Up until now, everyone has been used to using a standard C array to access their RGB objects. This lets you do things like the following:

for(int i = 0; i < NUM_LEDS; i++) { leds[i] = CRGB::Red; FastLED.delay(33); leds[i] = CRGB::Black; }

This moves a simple red dot down the line of leds. What if you wanted to make the dot two pixels wide? You used to have to do something like this:

for(int i = 0; i < NUM_LEDS-1; i++) { 
  leds[i] = CRGB::Red; 
  leds[i+1] = CRGB::Red;
  FastLED.delay(33); 
  leds[i] = CRGB::Black; 
  leds[i+1] = CRGB::Black;
}

Now, though, FastLED is adding a new container class, CRGBSet which allows you to work on portions of your led array in one shot. For example, if you want a 2 led wide dot, assuming leds is a CRGBSet you can just do:

for(int i = 0; i < NUM_LEDS-1; i++) { leds(i,i+1) = CRGB::Red; FastLED.delay(33); leds(i,i+1) = CRGB::Black; }

If you could do it to an individual pixel before, you can do it to a set of pixels now. In addition, CRGBSet can be passed into most places where you currently pass in CRGB*. So, starting at the beginning, with creating a CRGBSet - here's a simple demo program:

#include <FastLED.h>
#define NUM_LEDS 40

CRGBArray<NUM_LEDS> leds;

void setup() { FastLED.addLeds<APA102>(leds, NUM_LEDS); }
void loop() { static uint8_t hue=0; leds.fill_rainbow(hue++); FastLED.delay(30); }

This simply fills a rainbow in on the main CRGBSet called leds. What if you want a mirrored type effect? You could replace your loop line with something like this:

void loop() { 
  static uint8_t hue=0;
  leds(0,NUM_LEDS/2 - 1).fill_rainbow(hue++);  // fill the first 20 items with a rainbow
  leds(NUM_LEDS/2, NUM_LEDS-1) = leds(NUM_LEDS/2-1,0);
  FastLED.delay(30);
}

CRGBSet is a reference object. It doesn't have any actual pixel data of its own, rather, it's a reference to some set of other pixel data. How do you get other pixel data in there? There's a couple of ways. The first is you can have a pointer to CRGB objects or an array of CRGB objects that you then use to make your initial set out of. For example:

CRGB *realleds[NUM_LEDS];
CRGBSet leds(realleds, NUM_LEDS);

This creates an RGBSet that references the array of real leds. Or, alternatively (and preferably, going forward), you can use a type of RGBSet called CRGBArray - which is an RGBSet that has its own set of pixel data:

CRGBArray<NUM_LEDS> leds;

Now you can do things on all of your leds - or a subset of leds, quite easily. For example, you could fade all the leds to black:

leds.fadeToBlackBy(40);

or you could just do a subset:

leds(0,9).fadeToBlackBy(40);

If you are using an environment/compiler that supports C++11 (Arduino 1.6.6 for some platforms) you can even use the new ranged for loop to iterate over your pixels:

for(CRGB & pixel : leds) { pixel = CHSV(hue++,255,255); }

or you could just do something to a subset of your pixels:

for(CRGB & pixel : leds(8,10)) { pixel = CRGB::Black; } 

(of course, the above is equivalent to just saying leds(8,10) = CRGB::Black; - but this is just to give the basic idea for how it could be used). Note that in testing it is faster to use this style of loop vs. the more traditional

for(int i = 0; i < NUM_LEDS; i++) { leds[i] = CHSV(hue++,255,255); }

The loop overhead is reduced 10-20% when using CRGBSet iterators.

What's that, you say? Your compiler doesn't support C++11/14, and you want the faster loops? Don't worry - it takes a little bit more code to do - but you can get them as well:

    for(CRGBSet::iterator pixel=ml.begin(), end=ml.end(); pixel != end; ++pixel) { (*pixel) = CRGB::Black; }

I know, it's a little uglier. Get thee to C++11!

This is a work in progress - you can see some more ideas on things you do on the g+ post here - https://plus.google.com/102282558639672545743/posts/a3VeZVhRnqG - and I will try to update this page with more documentation and examples as I work on/expand this functionality.