How to generate 2 pixel BMP image with Awk?

sorry for the silly question, but I'm trying to create (generate) a simple BMP image 2x1, 24-bit bitmap with pixel format RGB24, using Awk.

Format is "BitmapFileHeader (2+4+4+4=14 bytes) + DIBHeader (4+4+4+2+2+4+4+4+4+4+4=40 bytes) = 54byte", then start of pixel array (bitmap data). Here is my simple script:

BEGIN {
ORS="";
filebmp="Image.bmp"
# BMP Header: 2+4+4+4=14 bytes
printf("%c%c",66,77)>filebmp;
printf("%c%c%c%c",62,0,0,0)>filebmp;
printf("%c%c%c%c",0,0,0,0)>filebmp;
printf("%c%c%c%c",54,0,0,0)>filebpm;

# DIB Header: 4+4+4+2+2+4+4+4+4+4+4=40 bytes
printf("%c%c%c%c",40,0,0,0)>filebmp;
printf("%c%c%c%c",2,0,0,0)>filebmp;
printf("%c%c%c%c",1,0,0,0)>filebmp;
printf("%c%c",1,0)>filebmp;
printf("%c%c",24,0)>filebmp;
printf("%c%c%c%c",0,0,0,0)>filebmp;
printf("%c%c%c%c",8,0,0,0)>filebmp;
printf("%c%c%c%c",19,11,0,0)>filebmp;
printf("%c%c%c%c",19,11,0,0)>filebmp;
printf("%c%c%c%c",0,0,0,0)>filebmp;
printf("%c%c%c%c",0,0,0,0)>filebmp;

# start of pixel array (bitmap data)
# Blu pixel
printf ("%c%c%c",127,0,0)>filebmp;
# Green pixel
printf ("%c%c%c",0,127,0)>filebmp;
# Padding
printf ("%c%c",0,0)>filebmp;
}

it's works (image below): it generates a BMP image 2x1 and 62 byte size.

http://ift.tt/1DEHTQR

But, now, if in the bitmap data I substitute 127 value with 128 (e.g. in blue pixel), same script Awk generates a BMP image 2x1 but 63 byte size and with "other colours" (see image below)!

http://ift.tt/1NcrOda

I wonder where I am going wrong...

...any ideas?

Thanks & Bye