I've been studying the way detail is rendered on a pixel level by different demosaicing algorithms and the various artifacts that each method produces, and how each renders noise grain textures. This in turn is amplified in different ways by different capture sharpening tools. In particular, almost all the sharpened examples I provide below are using Focus Magic, as this is the tool that is highly recommended by someone I highly respect, Bart van der Wolf.
Before someone comes along to say that I won't ever see any of these issues in print, for the record I do see them in prints. I have a bunch of images from years ago that are around 10-20MP, before I learned to stitch to get more native resolution. I have made average to medium-large sized prints (16x20", 20x25", 30x40") with them. I am interested to perfect these images on a pixel level so prints of those sizes can be made that will be largely artifact free.
Some time ago I transitioned from ACR to Raw Therapee to obtain the amazing AMaZE demosaicing. I found that on some of my images, where there are contrasting colors (yellow leaves against blue sky), I get significant mazing artifacts with ACR's demosaicing which I saw in my prints and that annoyed me. Textured surfaces like granite or tree bark or even skin texture in portraits do not reveal the deficiencies of the ACR demosaicing, and I had been quite happy in the beginning with ACR on more forgiving subjects such as these. Being a landscape photographer however, it is hard to avoid pictures where foilage might be set against a blue sky!
I have since noticed some disturbing artifacts with AMaZE demosaicing too - while it produces the best overall rendition of fine detail that has minimal mazing and zipper artifacts, some weird patterns form along diagonal edges of relatively high contrast. Note that you will not see these artifacts in edges that are vertical or horizontal or almost that. They are most obvious in diagonal edges (see first attached example, red arrows point to where one might see these artifacts). It is a sort of stair-stepped ringing artifact, with lines of varying contrast at exactly 45 degrees. I do not mean the different kind of digital jaggies, which is what happens with DCB demosaicing for example that is more coarse (you get 2x2 pixel blocks of steps rather than 1 pixel steps with AMaZE, which is the finest possible step). It's a single pixel wide line of darker dots, always a perfect 45 degree slant, and these lines often appear next to each other and overlap, similar to the kind of ringing artifacts one might see with Lanczos 8 interpolation. I wonder if the anti-zipper smoothing algorithm of AMaZE is the cause of this kind of artifact, making zipper artifacts - alternating dark pixels at a 45 degree angle - smooth perhaps means joining them up. Another artifact that is observed is a dark single pixel tends to appear in the corners of edges, like on the tip of a leaf. They are peppered throughout the provided example.
When I capture sharpen with Focus Magic, it enhances the stair-stepping to a very high degree, making edges look to have strong ringing artifacts. The black-dots artifact is also enhanced to a large degree. Using the blend-if sliders to tame the dark halo reduces the effect, but does not eliminate it as it is present already. I've noticed that using Topaz Detail's Deblur function, the ringing artifacts are hidden to a very large degree, except on some really problematic edges. I wonder if Focus Magic is creating these ringing artifacts - but a sanity check on a DCB demosaic version shows no such 45 degree diagonal lines. I also noticed that the thin-ness of edges, if there is indeed such a way of describing how crisp an edge is, is much finer than what I can achieve with even radius 1 in Focus Magic. The result is that edges appear to be more crisp, though at lower percentage viewing on monitors, the fatter halos with Focus Magic gives the sensation of a 'sharper' image, but when upsampled ~400% - 800% for printing larger and to serve 600ppi data to my Canon printer, Topaz Detail's Deblur (or Infocus) produces a visible cleaner edge which lends itself to more (better) output sharpening.
I was interested to see how good the latest Capture One V10 renders detail and grain pattern, as Bart has mentioned several times that the demosaicing has been significantly improved (since Ver 9 I believe). Indeed it has, but unfortunately in my comparisons I am not convinced that it is better overall than AMaZE. There is still quite some significant zipper artifacts (see second attached crop example), and the contrast of the zipper artifact pixels is quite high, so when sharpened, looks even worse than ACR in some regions (hunt for them in the full resolution tiff!). But it must be noted that ACR's demosaicing is significantly worse for mazing and zipper artifacts overall, with lots of false color, and has a weird soft look to the detail perhaps owing to what looks like noise filtering. Capture One and Raw Therapee (with AMaZE, DCB, LMMSE and AHD) all exhibit much more crisp rendition of detail with varying levels of mazing and zipper artifacts, but the grain structure is correspondingly crisper but harsher, requiring some luminance noise reduction to reduce it to ACR levels. I notice that Capture One V10's grain structure has the occasional relatively very light and very dark pixel that appears sporadically, and some correlations in the noise pattern occurs (with zero sharpening of any sort). I also see that AHD creates some kind of weird hash pattern (almost like a JPEG artifact) in the grain, which can be seen also with DCB and LMMSE, and AMaZE even, but it is less so in the latter. Capture One exhibits a small amount of the hashing pattern, visible in the second attachment where it is enhanced by the Focus Magic sharpening.
