Nonograms: working from the edges
A filled cell near the edge, or next to a cross, pins its block. The block can’t slide past the wall, so its reach is short.
How it works
Once a line has some filled cells, the overlap idea gets sharper. A filled cell belongs to some block. That block must include the cell, so it can only start a few places back.
Near the edge, the edge acts as a wall. The block can’t start before cell 1. So it has fewer places, and more of its cells are sure.
A crossed cell is a wall too. And any cell too far from the filled one for the block to reach is empty. Checking every placement of a line this way is what a line solver does.
A cross is a wall
Cell 3 is crossed and cell 4 is filled. The 3 includes cell 4 but can’t cross into cell 3, so it starts at cell 4. That fills 4 to 6 and empties everything else.
A finished clue empties the rest
When every block in a line is found, every other cell in that line is empty. Cross them at once. Those crosses become walls for the columns they cross.
Worth memorizing
A filled cell pins its block.
The block must include it.
Edges and crosses are walls.
A block can’t pass them.
Cells out of reach are empty.
Practice
0 of 5 rightWhich cells are sure to be filled?
What does the whole row become?
Cell 4 is crossed. Which cells does that let you fill?
Which cells are sure to be empty?
The 2 is already found. What happens to the other cells?