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POWhy is my image rotation algorithm not working?
 

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  1. POWhy is my image rotation algorithm not working?
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    <h3>Attempts 1 &amp; 2:</h3> <p><em>Note:</em> Removed first attempts to cut down on question size. See community wiki for previous attempts.</p> <h3>Attempt 3:</h3> <p>As per fuzzy-waffle's example, I have implemented the following, which doesn't appear to work correctly. Any ideas what I could be doing wrong?</p> <pre><code>ImageMatrix ImageMatrix::GetRotatedCopy(VDouble angle) { // Copy the specifications of the original. ImageMatrix &amp;source = *this; ImageMatrix &amp;target = CreateEmptyCopy(); double centerX = ((double)(source.GetColumnCount()-1)) / 2; double centerY = ((double)(source.GetRowCount()-1)) / 2; // Remember: row = y, column = x for (VUInt32 y = 0; y &lt; source.GetRowCount(); y++) { for (VUInt32 x = 0; x &lt; source.GetColumnCount(); x++) { double dx = ((double)x) - centerX; double dy = ((double)y) - centerY; double newX = cos(angle) * dx - sin(angle) * dy + centerX; double newY = cos(angle) * dy + sin(angle) * dx + centerY; int ix = (int)round(newX); int iy = (int)round(newY); target[x][y][0] = source[ix][iy][0]; } } return target; } </code></pre> <p>With this prototype matrix...</p> <pre><code>1 2 1 0 0 0 -1 -2 -1 </code></pre> <p>... prototype.GetRotatedCopy(0) (which is correct) ...</p> <pre><code>1 2 1 0 0 0 -1 -2 -1 </code></pre> <p>... prototype.GetRotatedCopy(90) (incorrect) ...</p> <pre><code>-2 0 0 -2 0 2 0 0 2 </code></pre> <p>... prototype.GetRotatedCopy(180) (incorrect - but sort of logical?) ...</p> <pre><code>0 -1 -2 1 0 -1 2 1 0 </code></pre> <p>... prototype.GetRotatedCopy(270) (incorrect - why is this the same as 0 rotation?) ...</p> <pre><code>1 2 1 0 0 0 -1 -2 -1 </code></pre> <h3>Solution:</h3> <p>As pointed out by Mark Ransom, I should be using radians, not degrees; I have adjusted my code as follows:</p> <pre><code>ImageMatrix ImageMatrix::GetRotatedCopy(VDouble degrees) { // Copy the specifications of the original. ImageMatrix &amp;source = *this; ImageMatrix &amp;target = CreateEmptyCopy(); // Convert degree measurement to radians. double angle = degrees / 57.3; // ... rest of code as in attempt #3 ... </code></pre> <p>Thanks for all your help guys!</p> <pre><code>1 2 1 0 0 0 -1 -2 -1 1 2 1 0 0 0 -1 -2 -1 -1 0 1 -2 0 2 -1 0 1 -1 -2 -1 0 0 0 1 2 1 1 0 -1 2 0 -2 1 0 -1 </code></pre>
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    1. USNick Bolton
    1. USNick Bolton
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      1. POWhy is my image rotation algorithm not working?
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    1. CODo you mean newRow and newColumn for your output (not targetRow and targetColumn)?
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    2. COAny chance you're using degrees instead of radians for the angle? Read fuzzy-waffle's answer more carefully.
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      1. USMark Ransom
    3. COAh, yes - just updated my question with the method calls. I will try with radians.
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