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    <p>The algorithm for this took three main stages:<br> 1) Translate the Y from top left to bottom left.<br> 2) Break up the X into word:bit values.<br> 3) Use Bresenham's algorithm to draw lines between the points. And then figure out how to <a href="https://stackoverflow.com/questions/1222713/how-do-i-create-a-line-of-arbitrary-thickness-using-bresenham/18950950#18950950">make the line thicker</a>.</p> <p>For <a href="https://forum.sparkfun.com/viewtopic.php?f=14&amp;t=36254&amp;sid=091846753e1e51bb2dc126d3597d0bfc&amp;start=15#p164392" rel="nofollow noreferrer">my exact case</a>, the target bitmap is 384x384, so requires 19k of SRAM to store in memory. I had to ditch the "lame" Arduino Mega and upgrade to the <a href="http://www.adafruit.com/products/949" rel="nofollow noreferrer">ChipKIT uC32</a> to pull this off, 32k of RAM, 80 MHz cpu, &amp; twice the I/O!</p> <p>The way I figured out this was to base my logic on <a href="https://github.com/adafruit/Adafruit-Thermal-Printer-Library" rel="nofollow noreferrer">Adafruit's Thermal library</a> for Arduino. In their examples, they include how to convert a 1-bit bitmap into a static array for printing. I used their GFX library to implement the setXY function as well as their GFX Bresenham's algorithm to draw lines between (X,Y)s using my <code>setXY()</code>.</p> <p>It all boiled down to the code in this function I wrote:</p> <pre class="lang-c prettyprint-override"><code>// *bitmap is global or class member pointer to byte array of size 384/8*384 // bytesPerRow is 384/8 void setXY(int x, int y) { // integer divide by 8 (/8) because array size is byte or char int xByte = x/8; // modulus 8 (%8) to get the bit to set uint8_t shifty = x%8; // right shift because we start from the LEFT int xVal = 0x80 &gt;&gt; shifty; // inverts Y from bottom to start of array int yRow = yMax - y; // Get the actual byte in the array to manipulate int offset = yRow*bytesPerRow + xByte; // Use logical OR in case there is other data in the bitmap, // such as a frame or a grid *(bitmap+offset)|=xVal; } </code></pre> <p>The big point is to remember with an array, we are starting at the top left of the bitmap, going right across the row, then down one Y row and repeating. The <code>gotchya's</code> are in translating the X into the word:bit combo. You've got to shift from the left (sort-of like translating the Y backwards). Another gotchya is one-off error in bookkeeping for the Y. </p> <p>I put all of this in a class, which helped prevent me from making one big function to do it all and through better design made the implementation easier than I thought it would be.</p> <p>Pic of the printout:<br> <img src="https://i.stack.imgur.com/U7hVC.jpg" alt="Output from TPS test bench I made"><br> <a href="http://forums.adafruit.com/viewtopic.php?f=22&amp;t=44710" rel="nofollow noreferrer">Write-up of the project is here</a>.</p>
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