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

1 files changed, 10 insertions, 0 deletions

diff --git a/src/GPU3D_Soft.cpp b/src/GPU3D_Soft.cpp
index 10ad1d6..1323f81 100644
--- a/src/GPU3D_Soft.cpp
+++ b/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,