Thanks , I am having a clearer picture now as what you say seems to be exactly what the show() function does. Then, lets say I want to control pixel number three, I will need to send 9 bytes, 3 for the first IC, three for the second and the last three will go to pixel 3. However, the show() routine will go to ALL the pixels , then I imagine I will send unchanged data to the pixels I do not want to undergo any change.
In summary, even if I want to modify the color of pixel three, the routine will fill ALL the pixels with data then I need to keep same value for the pixels I want to leave unchanged. The more difficult to understand is the latching; provided there are less than 128 pixels, show() will send three zeroes at the end of the sequence. Then it must be that when the first chip receives 3 consecutive zeroes , it will latch and pass them thru and so on and so forth until the last chip. But if there are 128 to 192 pixels in the strip, then show() will send SIX zeroes at the end of the transfer. So the total number of pixels seems to affect the way the latch command ( zeroes) is decoded.
Thanks !