This is the fourth in a series of posts showing images that have been downsampled using several different algorithms
- Photoshop’s bilinear interpolation
- Ps bicubic sharper
- Lightroom export with no sharpening
- Lr export with Low, Standard, and High sharpening for glossy paper
- A complicated filter based on Elliptical Weighted Averaging (EWA), performed at two gammas and blended at two sharpening levels
The last algorithm is what I consider to be the state of the art in downsampling, although it is a work in progress. It’s implemented using a script that Bart van der Wolf wrote for ImageMagick, an image-manipulation program with resampling software written by Nicholas Robidoux and his associates.
This post using a Fuji demonstration image. This is the first target image that is actually photographic, in that it was captured by an actual, not a simulated, camera.
Here’s the whole target:
Now I’ll show you a series of images downsampled to 15% of the original linear dimensions with each of the algorithms under test, blown up again by a factor of 4 using nearest neighbor, with my comments under each image.
Bilinear interpolation, as implemented in Photoshop, is a first-do-no-harm downsizing method. It’s not the sharpest algorithm around, but it hardly ever bites you with in-your-face artifacts. That’s what we see here.
Photoshop’s implementation of bicubic sharper, on the other hand, is a risky proposition. Look at the halos around the flower stems, the flowers themselves, the clock, and just about everywhere.
With the sharpening turned off, Lightroom’s export downsizing is, as usual, a credible performer. It’s a hair sharper than bilinear — though in this image the two are very close — and shows no halos, or any other artifacts that I can see.
I’ll skip over the various Lightroom sharpening options, and just include the images at the end. We’ve seen before that these don’t provide better performance than no sharpening when examined at the pixel-peeping level, although they might when printed.
For this crop, EWA looks a lot like Lightroom’s export processing, but with some lightening of the first third of the tone curve in high-spatial frequency areas. Look at the clock near the white flower, and the green stems near the yellow flower at the upper left corner.
Withe the deblur dialed up to 100, the image crisps up nicely. The downside is mild haloing around the clock and the stems.
In general, the differences with this scene are less striking than with the artificial targets used in previous posts.