c++ - An algorithm for iterating over a rectangular area inside a 1 dimensional array (bitmapping) -

this odd question had tough time writing title for.

i'm working pixels (bitmaps, more specifically) , can't figure out (simple) maths pragmatically accessing each array cell.

my canvas [n16 x 16] pixels, n 1 or greater.

here's photo of basic n = 2 canvas:

http://i.imgur.com/mabwqfj.png

what want magical algorithm run 0 495 without touching lighter grey area go 16 512 (which cell 511, bad) without touching dark grey area.

so, 0 15, skip 16 31 followed 32 47, etc.

and n = 3:

http://i.imgur.com/tqjmwl6.png

in case 0-735 skipping lighter grey areas, 16-751 skipping areas on each side , 32-767 skipping darker grey areas.

what tried:

here's extract code, it's useful , shows tried already. it's part figures out value 'idxpos'.

// let's length = 3 now. (int character = 0; character < length; ++character) {     // in case you're wondering, grabs 16x16 characters ascii spritesheet     charpos = (string[character] - ' ') * 16 * 16;      // runs through spritesheet character map     // huge 16x1520 bitmap.     (int pixel = 0; pixel < 16 * 16; ++pixel)     {         // ignore this, me messing around pixel tinting         r = (((charmap[charpos + pixel] >> 0) & 0xff) + 255 - u);         g = (((charmap[charpos + pixel] >> 8) & 0xff) + 255 - v);         b = (((charmap[charpos + pixel] >> 16) & 0xff) + 255 - w);         newcolour = rgb(r, g, b);          // part stuck on:         idxpos = pixel + (character * 16 * 16);          bitmap[idxpos] = charmap[charpos + j];     } } 

you idea. sounds dead simple me can't figure out.

oh, , i'm not interested in magical library can handle bitmap stuff me, i'm not in position can use one.

if question properly, want visit them in order mentioned. here's code (given n):

for(int = 0; < n; i++) //which section going through {   for(int row = 0; row < size; row++) //size = 16, better use on of constants   {      for(int col = 0; col < size; col++)     {       int pixelindex = size * (row * n) + col + size * i;        /*the last 1 offset - moves        index right far need.       if need 2 coordinates (as in (x,y))       instead of 1 number, is: */       int x = row, y = col + size * i;       dosomethingwithpixel(pixelindex);     }   } } 

hope helps.