Author: Andrey Bogdanov
Place the given elements into the grid. Elements may be rotated and reflected. Elements can not touch each other, not even diagonally. Grid numbers show the total horizontally and vertically visible white cells from that cell, including itself, with elements blocking the line of sight.
Grid control: Click the mouse button at the selected cell to change the color of the cell. Second click will put auxiliary cross sign, which doesn't affect correctness of the solution, but helps visually. You can also drag the content of one cell to another with the mouse. You can also click on the given tetromino figures to mark them out. More details about grid control are at the help page.