ebiten: add (*Image).ReadPixels

Closes #1995

image.go

@@ -777,6 +777,37 @@ func (i *Image) ColorModel() color.Model {
 	return color.RGBAModel
 }
 
+// ReadPixels reads the image's pixels from the image.
+//
+// ReadPixels loads pixels from GPU to system memory if necessary, which means that ReadPixels can be slow.
+//
+// ReadPixels always sets a transparent color if the image is disposed.
+//
+// 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 {
+	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)
+	}
+
+	if i.isDisposed() {
+		for i := range pixels {
+			pixels[i] = 0
+		}
+		return nil
+	}
+
+	i.resolveSetVerticesCacheIfNeeded()
+
+	x, y := i.adjustPosition(b.Min.X, b.Min.Y)
+	return i.image.ReadPixels(pixels, x, y, b.Dx(), b.Dy())
+}
+
 // At returns the color of the image at (x, y).
 //
 // At loads pixels from GPU to system memory if necessary, which means that At can be slow.
@@ -819,9 +850,7 @@ 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]
 	}
-	var pix [4]byte
+	return i.image.At(x, y)
-	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

@@ -110,6 +110,21 @@ func TestImagePixels(t *testing.T) {
 			}
 		}
 	}
+
+	pix := make([]byte, 4*w*h)
+	if err := img0.ReadPixels(pix); err != nil {
+		t.Fatal(err)
+	}
+	for j := 0; j < h; j++ {
+		for i := 0; i < w; i++ {
+			idx := 4 * (j*w + i)
+			got := color.RGBA{pix[idx], pix[idx+1], pix[idx+2], pix[idx+3]}
+			want := color.RGBAModel.Convert(img.At(i, j))
+			if got != want {
+				t.Errorf("(%d, %d): got %v; want %v", i, j, got, want)
+			}
+		}
+	}
 }
 
 func TestImageComposition(t *testing.T) {

internal/ui/image.go

@@ -75,21 +75,25 @@ 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) {
+func (i *Image) ReadPixels(pixels []byte, x, y, width, height int) error {
+	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
+		return 0, 0, 0, 0
 	}
 
-	if err := theUI.readPixels(i.mipmap, pixels, x, y, width, height); err != nil {
+	var pix [4]byte
+	if err := theUI.readPixels(i.mipmap, pix[:], x, y, 1, 1); err != nil {
 		if panicOnErrorOnReadingPixels {
 			panic(err)
 		}
 		theGlobalState.setError(err)
-		for i := range pixels {
+		return 0, 0, 0, 0
-			pixels[i] = 0
-		}
 	}
+	return pix[0], pix[1], pix[2], pix[3]
 }
 
 func (i *Image) DumpScreenshot(name string, blackbg bool) error {