I would like to "write" on to a matrix using the mouse. That is, I would like to use the integer value of Locator coordinates (p) to index a matrix. Then, if the mouse button is down, write a 1 in that location. That approach doesn't seem to work since, when attempting to do something like M[[Sequence @@ Round [p]]], the coordinate p is actually a dynamic quantity, which can't be converted to two integer indices by Sequence @@ Round.
Is there any way to do this? or, Is there an equivalent workaround that gets the same result. The simplest example would be using the mouse to input a freehand black and white drawing as pixels on a matrix, which could later be subject to image processing algorithms.