Using byteArray with getPixel (no 's') then setPixelS - actionscript-3

Question

Here is what I am trying to do:
one byteArray (colorsByteArray) holds color values, that I created with the "getPixels" method;
for every pixel on screen, I make calculations and depending on that I get a rank that I use to retrieve the proper color value inside colorsByteArray;
finally, I write inside a second byteArray (pixelsByteArray) the color I just retrieved. Then I 'draw' this byteArray using the "setPixels" method.
Problem is I do not find the same image as if I simply wrote a "setPixel" for every pixel in my loop. The image I get is missing one line out of two, and is kind of repeated three times horizontally. Of course I guess I must be writing (or reading) something wrong from my second byteArray, but can someone explain me what I am doing wrong? Or even if my efforts are worth anything actually :-) cause I didn't find anything quite like this so far...
Thanks!
Here is a bit of my code to make things clearer:
colorsByteArray = bitmDext.getPixels(new Rectangle(0, 0, myShadeBMD.width, 1));
canvas.lock();
for(var i:int = 0; i < STAGE_WIDTH; i++)
{
for(var j:int = 0; j < STAGE_HEIGHT; j++)
{
//make some calculation on each pixel to get the 'colorRank' uint value
colorsByteArray.position = colorRank * 4;//unsigned int is 32 bytes, representing 4 slots of a byteArray
var colorValue:uint = colorsByteArray.readUnsignedInt();
//canvas.setPixel(i, j, colorValue);//works just fine
pixelsByteArray.writeUnsignedInt(colorValue);
}
}
pixelsByteArray.position = 0;
var myRect:Rectangle = new Rectangle(0, 0, STAGE_WIDTH, STAGE_HEIGHT);
canvas.setPixels(myRect, pixelsByteArray);
canvas.unlock();
added a picture that may help:
http://www.oghel.com/pictures/stackOverflow/example3
the expected part is on the right, the wrong one on the left.