How can I get high and low value of a histogram?

Actually you don't need a mozaic effect, the sampleImage expression can specify a (sampling) radius.

The method would be to

1- add a slider control to the layer,

2- in a loop over x and over y (the coordinates of the center of the sampling point),

         - set the slider control expression to

rgbToHsl(sampleImage([x,y], radius, postEffect, time))[2];

          (this first evaluates the [R,G,B,Alpha] color of the layer in a rectangle of half size [radius[0], radius[1]] centered at [x,y], at the time specified, pre or post effect depending on postEffect (true/false),

          then converts it to [H,S,L, Alpha], picks the "L" entry (a number in [0,1]);

          - harvest the result of the expression,

          - compare to previous values: if below min or above max, store new min or max, and coordinates.

3- when it's done, remove the slider control.

For the previous post i had used radius = [0.5, 0.5] so you get pixel accuracy (as in your original post).

But increasing the radius can speed things up greatly.

Dividing a 1024x768 image into C columns (and roughly C*layer.height/layer.width rows), i got these times:

C = 100 (num samples: 7500) time 1.7 sec

C = 50 (num samples: 1900) time 0.45 sec

C = 25 (num samples: 475) time: 0.13 sec

I think it is slow because the sampleImage expression itself  is a little bit slow, but maybe also because of all the history entries (undos) such a script creates.

Putting everything in an undoGroup speed things up a bit, but not much.

Xavier.