Why does an ideal high pass filter on this image generate gray values As seen from the image below, an ideal high pass filter was applied. Hence, stark changes in pixels from lighter to darker or vice versa show up on the filtered image. However, slower changes such as on the cheekbone appear to be this gray color.

What is this gray color? It seems to be quite prevalent in all my grayscale images.
Extreme positive values are black and extreme negative values are white. Values near zero are grey. Where there is not much change the high pass filter outputs values near zero. Hence the grey regions in the image.