I am providing here a few full resolution examples to show what I am referring to, including links to the original raw test image. These are layered tiffs (warning: huge) containing the full resolution renditions, converted with different demosaicing from ACR 9.1.1, Raw Therapee and Capture One V10, and then a copy of each with Focus Magic sharpening applied. The layer names should be self-explanatory. The bottom set of layers, without the 'FM 1 300%' suffix in the layer name, is just converted from raw, with the white balance and exposures matched as best as I can, with zero curves, sharpening, noise reduction and all other adjustments zeroed out. You can try your own editing on the sample raw file if you wish, if you don't trust my layered tiff and wish to come to your own conclusions.
Download Links:
Raw image sample. Canon 5D II capture. For personal use only please.
https://drive.google.com/open?id=0B5AXKSbQEPuFODBkVjBwLWlwRFkLayered Tiff (1.3GB) This is the above raw image converted. I included the AMaZE rendition sharpened with Topaz Detail's Deblur as a comparison to the Focus Magic Sharpened layer. Toggle the layer visibility for best visual comparison (easier to see than the side-by-side example crops below) Note that I used Focus Magic at 300% amount to enhance the artifacts of the sharpening process, and I NEVER use such a high amount ever for capture sharpening, but the artifacts are still there, just less visible/amplified. That is, the width of the halos, and the amount of ringing, is the same but with less contrast only at lower amount percentages. Pan about at 300% view or even more - there is a fascinating minefield of artifacts to look at. Flip between the unsharpened and no noise reduction originals (bottom layers) and the sharpened layers. It's interesting to see how each demosaicing algorithm renders detail on a real world image, rather than on some synthetic target. Some leaves show significant mazing around them in the blue sky region, some leaves show significant zipper artifacts. Some don't show any. Complex results.
https://drive.google.com/open?id=0B5AXKSbQEPuFU1hNMnhxNXQ5UjgLayered Tiff (2GB) This is a different example, a frame from a Sony A7R II, underexposed and with extremely low contrast and shot at ISO 100 which makes it a good candidate to look at fine grained noise when the data is stretched quite a lot with a curves adjustment layer. AMaZE renders the grain of the image quite harshly compared to ACR, with random extra light and dark pixels peppered about. ACR has perhaps the best noise grain pattern, which is not just softer but the intensity of the salt and pepper noise is leveled out to around the average deviation. It looks like ACR has some noise filtering going on in the demosaicing, despite the noise reduction settings zeroed out. This example has large flat gray regions of almost no detail to allow a study of the rendition of grain patterns by the various demosaicing algorithms, but also some contrasty detail in the bottom corner to look at artifacts on high contrast edges too. Not as good as above example for looking at edge artifacts however.
https://drive.google.com/open?id=0B5AXKSbQEPuFbGJjWVBVZXNIdUUIt would be interesting to hear if anyone has found ways to mitigate such artifacts, whist maintaining the highest possible amount of detail and crisp edges that looks good. Bart, if you happen to see this and have the time to comment, it would be most appreciated. It is always enlightening to read your thoughts on achieving high quality results. I find it interesting that I prefer Deblur's rendering of edges over Focus Magic, which I have long held as the better capture sharpening tool because you said so on several occasions. I also find it weird that I cannot find information detailing the issue of diagonal patterns with AMaZE demosaicing.
I suppose overall I find AMaZE to give the best rendition of fine detail, but it needs to be coupled with Topaz Detail's Deblur or Topaz Infocus rather than Focus Magic to avoid making the majority of the 45 degree diagonal ringing lines visible, and one can use the blend-if sliders to tame the peppering of dark almost-black pixels on certain edges. I just wish AMaZE will not generate the disgusting diagonal lines!
