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<HEAD><TITLE>PRINT_SWEEP specification</TITLE></HEAD>
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<H2>Sweep data explanation</H2>
<p>It took me quite a while to understand the explanation of the sweep
data from the HP Journal articles, so I will try to explain it in a
little clearer terms here.
<P>The swing buffer separation applies for black as well as CMY
sweeps. I have only deciphered the ordering for the black data,
however.
<H3>What's a sweep?</H3>
<P>Let's first start off by describing how the nozzles on the
printhead are arranged.
<P>For the black nozzles, there are 300 total nozzles. They are
numbered from 1 to 300. There are two columns of nozzles - odd and
even. The odd colum consists of 150 nozzles separated by 1/300 of an
inch. The even column is similar, but it is shifted down 1/600 of an
inch from the odd column. Thus, we can get 300 dpi by printing with
one column and 600 dpi by printing "between the lines" with the other
column.
<P>For the color nozzles, each color (CMY - cyan, magenta, yellow) has
64 nozzles. They are split into two columns of 32 nozzles each. The
odd column has nozzles separated by 1/150 of an inch, and the even
column has the same set of nozzles, but shifted 1/300 of an inch
down. This allows us to get 300 dpi color. (This may differ slightly
on printers other than the 820Cse)
<P>When the printer prints a graphical image, it must convert the
image data into a form that it can understand. The first task is to
separate the image into <b>swaths</b>. Let us assume that we have a
black-and-white bitmap that is X pixels wide and Y pixels tall that we
want to send to the printer. We must cut this into several swaths,
each of a certain height. For example, we might choose to cut it into
swaths that are 300 pixels high (1/2 an inch). This produces a series
of bitmaps that are X by 300 pixels in dimension.
<P>With this swath data, we can now produce a series of
<b>sweeps</b>. A sweep is simply a set of data that tells the
printhead how to move across the page and what data to print. For
each swath, there may be only one sweep, or there may be multiple
sweeps.
<H3>Formatting the sweep data</H3>
<P>The sweep data is formatted in a very specific way so that the
printer can understand it. The first thing that we need to do is
convert the bitmap data into the correct form. The form that is
required looks like this:
<p><table border>
<tr><td><b>Columns 0-7</b></td><td><b>Columns
8-15</b></td><td><b>...</b></td> <td><b>Columns ceil(X/8)*8-8 to
ceil(X/8)*8-1</b></td></tr>
<tr><td>Byte 0</td><td>Byte 1</td><td>...</td><td>Byte
ceil(X/8)-1</td></tr>
<tr><td>Byte ceil(X/8)</td><td>Byte
ceil(X/8)+1</td><td>...</td><td>Byte 2*ceil(X/8)-1</td></tr>
<tr><Td>...</td><td>...</td><td>...</td><td>...</td></tr></table>
<P>Each of the 8 bit values are packed into a byte in big-endian
order.
<P>To give a more concrete example, assume we have a sweep that is 64
pixels wide and 8 pixels tall. Here is what our data organization
will look like:
<p><table border>
<tr><td></td><td><b>Col 0-7</b></td><td><b>Col 8-15</b></td><td><b>Col
16-23</b></td><td><b>Col 24-31</b></td><td><b>Col
32-39</b></td><td><b>Col 40-47</b></td><td><b>Col
48-55</b></td><td><b>Col 56-63</b></td></tr>
<tr><td><b>Row 0</b></td>
<td>Byte 0</td><td>Byte 1</td><td>Byte 2</td><td>Byte
3</td><td>Byte 4</td><td>Byte 5</td><td>Byte 6</td><td>Byte
7</td></tr>
<tr><td><b>Row 1</b></td><td>Byte 8</td><td>Byte 9</td><td>Byte 10</td><td>Byte
11</td><td>Byte 12</td><td>Byte 13</td><td>Byte 14</td><td>Byte
15</td></tr>
<tr><td><b>Row 2</b></td><td>Byte 16</td><td>Byte 17</td><td>Byte 18</td><td>Byte
