[Image-SIG] Re: aliasing on resize or thumnail?

[Clover Andrew]

David Jeske <jeske@chat.net> wrote:

> As near as I can tell, the filter paramaters in resize() have no
> effect.

They do blur the picture a little for me, in comparison if NEAREST. I
believe what is happening - by design - is that BILINEAR and BICUBIC are
just exactly that: they calculate each pixel color from the four pixels
closest to it in the original image. This is fine for scaling up of
course, but to get a smooth downsampling you need to average out every
pixel in the source image covered by the destination pixel, not just the
four at the corners.

These options work fine if you're reducing an image to one-half
(linearly) of its current size, but are pretty poor for any other scale
factor below 1. It would be very handy to have some new filter options
for scaling down, since thumbnails are indeed an extremely common
application of scripted image processing.

--=20
Andrew Clover
Technical Consultant
1VALUE.com AG