In our last workshop in the Bristlecone Pines, several students were concerned that their star trail photos showed strange blurring and “jaggedness”. Before I show you any of the photos, can you guess what the cause might be? I can assure you that their photos were pleasingly sharp and there was no motion blur.
There are two primary causes of the jaggies, and both are easily rectified. The artifact occurs due to Aliasing which produces a Moire-like pattern. Say What? Take a large image with parallel or concentric lines, try to fit it onto a smaller image area and strange patterns may occur.
See the image below for an example. It looks like the star trails have turned to mush in the selected area. Meanwhile just below, it appears the star trails look like the edge of a saw blade. What is that about?
This is a big file. It’s 5616 x 3744 pixels, and we are viewing it at only 440 x 330 pixels. To show the whole image on a small space many pixels have to be skipped or dropped. When scaling down like this – for the DISPLAY – we got a Moire pattern due to aliasing – just like the trippy effect when the weatherman wears pinstripes. The effect can also be caused by viewing the file at a different aspect ratio, and some programs show even more harsh moire pattern.
But before we panic, let’s zoom in to 50%. For this view the program has had to mash two pixels together to see one pixel on the screen. Even now we still see a little bit of the effect.
Only at 100% do we notice that there really is no smeary pattern after all! In fact, that’s when we notice that we have tiny gaps in our star trail. Do we worry about those gaps… nah. Not only are they easy to fix, but as we’ve already seen by the example above, we might not see them in the finished image anyway! If after zooming in you STILL see the blur or jaggies, you might have problem number two (see below). OR it could be that you’ve downsized the image so much that the aliasing artifacts have shown up.
The other cause of jaggies is not using your display at its native resolution. So, for example if you have a 1920 x 1200 (1.6 aspect ratio) monitor but are displaying your photo at 1280 x 960 (1.42 aspect ratio) your *display* is likely to look distorted. The squashing or stretching of pixels on the display will increase the strange effect. The fix is easy: Use the native resolution of your display! Can’t speak for you Mac people, but on most windows machines, the “recommended” resolution is the best for proper display.
What if The Blur Or Jaggies are Visible at 100%?
If after looking at a 100% view on a properly configured display the problem is still noticeable, there is no hope except to start over with the full size image and try a different downsizing algorithm. E.g. Bicubic smoother instead of sharper, or “bilinear” or “nearest neighbor” – all of these options appear in the Image Size for resampling of Photoshop. The latest version of Photoshop (CC) seems to pick a good algorithm if left in the “Automatic” mode.
Here is one of the images that student, Susan Starr, noticed jaggies with. I told the browser to shrink the vertical size – but I did not change the actual image size, The effect is not very noticeable on my display. Click the image for the correct aspect image size.
you showed us the problem, but didn’t tell us how to fix it. Is there a way to make the anti-aliasing “go away” or “work for us”?
But Burt, I explained the problem is not a problem at all! It’s an artifact of viewing a large document in a small space. I did notice that my second cause was missing… so I added it. It’s a matter of getting the resolution correct on the monitor.
If after looking at a 100% view on a properly configured display the problem is still noticeable, there may be no hope except to start over with the full size image and try a different downsizing algorithm. E.g. Bicubic smoother instead of sharper, or “bilinear” or “nearest neighbor” – all options in the Image Size for resampling in Photoshop. The latest version of Photoshop (CC) seems to pick a good algorithm if left in the “Automatic” mode.
hmmm… I would have thought there would be a solution in something like Photoshop resizing. I understand it is aliasing, but that doesn’t change the fact that it looks bad at the size being presented.
Just as there are algorithms and plugins to intelligently enlarge an image, creating ‘new pixels’ by interpolation, I would have thought there would be an intelligent reduction mechanism to avoid the aliasing.
In truth I’ve not noticed a problem when downsizing images using “automatic” in Photoshop CC. Automatic is the equivalent of “bicubic sharper” in any prior version. I trust Photoshop to do a better job downsizing than allowing say a browser or smartphone to do it. But if you’re using say flickr or 500px or Facebook for example, you don’t have much say in whatever downsizing those things do to your large images. You can reduce the moire by blurring the image, but that’s not a very nice solution.
Do you have an example of an image with an aliased star trail pattern when viewed at “full size” after downsizing?
All the examples I’ve been asked about were cases where people were viewing large documents in a small window. When viewing a large document in a small window the obvious solution is to not do that – and that was the thrust of the article.
The choice for a downsampling algorithm goes with what you want to do and the goal at hand. Bicubic spline algorithms are fine, but they’re crap when the downsampling is high. If you want a very smooth aliasing-free result (but losing sharpness considerably), go for something like cubic B-spline. If you want to preserve sharpness without the dreaded “stairs” effect, use an algorithm based in Mitchell-Netravali or Catmull-Rom.
Thanks, Rogelio. For the readers who are wondering… the resampling alogrithms that Rogelio is mentioning are not natively present in Photoshop. Here is an interesting, not too technical article comparing upsizing using the different algorithms. To get significantly different resizing algorithms you’ll need to use GIMP (free) ImageMagick (free), or PixInsight, or one of the resizing plugins available for Photoshop.
Another case of jaggies (moire, actually) may be due to using Lens Correction. See this article: