1. Open the material, create a new layer, and fill it with 50% gray using shift+F5.
2. Change the filled gray layer to the "Difference" blending mode.
3. Add a threshold adjustment layer and adjust the slider until there are fewer black blocks in the image.
4. The black part in the picture is the gray field. Create a new layer and mark it with a red circle.
5. Hide the gray and threshold layers and return to the background layer
