mipmap: Bug fix: Scale could be Inf/0 and caused a forever loop

Fixes #1398

image_test.go

@@ -2128,3 +2128,24 @@ func TestImageZeroTriangle(t *testing.T) {
 	is := []uint16{}
 	dst.DrawTriangles(vs, is, src, nil)
 }
+
+// Issue #1398
+func TestImageDrawImageTooBigScale(t *testing.T) {
+	dst := NewImage(1, 1)
+	src := NewImage(1, 1)
+
+	op := &DrawImageOptions{}
+	op.GeoM.Scale(1e20, 1e20)
+	dst.DrawImage(src, op)
+}
+
+// Issue #1398
+func TestImageDrawImageTooSmallScale(t *testing.T) {
+	dst := NewImage(1, 1)
+	src := NewImage(1, 1)
+
+	op := &DrawImageOptions{}
+	op.Filter = FilterLinear
+	op.GeoM.Scale(1e-10, 1e-10)
+	dst.DrawImage(src, op)
+}

internal/mipmap/mipmap.go

@@ -277,6 +277,8 @@ func (m *Mipmap) disposeMipmaps() {
 
 // mipmapLevel returns an appropriate mipmap level for the given distance.
 func mipmapLevelFromDistance(dx0, dy0, dx1, dy1, sx0, sy0, sx1, sy1 float32, filter driver.Filter) int {
+	const maxScale = 6
+
 	if filter == driver.FilterScreen {
 		return 0
 	}
@@ -288,6 +290,19 @@ func mipmapLevelFromDistance(dx0, dy0, dx1, dy1, sx0, sy0, sx1, sy1 float32, fil
 	}
 	scale := d / s
 
+	// Scale can be infinite when the specified scale is extremely big (#1398).
+	if math.IsInf(float64(scale), 0) {
+		if filter == driver.FilterNearest {
+			return -maxScale
+		}
+		return 0
+	}
+
+	// Scale can be zero when the specified scale is extremely small (#1398).
+	if scale == 0 {
+		return 0
+	}
+
 	// Use 'negative' mipmap to render edges correctly (#611, #907).
 	// It looks like 128 is the enlargement factor that causes edge missings to pass the test TestImageStretch,
 	// but we use 32 here for environments where the float precision is low (#1044, #1270).
@@ -316,10 +331,10 @@ func mipmapLevelFromDistance(dx0, dy0, dx1, dy1, sx0, sy0, sx1, sy1 float32, fil
 			}
 		}
 
-		// If tooBigScale is 64, level -6 means that the maximum scale is 64 * 2^6 = 4096. This should be
+		// If tooBigScale is 32, level -6 means that the maximum scale is 32 * 2^6 = 2048. This should be
 		// enough.
-		if level < -6 {
+		if level < -maxScale {
-			level = -6
+			level = -maxScale
 		}
 		return level
 	}
@@ -352,8 +367,8 @@ func mipmapLevelFromDistance(dx0, dy0, dx1, dy1, sx0, sy0, sx1, sy1 float32, fil
 		}
 	}
 
-	if level > 6 {
+	if level > maxScale {
-		level = 6
+		level = maxScale
 	}
 
 	return level