Draw entities back to front within tiles (#705)

* Draw entities per subtile for correct depth order

* Use a temporary entity slice for depth sorting

Speeds things up quite a bit

d2common/d2math/d2vector/position.go

@@ -68,11 +68,11 @@ func (p *Position) Tile() *Vector {
 // and to the lower left diagonal ('left') of the isometric grid. This renders the target one index above which visually
 // is one tile below.
 func (p *Position) RenderOffset() *Vector {
-	return p.subTileOffset().AddScalar(1)
+	return p.SubTileOffset().AddScalar(1)
 }
 
 // subTileOffset is the offset from the current map tile in sub tiles.
-func (p *Position) subTileOffset() *Vector {
+func (p *Position) SubTileOffset() *Vector {
 	t := p.Tile().Scale(subTilesPerTile)
 	c := p.Clone()
 

d2core/d2map/d2maprenderer/renderer.go

@@ -197,6 +197,8 @@ func (mr *MapRenderer) renderPass2(target d2interface.Surface, startX, startY, e
 		for tileX := startX; tileX < endX; tileX++ {
 			mr.viewport.PushTranslationWorld(float64(tileX), float64(tileY))
 
+			tileEnt := make([]d2interface.MapEntity, 0)
+
 			// TODO: Do not loop over every entity every frame
 			for _, mapEntity := range mr.mapEngine.Entities() {
 				pos := mapEntity.GetPosition()
@@ -211,10 +213,23 @@ func (mr *MapRenderer) renderPass2(target d2interface.Surface, startX, startY, e
 					continue
 				}
 
+				tileEnt = append(tileEnt, mapEntity)
+			}
+
+			for subY := 0; subY < 5; subY++ {
+				for subX := 0; subX < 5; subX++ {
+					for _, mapEntity := range tileEnt {
+						pos := mapEntity.GetPosition()
+						if (int(pos.SubTileOffset().X()) != subX) || (int(pos.SubTileOffset().Y()) != subY) {
+							continue
+						}
+
 						target.PushTranslation(mr.viewport.GetTranslationScreen())
 						mapEntity.Render(target)
 						target.Pop()
 					}
+				}
+			}
 
 			mr.viewport.PopTranslation()
 		}
@@ -229,6 +244,8 @@ func (mr *MapRenderer) renderPass3(target d2interface.Surface, startX, startY, e
 			mr.viewport.PushTranslationWorld(float64(tileX), float64(tileY))
 			mr.renderTilePass2(tile, target)
 
+			tileEnt := make([]d2interface.MapEntity, 0)
+
 			// TODO: Do not loop over every entity every frame
 			for _, mapEntity := range mr.mapEngine.Entities() {
 				pos := mapEntity.GetPosition()
@@ -243,10 +260,23 @@ func (mr *MapRenderer) renderPass3(target d2interface.Surface, startX, startY, e
 					continue
 				}
 
+				tileEnt = append(tileEnt, mapEntity)
+			}
+
+			for subY := 0; subY < 5; subY++ {
+				for subX := 0; subX < 5; subX++ {
+					for _, mapEntity := range tileEnt {
+						pos := mapEntity.GetPosition()
+						if (int(pos.SubTileOffset().X()) != subX) || (int(pos.SubTileOffset().Y()) != subY) {
+							continue
+						}
+
 						target.PushTranslation(mr.viewport.GetTranslationScreen())
 						mapEntity.Render(target)
 						target.Pop()
 					}
+				}
+			}
 
 			mr.viewport.PopTranslation()
 		}