19</td><td>Byte 20</td><td>Byte 21</td><td>Byte 22</td><td>Byte
23</td></tr>
<tr><td><b>Row 3</b></td><td>Byte 24</td><td>Byte 25</td><td>Byte 26</td><td>Byte
27</td><td>Byte 28</td><td>Byte 29</td><td>Byte 30</td><td>Byte
31</td></tr>
<tr><td><b>Row 4</b></td><td>Byte 32</td><td>Byte 33</td><td>Byte 34</td><td>Byte
35</td><td>Byte 36</td><td>Byte 37</td><td>Byte 38</td><td>Byte
39</td></tr>
<tr><td><b>Row 5</b></td><td>Byte 40</td><td>Byte 41</td><td>Byte 42</td><td>Byte
43</td><td>Byte 44</td><td>Byte 45</td><td>Byte 46</td><td>Byte
47</td></tr>
<tr><td><b>Row 6</b></td><td>Byte 48</td><td>Byte 49</td><td>Byte 50</td><td>Byte
51</td><td>Byte 52</td><td>Byte 53</td><td>Byte 54</td><td>Byte
55</td></tr>
<tr><td><b>Row 7</b></td><td>Byte 56</td><td>Byte 57</td><td>Byte 58</td><td>Byte
59</td><td>Byte 60</td><td>Byte 61</td><td>Byte 62</td><td>Byte
63</td></tr>
</table>
<P>Our next task is to separate this data into <b>swing buffers</b>. Since
the odd and even nozzles on the printhead are physically separated, we
must extract the appropriate rows of pixels to give to the printer. Two
swing buffers are created for each column of bytes. The first is the
odd swing buffer, and the second is the even swing buffer. These
correspond to which pins will be printing the data. Thus, the odd
swing buffer for the first column in the above example would look like
this: <b>(Note added: from inspection of the
pbm2ppa and pnm2mma code, this seems to be
the even swing buffer, not the odd one (?))</b>
<p><table border>
<tr><td>Byte 0</td></tr>
<tr><td>Byte 16</td></tr>
<tr><td>Byte 32</td></tr>
<tr><td>Byte 48</td></tr>
</table>
<P>And the even swing buffer looks like this:
<b>(Note added: this seems to be the odd swing buffer (?))</b>
<p><table border>
<tr><td>Byte 8</td></tr>
<tr><td>Byte 24</td></tr>
<tr><td>Byte 40</td></tr>
<tr><td>Byte 56</td></tr>
</table>
<H3>Ordering the swing buffers</H3>
<p>Now we have odd and even swing buffers for each of the columns in
the print sweep. The next thing for us to do is order these swing
buffers and send them to the printer.
<p>From experimentation, I have determined the ordering for the black
data on the 820Cse.
<ol>
<li>First, the odd swing buffers for the left 12 columns are sent as
sweep data.
<li>Next, the odd swing buffer for the 13th column is sent, followed by
the even swing buffer for the first column. This alternates until we
run out of odd swing buffers.
<li>Finally, the even swing buffers for the last 12 columns are sent.
</ol>
<p>From experimentation, it seems that the values of the first and
last 12 columns of data MUST be all 0's. Otherwise, strange results
occur, including the repetition of a single swing buffer across the
whole sweep.
<P>Here is a quick reference for the ordering of the swing buffer
data, assuming X columns of data.
<p><table border>
<tr><td>Odd swing buffer for column 0</td></tr>
<tr><td>Odd swing buffer for column 1</td></tr>
<tr><td>...</td></tr>
<tr><td>Odd swing buffer for column 11</td></tr>
<tr><td>Odd swing buffer for column 12</td></tr>
<tr><td>Even swing buffer for column 0</td></tr>
<tr><td>Odd swing buffer for column 13</td></tr>
<tr><td>Even swing buffer for column 1</td></tr>
<tr><td>...</td></tr>
<tr><td>Odd swing buffer for column X-1</td></tr>
<tr><td>Even swing buffer for column X-13</td></tr>
<tr><td>Even swing buffer for column X-12</td></tr>
<tr><td>Even swing buffer for column X-11</td></tr>
<tr><td>...</td></tr>
<tr><td>Even swing buffer for column X-1</td></tr>
</table>
<p>For different printers and color printing, the number of columns to
send before we start interleaving seems to vary.
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