graphics: Restrict QuadVertices to allow only power of 2 sizes

Now QuadVertices can skip calculating power of 2 values.

internal/graphicsutil/vertices.go

@@ -47,7 +47,18 @@ type GeoM interface {
 	Elements() (a, b, c, d, tx, ty float32)
 }
 
+func isPowerOf2(x int) bool {
+	return (x & (x - 1)) == 0
+}
+
 func QuadVertices(width, height int, sx0, sy0, sx1, sy1 int, geom GeoM) []float32 {
+	if !isPowerOf2(width) {
+		panic("not reached")
+	}
+	if !isPowerOf2(height) {
+		panic("not reached")
+	}
+
 	if sx0 >= sx1 || sy0 >= sy1 {
 		return nil
 	}
@@ -55,18 +66,8 @@ func QuadVertices(width, height int, sx0, sy0, sx1, sy1 int, geom GeoM) []float3
 		return nil
 	}
 
-	// it really feels like we should be able to cache this computation
-	// but it may not matter.
-	w := 1
-	h := 1
-	for w < width {
-		w *= 2
-	}
-	for h < height {
-		h *= 2
-	}
-	wf := float32(w)
-	hf := float32(h)
+	wf := float32(width)
+	hf := float32(height)
 	u0, v0, u1, v1 := float32(sx0)/wf, float32(sy0)/hf, float32(sx1)/wf, float32(sy1)/hf
 	return quadVerticesImpl(float32(sx1-sx0), float32(sy1-sy0), u0, v0, u1, v1, geom)
 }

internal/restorable/image.go

@@ -22,6 +22,7 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/affine"
 	"github.com/hajimehoshi/ebiten/internal/graphics"
 	"github.com/hajimehoshi/ebiten/internal/graphicsutil"
+	"github.com/hajimehoshi/ebiten/internal/math"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 )
 
@@ -53,6 +54,9 @@ type Image struct {
 
 	// screen indicates whether the image is used as an actual screen.
 	screen bool
+
+	w2 int
+	h2 int
 }
 
 var dummyImage = newImageWithoutInit(16, 16, false)
@@ -106,6 +110,16 @@ func (i *Image) Size() (int, int) {
 	return i.image.Size()
 }
 
+// SizePowerOf2 returns the next power of 2 values for the size.
+func (i *Image) SizePowerOf2() (int, int) {
+	if i.w2 == 0 || i.h2 == 0 {
+		w, h := i.image.Size()
+		i.w2 = math.NextPowerOf2Int(w)
+		i.h2 = math.NextPowerOf2Int(h)
+	}
+	return i.w2, i.h2
+}
+
 // makeStale makes the image stale.
 func (i *Image) makeStale() {
 	i.basePixels = nil

internal/shareable/shareable.go

@@ -164,7 +164,7 @@ func (i *Image) DrawImage(img *Image, sx0, sy0, sx1, sy1 int, geom graphicsutil.
 	}
 
 	dx, dy, _, _ := img.region()
-	w, h := img.backend.restorable.Size()
+	w, h := img.backend.restorable.SizePowerOf2()
 	vs := graphicsutil.QuadVertices(w, h, sx0+dx, sy0+dy, sx1+dx, sy1+dy, geom)
 	i.backend.restorable.DrawImage(img.backend.restorable, vs, quadIndices, colorm, mode, filter)
 }