Description:
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Drawing an arc with an image thickness of other than 1 produces very strange results. The thicker the line specified, the "hairier" it gets. At 5px it's uneven, 25px quite ragged, at 50px and above it gets very non-round. The best way I can show the problem is with some sample images:
http://www.silisoftware.com/temp/imagearc/

Not sure if it's a related issue, but I notice if I use the same code to draw an arc from 1-360 degrees, it looks like a big hairy "+" instead of a "o", but if I draw from 0-360 degrees it becomes a 1px circle, no matter what I specify for ImageSetThickness.

Reproduce code:
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$imagethickness = 100;

$gdimg_output = ImageCreateTrueColor(400, 400);
$color_bg = ImageColorAllocate($gdimg_output,   0, 0, 255);
$color_fg = ImageColorAllocate($gdimg_output, 255, 0,   0);
ImageSetThickness($gdimg_output, $imagethickness);
ImageFilledRectangle($gdimg_output, 0, 0, 400, 400, $color_bg);
ImageArc($gdimg_output, 200, 200, 200, 200, 90, 360, $color_fg);
ImagePNG($gdimg_output);


Expected result:
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100px thick, 270-degree red arc on blue background

Actual result:
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This: http://www.silisoftware.com/temp/imagearc/100.png

270-degree red arc on blue background, but thickness varies from 1px to 100px, it looks "hairy", with a tendency to elongate into a "+" rather than a "o" (more pronounced the thicker the line is set to).