ebiten: remove returning error from ReadPixels

- As ReadPixels should often be used at Draw, error handling would be hard.
- Make the API consistent with ReplacePixels.

Updates #1995

image.go

@@ -787,31 +787,29 @@ func (i *Image) ColorModel() color.Model {
 //
 // ReadPixels always sets a transparent color if the image is disposed.
 //
-// ReadPixels returns an error when an error occurs during reading pixels from GPU.
+// len(pixels) must be 4*width*height. If the sizes don't match, ReadPixels panics.
-//
-// len(pixels) must be 4*width*height. If the sizes don't match, ReadPixels returns an error.
 //
 // Note that an important logic should not rely on values returned by ReadPixels, since
 // the returned values can include very slight differences between some machines.
 //
 // ReadPixels can't be called outside the main loop (ebiten.Run's updating function) starts.
-func (i *Image) ReadPixels(pixels []byte) error {
+func (i *Image) ReadPixels(pixels []byte) {
 	b := i.Bounds()
 	if got, want := len(pixels), 4*b.Dx()*b.Dy(); got != want {
-		return fmt.Errorf("ebiten: len(pixels) must be %d but %d at ReadPixels", want, got)
+		panic(fmt.Sprintf("ebiten: len(pixels) must be %d but %d at ReadPixels", want, got))
 	}
 
 	if i.isDisposed() {
 		for i := range pixels {
 			pixels[i] = 0
 		}
-		return nil
+		return
 	}
 
 	i.resolveSetVerticesCacheIfNeeded()
 
 	x, y := i.adjustPosition(b.Min.X, b.Min.Y)
-	return i.image.ReadPixels(pixels, x, y, b.Dx(), b.Dy())
+	i.image.ReadPixels(pixels, x, y, b.Dx(), b.Dy())
 }
 
 // At returns the color of the image at (x, y).
@@ -858,7 +856,9 @@ func (i *Image) at(x, y int) (r, g, b, a byte) {
 	if c, ok := i.setVerticesCache[[2]int{x, y}]; ok {
 		return c[0], c[1], c[2], c[3]
 	}
-	return i.image.At(x, y)
+	var pix [4]byte
+	i.image.ReadPixels(pix[:], x, y, 1, 1)
+	return pix[0], pix[1], pix[2], pix[3]
 }
 
 // Set sets the color at (x, y).

image_test.go

@@ -112,9 +112,7 @@ func TestImagePixels(t *testing.T) {
 	}
 
 	pix := make([]byte, 4*w*h)
-	if err := img0.ReadPixels(pix); err != nil {
+	img0.ReadPixels(pix)
-		t.Fatal(err)
-	}
 	for j := 0; j < h; j++ {
 		for i := 0; i < w; i++ {
 			idx := 4 * (j*w + i)

internal/ui/image.go

@@ -75,25 +75,18 @@ func (i *Image) ReplacePixels(pix []byte, x, y, width, height int) {
 	i.mipmap.ReplacePixels(pix, x, y, width, height)
 }
 
-func (i *Image) ReadPixels(pixels []byte, x, y, width, height int) error {
+func (i *Image) ReadPixels(pixels []byte, x, y, width, height int) {
-	return theUI.readPixels(i.mipmap, pixels, x, y, width, height)
-}
-
-func (i *Image) At(x, y int) (r, g, b, a byte) {
 	// Check the error existence and avoid unnecessary calls.
 	if theGlobalState.error() != nil {
-		return 0, 0, 0, 0
+		return
 	}
 
-	var pix [4]byte
+	if err := theUI.readPixels(i.mipmap, pixels, x, y, width, height); err != nil {
-	if err := theUI.readPixels(i.mipmap, pix[:], x, y, 1, 1); err != nil {
 		if panicOnErrorOnReadingPixels {
 			panic(err)
 		}
 		theGlobalState.setError(err)
-		return 0, 0, 0, 0
 	}
-	return pix[0], pix[1], pix[2], pix[3]
 }
 
 func (i *Image) DumpScreenshot(name string, blackbg bool) error {