use a zero waveform image with single horizontal line when none exists

backend/playbackmanager.go

@@ -190,6 +190,15 @@ func (p *PlaybackManager) handleWaveformImageSongChange(item mediaprovider.Media
 			}
 		}
 	}
+
+	if item == nil || item.Metadata().Type != mediaprovider.MediaItemTypeTrack {
+		// set a zero waveform image when we're not playing anything
+		// or playing a media type we can't derive a waveform from (e.g. radio)
+		img := NewWaveformImage()
+		for _, cb := range p.onWaveformImgUpdate {
+			cb(img)
+		}
+	}
 }
 
 func (p *PlaybackManager) ScanRemotePlayers(ctx context.Context, fastScan bool) {

backend/waveformimage.go

@@ -28,7 +28,16 @@ type WaveformImageGenerator struct {
 type WaveformImage = image.NRGBA
 
 func NewWaveformImage() *WaveformImage {
-	return image.NewNRGBA(image.Rect(0, 0, 1024, 48))
+	img := image.NewNRGBA(image.Rect(0, 0, 1024, 48))
+	centerTop := img.Rect.Dy() / 2 // 24
+	centerBottom := centerTop - 1  // 23
+
+	// color in center line
+	for x := 0; x < img.Bounds().Dx(); x++ {
+		setPixel(img, x, centerTop, color.NRGBA{R: 255, G: 255, B: 255, A: 255})
+		setPixel(img, x, centerBottom, color.NRGBA{R: 255, G: 255, B: 255, A: 255})
+	}
+	return img
 }
 
 type WaveformImageJob struct {