Named features -- accessible route (no image required)

Tones run 0 (lightest) to 4 (darkest). Every value below is the
actual computed tone at that cell -- nothing here has been rounded for effect.

f1 -- Top-left interior (row 0, col 0; inside a block whose four source cells are all equal)
  source tone 1; degraded tone 1; restored tone 1

f2 -- Top-right interior (row 1, col 6; inside a block whose four source cells are all equal)
  source tone 3; degraded tone 3; restored tone 3

f3 -- Fine anomaly cell (row 2, col 2; the one cell in its block that differs from its three neighbours)
  source tone 4; degraded tone 2; restored tone 1

f4 -- Anomaly block, adjacent cell (row 2, col 3; shares a block with f3 but was not itself the odd one out)
  source tone 1; degraded tone 2; restored tone 1

f5 -- Diagonal detail, first cell (row 4, col 4; one of four cells forming a one-cell-wide diagonal line)
  source tone 0; degraded tone 2; restored tone 2

f6 -- Diagonal detail, last cell (row 7, col 7; the diagonal line's other end, a different block from f5)
  source tone 0; degraded tone 2; restored tone 2

Full 64-cell grid for source, degraded and restored is in pixels.csv.