Re: [GM-help] Jittery rotate operation -- help with SRT operations

[Bob F. <bfr...@si...>]

On Tue, 10 Sep 2013, Jonathan Haws wrote:

> All,
>
> I am having troubles using Magick++ to rotate an image about its center.  Here is what I have right now:
>
>    img.rotate(rotation);
>    uint32_t xoff = (img.size().width() - m_image.img.size().width()) / 2;
>    uint32_t yoff = (img.size().height() - m_image.img.size().height()) / 2;
>    Magick::Geometry newsz(m_image.img.size().width(), m_image.img.size().height(), xoff, yoff);
>    img.crop(newsz);
>    img.write(&blob);
>
> However, what I am seeing is what is shown in this ImageMagick forum post:
>
> http://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=17792#p67335
>
> Notice the jitter in the first Koala...I am seeing exactly that.
>
> However, what I am having trouble finding is information on how to perform a SRT operation on the image to remove that.  Can anyone here help point me in the right direction?

GraphicsMagick does not (and likely will never) support ImageMagick's 
distortion algorithm since it was developed by Anthony Thyssen over 
several years strictly for ImageMagick and not contributed to 
GraphicsMagick.

I expect that by understanding the image output by GraphicsMagick's 
rotate (it creates a larger square image wholely containing the 
rotated original) that an intelligent crop can be added to eliminate 
the jitter.  The key is to compute the coordinates of the center so 
the crop is correct.  First one would want to composite the original 
image onto the center of a larger square canvas (e.g. white) image so 
that the image can be repeatedly rotated and cropped without losing 
any of the original image content.  I think that you would want to 
perform the same composite, rotate, crop operation for each step of 
the rotation.

ImageMagick's distortion/warping algorithm is exceedingly slow so a 
relatively clumsy algorithm based on rotate (which is itself a bit 
clumsy, as Anthony describes in his forum posting) is likely to still 
be considerably faster.

Bob
-- 
Bob Friesenhahn
bfr...@si..., http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/