How to interactively draw a bitmap with mouse?

Question

I cannot find the MNIST dynamic activation example I strongly believe I have seen in the documentation.

What would be the most efficient way to

1. draw a black & white bitmap interactively
2. have each edit evaluate a function (/ dynamic value)

?

---

There was a much more robust version of the below somewhere in the documentation I believe:

c // ClearAll
c[m_] := With[{w = Clip[m, {1, s}]}, a[[s - w[[2]], w[[1]]]] = 0];
a = ConstantArray[1, {s, s}];
DynamicModule[{},
  EventHandler[Dynamic@Image[a], {
    "MouseDragged" :> (c@MousePosition["Graphics"])
    }]
  ]

Answer 1

With Szabolcs' reference: https://reference.wolfram.com/language/ref/Classify#1782628206

s = 64;
a = ConstantArray[1, {s, s}];
DynamicModule[{p1, p2},
 Grid[{
   {
    EventHandler[Dynamic[Image[a], TrackedSymbols :> {a}], {
      "MouseDown" :> (p1 = p2 = PixelPos[]; PaintDot[a, p1]),
      "MouseDragged" :> (p1 = p2; p2 = PixelPos[]; 
        PaintLine[a, p1, p2])
      }]
    },
   {Button["clear", a = ConstantArray[1, {s, s}]]}}
  , Frame -> True
  ]
 ]

where (need to be defined first; kindly extracted by Szabolcs from the above example):

PixelPos[] := 
 Replace[MousePosition["Graphics"], {{i_, j_} :> Round[{64 - j, i}], 
          _ :> None}]

Attributes[PaintDot] = {HoldFirst};
PaintDot[data_Symbol, p : {i1_, j1_}] := Block[{dim = Length[data]}, 
        Do[
   If[EuclideanDistance[N[{i, j}], N[p]] < 2.5, data[[i, j]] = 0.], 
          {i, Max[i1 - 3, 1], Min[i1 + 3, dim]}, {j, Max[j1 - 3, 1], 
    Min[j1 + 3, dim]}]]

Attributes[PaintLine] = {HoldFirst};
PaintLine[data_Symbol, {i1_, j1_}, 
        {i2_, j2_}] := Block[{dim, indices, ib, ie, jb, je}, 
        indices = InterpolatePoints[N[{i1, j1}], N[{i2, j2}], 2.5]; 
         {ib, ie} = Sort[{i1, i2}]; {jb, je} = 
   Sort[{j1, j2}]; {{ib, jb}, {ie, je}} = 
           Clip[{{ib, jb} - 3, {ie, je} + 3}, {1, Length[data]}]; 
         Quiet[
   Do[If[Min[(EuclideanDistance[N[{i, j}], #1] & ) /@ indices] < 
      2.5, 
               data[[i, j]] = 0.], {i, ib, ie}, {j, jb, je}]]; ]

InterpolatePoints[start$_, stop$_] := Module[{dist$, unit$}, 
        dist$ = N[EuclideanDistance[start$, stop$]]; 
  If[dist$ < 3, 
           Return[{start$, stop$}]
   ];
   unit$ = Normalize[stop$ - N[start$]];
    Append[stop$][Table[start$ + i*unit$, {i, 0, dist$, 3}]]
  ]

InterpolatePoints[p1_, p2_, r_] := 
 Module[{d, v}, d = EuclideanDistance[p1, p2]; 
         If[d < 2*r, Return[{p1, p2}]]; v = Normalize[p2 - p1]; 
         Developer`ToPackedArray[
   Append[p2][Table[p1 + i*v, {i, 0., d, r}]], Real]]
```