[magick-users] disable antialias on rotations with perlMagick

[Anthony Thyssen]

"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>
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    It is a pretty smart tree that can outsmart the average kite flyer.
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