It is generally not a good idea to rotate raster images (bitmaps, such as TIFF, PCX, BMP, GIF, and PNG) at anything other than 90 degree increments because it causes pixel distortion. The same sort of distortion can occur when enlarging raster images.
However, sometimes other orientations are necessary. When you must rotate, heed these tips:
- Always save an original version of the image and rotate a copy so you have a 'clean' copy to go back to if the distortion is too much.
- If you rotate once but then decide you want to rotate it just a little more or less, go back to the original orientation and rotate the desired amount (that is instead of one rotation of 20 degrees then a second rotation of 5 degrees, go back to the original orientation and rotate once to 25 degrees). This way you limit the amount of distortion.
- Use blur, sharpen, or other filter effects selectively on those areas of the image that need a little tweaking after rotation (to minimize the distorted pixels).