How do I prevent interpolation when rotating an image.

[rmcooke]

I am something of an Imagemagick noob and have been attempting to resize and rotate images with ImageMagick in the command line using this command:

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convert -resize 230x330 -rotate 15 -transparent white laugh.png laugh_skewed.png

At the same time, I want to make the resulting white background transparent.

The whole thing works pretty well, except I seem to end up with an interpolated border merging the white background produced by the rotation into the original image.

Is there any way to turn off this effect so that only the background and the transposed pixels from the original image are produced?

Here are the before and after images.



The effect is difficult to see on a light background, but the site design requires the image to be overlaid on a black background so I am attaching a screenshot clip below.




It occurred to me that first adding a black border to the original image would alleviate the problem, but it did not. The jagged light border still appeared outside the black border.

Many thanks for any help you can give.


Richard

[rmcooke]

Apologies for answering my own post so soon after asking it.

The solution I found was this:

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convert -background black -bordercolor black -border 2x2 -resize 230x330 -rotate 15 -transparent black laugh.png laugh_skewed.png

[anthony]

The actual answer for, what you asked, and not, what you wanted.
(for others who are looking at this topic).

Your solution is not turning off interpolation in rotate....

Rotate is a very old operator in IM, and is based on a method involving shearing the image three times.
A technique known as 'Rotate by Shear' (RBS) and first publish in research papers by Alan Paeth (Paeth Rotate), in Graphics Gems, p. 179, edited by Andrew Glassner, published by Academic Press, 1990.
See Leptonica Rotation for more details.

See IM examples, Rotate which explains the above (with an example of a thin line)
http://www.imagemagick.org/Usage/warping/#rotate

Shears however involve sliding rows and columns of pixels, and this has hardcoded interpolation of the translated pixels.


If you want true no-interpolated rotation. You will need to use Distort with settings that turn off its Area Resampling, and the its Interpolated Lookup (set that to Interger, or NearestNeighbour) You also need to set how you want Virtual Pixels (pixel that are outside the original image) to be interpreted. You cna then rotate using SRT distortion.

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   convert input.png -alpha set \
               -filter point -interpolate NearestNeighbor  -virtual-pixel transparent \
               -distort SRT 30     output.png

See IM Exmaples Image Distortion (in general for effects of 'mapped distortions')
http://www.imagemagick.org/Usage/distorts/
and more specifically SRT (Scale-Rotate-Translate) distortion
http://www.imagemagick.org/Usage/distorts/#srt