graphics: Add comments to At

examples/drag/main.go

@@ -46,6 +46,10 @@ type Sprite struct {
 func (s *Sprite) In(x, y int) bool {
 	// Check the actual color (alpha) value at the specified position
 	// so that the result of In becomes natural to users.
+	//
+	// Note that this is not a good manner to use At for logic
+	// since color from At might include some errors on some machines.
+	// As this is not so important logic, it's ok to use it so far.
 	return s.image.At(x-s.x, y-s.y).(color.RGBA).A > 0
 }
 

image.go

@@ -246,6 +246,9 @@ func (i *Image) ColorModel() color.Model {
 //
 // At always returns a transparent color if the image is disposed.
 //
+// Note that important logic should not rely on At result since
+// At might include a very slight error on some machines.
+//
 // At can't be called before the main loop (ebiten.Run) starts (as of version 1.4.0-alpha).
 func (i *Image) At(x, y int) color.Color {
 	if i.isDisposed() {