Colour a subregion of four connected cells in every bounded region. These subregions are not allowed to touch other identically shaped subregions with a side. Rotations and reflections count as the same shape. All coloured cells have to be connected. No four coloured cells are allowed to form a 2x2 square.
Five different shapes are distinguished: The 2x2 square (but this one will never be used since coloured 2x2 squares are not allowed to occur anywhere, so also not within bounded regions), an L-shape, an I-shape, a T-shape and an S-shape (this explains the name of the genre). The Z-shape is regarded as an S-shape in this type of puzzles.
All three areas in the example puzzle need to get four connected coloured cells. In both the area on the left and the one in the bottom right we find can three cells that in case they would not be coloured, would make it impossible to colour a connected subregion of size four in there at all. Those cells have to become coloured for sure.
The cell in the third column on the third row is not allowed to be coloured, since it would create a a coloured 2x2 square. Therefore, using the right mouse button, we can mark it as permanently uncoloured. At this point in the shape that includes the cell in the upper right, there are also three cells which would render the puzzle unsolvable in case they were left blank, so they can also be coloured.
The second cell from the left on the top row, needs to remain blank for sure now, to avoid coloured 2x2 squares. As a direct consequence the cell in the lower left has to be coloured. Also the second cell from the left on the third row has to remain empty, from which it can be immediately concluded that the cell in the upper right does have to be coloured, the cell directly below it should not, and thus the second cell on the bottom row should be.
The puzzle is now solved. All coloured cells are connected, there are no coloured 2x2 squares present and the three shapes within the different bounded areas do not touch similar shapes in other areas (there are no similar shapes at all in this case).
This genre was formerly known as Nuruomino. It was invented by Naoki Inaba from Japan.
Puzzles in this genre