 : 0;
-                else                     dx = -0x40000;
+                else                     dx = 0;
             }
             else
             {