internal/atlas: ensure the given sizes must be power of 2

Updates #2519

internal/atlas/export_test.go

@@ -60,3 +60,5 @@ func (i *Image) EnsureIsolatedForTesting() {
 }
 
 var FlushDeferredForTesting = flushDeferred
+
+var ToPowerOf2 = toPowerOf2

internal/atlas/image.go

@@ -728,6 +728,17 @@ func EndFrame(graphicsDriver graphicsdriver.Graphics) error {
 	return nil
 }
 
+func toPowerOf2(x int) int {
+	if x <= 0 {
+		return 0
+	}
+	p2 := 1
+	for p2*2 <= x {
+		p2 *= 2
+	}
+	return p2
+}
+
 func BeginFrame(graphicsDriver graphicsdriver.Graphics) error {
 	defer backendsM.Unlock()
 
@@ -746,7 +757,7 @@ func BeginFrame(graphicsDriver graphicsdriver.Graphics) error {
 			minSize = 1024
 		}
 		if maxSize == 0 {
-			maxSize = restorable.MaxImageSize(graphicsDriver)
+			maxSize = toPowerOf2(restorable.MaxImageSize(graphicsDriver))
 		}
 	})
 	if err != nil {

internal/atlas/image_test.go

@@ -736,4 +736,52 @@ func TestImageWritePixelsModify(t *testing.T) {
 	}
 }
 
+func TestPowerOf2(t *testing.T) {
+	testCases := []struct {
+		In  int
+		Out int
+	}{
+		{
+			In:  1023,
+			Out: 512,
+		},
+		{
+			In:  1024,
+			Out: 1024,
+		},
+		{
+			In:  1025,
+			Out: 1024,
+		},
+		{
+			In:  10000,
+			Out: 8192,
+		},
+		{
+			In:  16384,
+			Out: 16384,
+		},
+		{
+			In:  1,
+			Out: 1,
+		},
+		{
+			In:  0,
+			Out: 0,
+		},
+		{
+			In:  -1,
+			Out: 0,
+		},
+	}
+
+	for _, tc := range testCases {
+		got := atlas.ToPowerOf2(tc.In)
+		want := tc.Out
+		if got != want {
+			t.Errorf("packing.ToPowerOf2(%d): got: %d, want: %d", tc.In, got, want)
+		}
+	}
+}
+
 // TODO: Add tests to extend image on an atlas out of the main loop

internal/packing/packing.go

@@ -31,7 +31,26 @@ type Page struct {
 	maxSize int
 }
 
+func isPositivePowerOf2(x int) bool {
+	if x <= 0 {
+		return false
+	}
+	for x > 1 {
+		if x/2*2 != x {
+			return false
+		}
+		x /= 2
+	}
+	return true
+}
+
 func NewPage(initSize int, maxSize int) *Page {
+	if !isPositivePowerOf2(initSize) {
+		panic(fmt.Sprintf("packing: initSize must be a positive power of 2 but %d", initSize))
+	}
+	if !isPositivePowerOf2(maxSize) {
+		panic(fmt.Sprintf("packing: maxSize must be a positive power of 2 but %d", maxSize))
+	}
 	return &Page{
 		width:   initSize,
 		height:  initSize,
@@ -263,7 +282,7 @@ func (p *Page) extendFor(width, height int) bool {
 
 			if newWidth > p.maxSize || newHeight > p.maxSize {
 				if newWidth > p.maxSize && newHeight > p.maxSize {
-					panic(fmt.Sprintf("packing: too big extension: (%d, %d)", newWidth, newHeight))
+					panic(fmt.Sprintf("packing: too big extension: allocating size: (%d, %d), current size: (%d, %d), new size: (%d, %d), (i, j): (%d, %d), max size: %d", width, height, p.width, p.height, newWidth, newHeight, i, j, p.maxSize))
 				}
 				continue
 			}