Update mapping.md - improve description of how LEDMAPs work, and how to use them

docs/advanced/mapping.md

@@ -5,7 +5,7 @@ hide:
   # - toc
 ---
 
-WLED now has the ability to remap your LED strip programmatically.
+WLED has the ability to remap your LED strip programmatically.
 
 ### What is it?
 This allows us to treat the WLED strip as if it is wired in any way - we can then use the mapping feature to address the strip in any order. This allows for matrix support, serpentine runs and such.
@@ -24,6 +24,32 @@ Multiple maps are supported in the latest versions by using `ledmapx.json` where
 
 Use -1 in the map for gaps/blank/null LEDs.
 
+The ledmap also allows defining the dimensions of a 2D matrix.
+In such case you do not need to set up 2D in settings.
+By specifying the `width` and `height` fields, these dimensions are used as default matrix size when the ledmap is applied.
+
+#### Mapping Requirements
+
+Mapping is an advanced feature, and for the correct results the following requirements must be fulfilled:
+
+* A same value must not appear multiple times in the mapping, except for -1.
+
+  Mapping works by reordering the  physical LEDs.
+  When an effect is rendered, the effect sets colors of a LEDs.
+  For example, a rainbow effect might set first LED of the strip as Red, the second LED as Yellow and the third LED as Blue.
+  Let's denote these physical LEDS as `LED[0]`, `LED[1]` and `LED[2]`.
+  Mapping changes these indices to `LED[map[0]]`, `LED[map[1]]` and `LED[map[2]]`, and by setting a map of `[2, 1, 0]` we can reverse the order.
+  But if the mapping contained the same value multiple times, for example `map=[0, 0, 0]`, then the effect would set the
+  colors of the LEDs `LED[map[0]] = LED[0]`, `LED[map[1]] = LED[0]` and `LED[map[2]] = LED[0]`, i.e. we would be setting the value of `LED[0]` multiple times.
+  In this case, the resulting color of the `LED[0]` will the last value set to it.
+
+* The ledmap should include a mapping for every physical led, that is to say, the length of the `map` array must be as long as the number of total LEDs in the system.
+
+  As described above, the mapping works by changing the LEDs from `LED[0], LED[1], LED[2]...` to `LED[map[0]], LED[map[1]], LED[map[2]]...`.
+  If the `map` array is shorter than the number of LEDs, the mapping is assumed to be `map[N] = N` for the missing elements.
+  Note that the total number of LEDs in the system is the number of LEDs in the system, which may be larger than the product of the `width` and `height` fields.
+  Specifically if you use sacrificial leds, i.e. LEDs, that are not present in the mapping, you may need to add trailing `-1` elements to your `map`.
+
 ### Examples 
 In the below example (formatted multiple ways), we remap a strip of four LEDs from a physical order of 0 1 2 3 into a new order of 0 2 1 3.
 
@@ -81,3 +107,23 @@ Note that if a `ledmap.json` file exists, the `2d-gaps.json` file will be ignore
 ```
 
 ![wiring diagram of the double ∞ shape mapping](mapping/mapping_infinity_shape.png)
+
+The following example demonstrates that when using sacrificial leds, you must include a mapping for all LEDs.
+
+Here, we have a serpentine of LEDs in 5 columns and 3 rows, and a sacrifical led between each row (in positions 5 and 11).
+Since the total number of LEDs in this system is 15 + 2 = 17, the `map` must also be 17 elements long.
+We fill those elements with -1.
+
+```json
+{
+  "map": [
+     0,  1,  2,  3,  4,
+    10,  9,  8,  7,  6,
+    12, 13, 14, 15, 16,
+
+    -1, -1
+  ],
+  "width":  5,
+  "height": 3
+}
+```