Explain in detail how the GW command functions 

Facts:  

EPL Command used to download graphics

Details

Here is a very simple test file to illustrate the use of the GW command. It draws a 16x16 dot box at coordinates 100,100.

hex                           hex                                                                ascii
address                byte values                                                   equivalent
=======                ===========                                                ==========

                    0 1   2 3   4 5      6 7   8 9    A B   C D    E F        0123456789ABCDEF

The GW command basically allows you to send a bitmapped graphic directly to the image buffer.

The EPL2 Programmer's Guide does have an error -- it should show either a comma or carriage return/line feed between p4 and DATA.

              The correct syntax is
             GWp1,p2,p3,p4,DATA
              -or-
              GWp1,p2,p3,p4[cr...]<lf>DATA

There is no need for a carriage return/line feed after the DATA, but it doesn't hurt either.

The p3 parameter is the width of the graphic in dots (which must be a multiple of 8) divided by 8 to convert it to bytes.

The graphic data consists of p3 * p4 binary bytes. Each byte has 8 bits, of course, and each bit represents one graphic dot. In the binary data, a 1 = white dot and 0 = black dot.

Each group of p3 bytes represents one "raster line" in the bitmap graphic, starting at the top and working down. The most significant bit in each byte represents the left most dot in that group of 8 dots.

Here is a conversion between the hexadecimal digits shown in the dump and their binary equivalents:
Hex Binary
0 0000
1 0001
2 0010
3 0011
4 0100
5 0101
6 0110
7 0111
8 1000
9 1001
A 1010
B 1011
C 1100
D 1101
E 1110
F 1111

To represent the 16x16 square box, the data (shown in boldface in the dump above) is as follows.

bytes  bits
00 00  0000000000000000
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
7F FE  0111111111111110
00 00  0000000000000000

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