36 lines
2.3 KiB
HTML
36 lines
2.3 KiB
HTML
<html>
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<head>
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<!-- __CINERA_INCLUDES__ -->
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</head>
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<body>
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<!-- __CINERA_MENUS__ -->
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<!-- __CINERA_PLAYER__ -->
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<article id="video-notes">
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<h1><!-- __CINERA_TITLE__ --></h1>
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<p>Masking the write:</p>
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<p>In SIMD, doing operations "4-wide" means that one wide (packed) operation operates on four pixels. So there's no
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difference between doing an operation on one pixel or two or three or four, except when it comes to reading and
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writing.</p>
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<p>The way we can make sure we only write the pixels we're actually operating on meaningfully is by masking out the ones we
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aren't. Instead of doing a conditional check every loop, we want to build a mask that's filled with 1s in the places
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where we'll keep the pixels, and 0s in the places where we'll throw out the pixels.
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If we're operating on four pixels at once and we're hanging 2 off the edge, the mask might look like:</p>
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<p>[0x00000000,0x00000000,0xFFFFFFFF,0xFFFFFFFF]</p>
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<p>By doing a bitwise AND with the pixel data we generate, we can mask out the values that are invalid, since the zeroes in
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the mask will knock out any bits set in our data. Likewise, the 1s will ensure any values we want to keep will remain in
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place.</p>
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<p>We still need to preserve the destination how it was, and the easiest way to do that is to remember what the destination
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looked like before, and use those values wherever we knocked out values in our data. So we generate an inverted mask
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that might look something like:</p>
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<p>[0xFFFFFFFF,0xFFFFFFFF,0x00000000,0x00000000]</p>
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<p>Using the same AND technique, we can grab out the destination values that should remain unchanged. Then, we can combine
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that with the set of valid pixel values we generated using the other mask using a bitwise OR. Since the places where the
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two sets of values overlap are set to 0s in one of them, the data will effectively just be copied from one onto the
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other with no interference.</p>
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</article>
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</body>
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</html>
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