Heteromino rules

I'm not sure what the best method for this. Using the example on that page (because I'm too lazy to make a new puzzle or find another example), and with R3 and C3:

Method 1: Shape of the trimino for each cell, like LITSO, using the characters I-rLJ7 (IrrI, 77rX)
Method 2: Like Method 1, but only I for straight and L for bend, not caring about orientation (ILLI, LLLX)
Method 3: Lengths of contiguous white squares, like shading puzzles but applied to borders (121, 21)
Method 4: O for centers of triminoes, X otherwise (XOXO, OXXX)

Or something else? Ideas?

Of course, for Method 1, we can use a different set of letters.

I prefer the 3rd one.

Advantages and disadvantages of each method in my opinion:

Method 1
+ Completely specifies each row/column
- Difficult to remember the 6-character legend

Method 2
+ Simplified version of Method 1
- L appears twice as often as I, making it somewhat guessable

Method 3
+ More consistent with other shading answer keys
- 1 and 2 can't quite differentiate I's and L's

Method 4
+ Centers are not easy to guess
- Probably difficult to notice

I myself seem to prefer Method 3 slightly, with Method 2 coming second.