Setting the value of the mask image
| x | Specifies the x coordinate (in the image coordinates of the layer) for which you want to set the value. |
| y | Specify the y coordinate (in the image coordinates of the layer) for which you want to set the value. |
| value | Specify the value to be set (0 to 255). |
Set a value (0 to 255) at any position on the layer mask image (the image that retains opacity).
An exception is thrown if you specify an invalid (out of range) position as the image coordinates.