Funny that my default choice is the opposite of his for the exact same reasons he mentions. But it may simply be that we represent differents use cases.
Bilinear interpolation is naive and only continuous to one level. The derivatives are not continuous. Therefore I do not advice using it for upscaling images.
Bicubic interpolation is continuous up to the derivatives. Therefore it gives smoother results when upscaling. Unfortunately it suffers from the Gibbs phenomen a.k.a 'ringing'. It generates 'halo' around the edges as seen on Graphics & Media lab: Image Resampling Algorithms and in the page Understanding Digital Interpolation linked in the comments of the blog post.
The only bad thing I'd say about the page I just mentionned is that it doesn't say that when the scaling ratio is < 1, the neighbours must themselves be and interpolation of neighbours pixels since the neigbourhood of the computed pixel extend beyond 1 pixel distance. ... I wish I knew if PhotoShop, or could force it to, use Mip-mapping.
I do NOT want the ringing ( or Gibbs phenomen : the high frequency bumps if you know a bit about frequency space and Fourier transformations ) produced by Bicubic, especially when downscaling an image. I want to control the sharpness of my images. That's why, depending on what I want to achieve, I take my Bilinear downscaled image and add a subtle pass of sharpen edge, or blend it with its Nearest Neighbour version, or with a its High-Pass version, or sharpen the image a bit before the Bilinear downscaling.
posted 07-08-2007 09:54
