ebiten: update comments

Updates #2471

image.go

@@ -103,8 +103,11 @@ type DrawImageOptions struct {
 	GeoM GeoM
 
 	// ColorScale is a scale of color.
-	// ColorScale is slightly different from ColorM's Scale in terms of alphas:
-	// ColorScale is applied to premultiplied-alpha colors, while ColorM is applied to straight-alpha colors.
+	//
+	// ColorScale is slightly different from colorm.ColorM's Scale in terms of alphas.
+	// ColorScale is applied to premultiplied-alpha colors, while colorm.ColorM is applied to straight-alpha colors.
+	// Thus, colorm.ColorM.Scale(r, g, b, a) equals to ColorScale.Scale(r*a, g*a, b*a, a).
+	//
 	// The default (zero) value is identity, which is (1, 1, 1, 1).
 	ColorScale ColorScale
 
@@ -188,9 +191,12 @@ func (i *Image) adjustedRegion() graphicsdriver.Region {
 // DrawImage works more efficiently as batches
 // when the successive calls of DrawImages satisfy the below conditions:
 //
-//   - All render targets are same (A in A.DrawImage(B, op))
-//   - All Blend values are same
-//   - All Filter values are same
+//   - All render targets are the same (A in A.DrawImage(B, op))
+//   - All Blend values are the same
+//   - All Filter values are the same
+//
+// A whole image and its sub-image are considered to be the same, but some
+// environments like browsers might not work efficiently (#2471).
 //
 // Even when all the above conditions are satisfied, multiple draw commands can
 // be used in really rare cases. Ebitengine images usually share an internal