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POBilinear Interpolation image resizing marks & 'splotches'
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  1. POBilinear Interpolation image resizing marks & 'splotches'
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    copied!<p>I'm making an image resizing program in C and at the moment I'm having trouble with the Bilinear Interpolation function (it's one of many I'm using). This problem only arises for 16-bit bitmaps, If I use 24-bit versions, it resizes them perfectly.</p> <p>Here's my code for the Bilinear Interpolation. n_w and n_h are the new widths and heights of the image:</p> <pre><code>#define getelm(x) (((pix+index)-&gt;x)*(1-xdiff)*(1-ydiff))+(((pix+index+1)-&gt;x)*(xdiff)*(1-ydiff))+(((pix+index+o_w)-&gt;x)*(1-xdiff)*(ydiff))+((pix+index+o_w+1)-&gt;x)*xdiff*ydiff int pad = (2*n_w) &amp; 3; if (pad) pad = 4-pad; uint16_t *buffer; if (buf) buffer = malloc(2*n_w); for (i = 0; i &lt; n_h; i++) { for (j = 0; j &lt; n_w; j++) { x = (int)(j*xrat); y = (int)(i*yrat); xdiff = (xrat*j)-x; ydiff = (yrat*i)-y; index = y*o_w+x; uint16_t container = 0; container |= (int)(round(getelm(b))) &lt;&lt; 11; container |= (int)(round(getelm(g))) &lt;&lt; 5; container |= (int)round(getelm(r)); if (buf) *(buffer+j) = container; else fwrite(&amp;container, 1, 2, dest); } if (buf) fwrite(buffer, 1, 2*n_w, dest); fwrite(&amp;pad, 1, pad, dest); } </code></pre> <p>My 24-bit version of this code (where the only difference is that the container is not used and instead 3 8-bit integers hold the RGB values) works beautifully.</p> <p>This code, however, gives weird results. Look at the image below:<img src="https://i.stack.imgur.com/Bk0Su.png" alt="Original image"></p> <p>When I resize this, it gives me this back: <img src="https://i.stack.imgur.com/XQVXy.png" alt="Resized image"></p> <p>I can't see why this would be happening, especially when it works for 24-bit bitmaps, and also that some other resizing algorithms (Nearest Neighbour for example) work with 16-bit in the same way that this should.</p> <p>EDIT: I don't think it's an overflow problem, because adding the following code gives no output when run:</p> <pre><code>if (MAX((int)(getelm(b)), 31) &gt; 31) printf("blue overflow: %.10f\n", (getelm(b))); if (MAX((int)(getelm(g)), 63) &gt; 63) printf("green overflow: %.10f\n", (getelm(g))); if (MAX((int)(getelm(r)), 31) &gt; 31) printf("red overflow: %.10f\n", (getelm(r))); </code></pre> <p>EDIT 2: I don't think it's an underflow problem either, this does nothing:</p> <pre><code>if ((getelm(b)) &lt; 0 || (getelm(g)) &lt; 0 || (getelm(r)) &lt; 0) printf("Underflow\n"); </code></pre>
 

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