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    <p>The difference between both lines is basically that Photoshop is re-considering the n-1 pixel drawn AFTER drawing the pixel n, and erasing it if it gives positive with one of the following masks: </p> <pre><code> x 1 x x 1 x 1 o x or x o 1 x x x x x x </code></pre> <p>or</p> <pre><code> x x x x x x 1 o x or x o 1 x 1 x x 1 x </code></pre> <p>x= Don't mind<br> o= The n-1 pixel<br> 1= Marked pixel after drawing pixel n </p> <p>Or written as logic:</p> <p>Suppose pixel n-1 is at a[i,j], so, after marking the pixel n, check: </p> <pre><code>If ( (a[i-1,j ]==1 &amp;&amp; a[i ,j-1]==1) || (a[i-1,j ]==1 &amp;&amp; a[i ,j+1]==1) || (a[i ,j-1]==1 &amp;&amp; a[i+1,j ]==1) || (a[i ,j+1]==1 &amp;&amp; a[i+1,j ]==1)) Then Unmark a[i,j]; </code></pre> <p>You may want your to draw your line delayed one pixel just for not showing your "disappearing" pixels (although I doubt it'll be noticeable at that scale).</p> <p><img src="https://i.stack.imgur.com/Dc9Kn.png" alt="alt text"><br> .</p>
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    1. PODrawing 1-pixel thick, aliased lines in real-time
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    1. COThanks for helping out with one of my questions again. The environment that I'm running this in very much favors a preemptive solution over a reactive one ("deleting" a pixel is an involved process, especially when you consider drawing over something that already exists). The issue here is deciding whether or not that one pixel should *really* be deleted. What if the user intended to draw the "L" shape in your diagram? The algorithm would blindly delete it. I'd find it hard to believe Adobe's implementation is insanely complicated with prediction and trajectory tracking...
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    2. CO@Xenethyl If the user is picky about the corners in Photoshop, he will probably use another tool, and not a freehand pencil. But I agree in that PShop may have better algorithms in there. As for drawing over something pre-existent, I thought you have an "undo" command, so Unmarking != Deleting.
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    3. COAgreed that user preference comes into this a little bit. Unfortunately, this issue is at the core of my drawing algorithm for any other drawing/painting tools (ie. pencil with different tips, paintbrush, etc). Every line that the user draws is treated as a 1px line, and if a tip (ex. a star) is being used it's just stamped at each pixel coordinate. Undo is not implemented yet, but the process would be similarly involved. Changing pixel data requires a copy, read, change, and write in this environment, unfortunately.
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