[magick-users] disable antialias on rotations with perlMagick

[Sergei Steshenko]

--- Anthony Thyssen <anthony at griffith.edu.au> wrote:

> "Ian" on  wrote...
> | Currently, I've got code like this:
> | --cut--
> | #!/usr/bin/perl
> | 
> | use warnings;
> | use strict;
> | use Image::Magick;
> | 
> | my $image = Image::Magick->new();
> | $image->Read('testimage.jpg');
> | $image->Set(antialias=>'False');
> | --cut--
> | 
> | Whenever I do a rotate, antialias still seems to mung up the image... It
> | get worse with consecutive rotates:
> | 
> | --cut--
> | #for a blurry mess:
> | for (my $i=0; $i<45; $i++)
> | {
> |   $image->Rotate(degrees=>1);
> | }
> | --cut--
> | 
> | I was wondering if there was a way to turn off antialiasing for rotation,
> | or a different method to rotate an image w/o otherwise altering the data?
> 
> The anti-aliasing involved with rotation isn't actually anti-aliasing,
> but a merging of parts of the pixels that go to making up the new pixel
> after the rotation.
> 
> There are three ways a rotation can select colors.
>    1/  Just take the color of the point that equates to the roated image
>        This will never add new colors to an image, but some pixels may
>        be duplicated, will other pixel may not ne used in the final
>        image.   Eg you loose information.
> 
>        This is what happens in the "-fx" 'distortion mapping technique
>    http://www.cit.gu.edu.au/~anthony/graphics/imagick6/distorts/#position_maps
> 
>        You could apply this method for implemented you 'aliased
>        rotations'.  I even have an example of a 45 degree rotation.
>        It is however slow, and it is basically a DIY, rotation using
>        lookup maps.
> 
>    2/  You can select the color of the point by mixing the colors of the
>        four pixels involved with this point in the new image (weighted
>        apporpaitally by distance).  This is called Interpolation.
> 
>        It also is not exact, and can produce errors and morie effects.
> 
>        Currently this is the only method used for 'Displacement maps'
>        See
>        http://www.cit.gu.edu.au/~anthony/graphics/imagick6/compose/#displace
> 
>    3/  The totally correct method is to reverse map the area of the
>        pixel you are trying to color in, back onto the original image
>        and figure out the amount of color that maps from the orignal
>        image into that pixel area.   This is what IM does, and it does
>        produce the most correct results.
> 
>        This is what was recently implemented onto Affine Transformations
>        (which can do rotations)
>        http://www.cit.gu.edu.au/~anthony/graphics/imagick6/distorts/#affine_rot
> 
> So you see, rotations don't have anti-aliasing.
> It only seems like it does :-)
> 
> PS: remember anything you do on the command line should be translatable
> somehow into a API such as PerlMagick.
> 
> 
>   Anthony Thyssen ( System Programmer )    <A.Thyssen at griffith.edu.au>
>  -----------------------------------------------------------------------------
>     It is a pretty smart tree that can outsmart the average kite flyer.
>                                                 --- Gary <gengvall at aol.com>
>  -----------------------------------------------------------------------------
>      Anthony's Home is his Castle     http://www.cit.gu.edu.au/~anthony/
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> 

Image rotation == resmapling by definition. I suggest to read
"The quest for the perfect resampler" article. You can
google for it, but the original site appears to be down.

Here is an alternative: http://citeseer.ist.psu.edu/630923.html
- look in the right side for "View or download:", a little bit
below and to the left "Cached:  PDF".

I've just checked it, the PDF file is still there.

A must read for anyuone dealing with image rotation, though
not an ImageMagick issue/article.

--Sergei. 

Applications From Scratch: http://appsfromscratch.berlios.de/

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