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<base href="https://bugs.freedesktop.org/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - cairo_rotate inherently inaccurate"
href="https://bugs.freedesktop.org/show_bug.cgi?id=89071">89071</a>
</td>
</tr>
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<th>Summary</th>
<td>cairo_rotate inherently inaccurate
</td>
</tr>
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<th>Product</th>
<td>cairo
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
</tr>
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<th>Hardware</th>
<td>Other
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>medium
</td>
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<th>Component</th>
<td>general
</td>
</tr>
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<th>Assignee</th>
<td>chris@chris-wilson.co.uk
</td>
</tr>
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<th>Reporter</th>
<td>terra@gnome.org
</td>
</tr>
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<th>QA Contact</th>
<td>cairo-bugs@cairographics.org
</td>
</tr></table>
<p>
<div>
<pre>cairo_rotate takes its argument in radians which is one of those
things that work better on a blackboard than on a computer.
Consider the angle M_PI/2. When this angle is rounded to its closest
"double" representative x, we have sin(x)=1 [good!] but cos(x)=6.12323e-17
[not good].
This is an API problem: the damage is already done when cairo_rotate
is entered.
Suggestions:
1. Add cairo_rotate_degrees and/or cairo_rotate_pi, the latter taking
an angle argument that is 2 for a full rotation.
2. Make the backend api use an angle argument that is 2 for a full turn.
3. Use the IEEE 754-2008 functions sinpi and cospi in the backends
where they currently use sin and cos.
<a href="http://www.math.fsu.edu/~gallivan/courses/FCM1/IEEE-fpstandard-2008.pdf.gz">http://www.math.fsu.edu/~gallivan/courses/FCM1/IEEE-fpstandard-2008.pdf.gz</a>
The sinpi and cospi functions are fairly easy to implement. Feel free
to grab code from
<a href="https://git.gnome.org/browse/goffice/tree/goffice/math/go-math.c">https://git.gnome.org/browse/goffice/tree/goffice/math/go-math.c</a>
lines 942-1043 which also has tanpi and atan2pi.
Benefits: the matrices will actually have zeros where you would expect them.
Rotating by 90 degrees four times will actually bring you back where you
started.</pre>
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</p>
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